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Studies covering combined effects of agronomic management strategies and their interactions for safer rice production in multiple heavy metal [cadmium (Cd) and lead (Pb)] and metalloid [(arsenic (As)] contaminated soil are rare. Therefore, the effectiveness of phosphorus (P) or silicon (Si) addition together with soil water management (flooded or aerobic) in a soil contaminated with multiple heavy metals was tested in a pot experiment for 8 weeks. Growth of rice was retarded under flooding (27%) compared to aerobic conditions, and application of P led to recover the reduction in growth. Aerobic and flooded water managements could decrease 66% As and 84% Cd concentrations in rice tissues, respectively. Applications of P or Si could decrease only 22% As (only under flooded condition) and 24% Cd (only under aerobic condition) in rice tissues. Accumulation of Pb in rice was not observed. Aerobic soil lowered the concentrations of As, Cd and Pb in the soil solution when compared to flooding. Moreover, As and Pb concentrations in soil solution increased with time while that of Cd declined. Appropriate water management was the most efficient in reducing heavy metals and metalloids accumulation in rice seedlings while the effects of P or Si were marginal. [ABSTRACT FROM AUTHOR]

Background: Pig slurries account for a large proportion of the organic manure available worldwide. Because of the large range of slurry compositions, farmers need fast and accurate alternatives to laboratory methods to assess nutrient concentrations in these farm‐based fertilizers. In this way, nutrient emissions into the environment can be reduced and targeted nutrition of crops can be implemented, while mineral fertilizers can be saved. Aim: The objective was to evaluate whether it is possible to rapidly determine ammonium and potassium concentrations in pig slurries on‐farm using ion‐selective electrodes (ISE) and to compare the measurement with established methods. Methods: A total of 391 pig slurry samples were collected from farms in north‐west Germany, and ammonium and potassium concentrations were analyzed in the laboratory. Then the composition of pig slurries was measured with ISE, the Quantofix‐N‐Volumeter (QV) and an electrical conductivity (EC) meter and regression models were used to evaluate the on‐farm quick tests. Results: For the 391 pig slurries studied, regressions with an R2 of 0.92 for ammonium and 0.91 for potassium were achieved for concentrations measured with the ISE in comparison with laboratory analyzes. The model fits for the electrodes were significantly higher than those based on the EC for ammonium and potassium, and it was equivalent to the QV method for the assessment of the ammonium concentration. Conclusion: Due to the simple and rapid measurement, ISE offer high potential for on‐farm quantification of ammonium and potassium concentrations in pig slurries. [ABSTRACT FROM AUTHOR]

Potassium uptake efficiency of safflower and sunflower was studied under semi-controlled conditions in loamy and sandy soils. Both species performed better in loamy soil. Safflower had higher agronomic efficiency and higher relative root length under suboptimal K supply. Safflower had higher specific root density and less root radius at all K levels. Safflower had higher relative root-shoot ratio under suboptimal K in loamy soil. Both species had similar K-influx at low and optimal K in loamy soil, while sunflower had higher influx under suboptimal and optimal supplies in sandy soil. Safflower had higher shoot demand in both soils under suboptimal and optimal K. Both species depleted similar amounts of soil solution-K under suboptimal K in sandy soil, while sunflower was more efficient under suboptimal levels in loamy soils. Sunflower depleted more extractable-K under both suboptimal and optimal K. Safflower could be considered K-uptake efficient crop. [ABSTRACT FROM AUTHOR]

Nitrogen (N) budgets at farm level are influenced by N fertilisation recommendations. In this study, we reviewed and analysed the underlying principles and methods of N fertilisation recommendations in 10 West European countries, to identify similarities and differences, and develop suggestions for reconsideration and improvement. An analysis of national official documents on N fertilisation recommendations revealed that there were three main categories of calculation methods: (i) 'N mass balances' (France, Italy, Spain), (ii) 'Corrected standards' (Germany, Netherlands, Switzerland, Luxembourg), and (iii) 'Pre‐parameterised calculations', which rely on a soil N supply typology (United Kingdom, Ireland, Belgium). In total 16 variables were identified in the calculation methods. The more complex methods use 10 (Italy, France), while the simplest only rely on 3 (Luxembourg). The most common variables include the availability of N in manure, the N uptake by a crop, and the N released by crop residues. Few countries explicitly consider N losses to ground and surface waters or to the atmosphere in the calculation methods. In some countries, the N fertilisation recommendation has a voluntary status, and in other countries, a legal one (caps on maximum allowable N rates). We compared the N fertiliser recommendations for a wheat crop grown on a farm with livestock, and for a farm with a diverse arable crop rotation without livestock. Across the 10 countries, large differences in the N fertilisation calculation methods and resulting N recommendations existed for the two management scenarios, ranging from almost no fertilisation to 135 kg N ha−1, and from 111 to 210 kg N ha−1, respectively. The differences were not accounted for by the complexity of the equations used, but rather resulted from contrasting reference values for N availability in manure, N uptake by crop and N leaching. However, the study concluded that standardisation of the method to calculate N fertilisation recommendations is likely to be counterproductive as there are no objective reasons to favour one method more than the others. Nonetheless, improvements in N use efficiency are necessary. Farm scale mass balance, combined with parameters such as minimum residual soil mineral N test at harvest, was suggested as being an important consideration. [ABSTRACT FROM AUTHOR]

A global data analysis shows that rice grain arsenic (As) concentrations increase with increasing soil As concentrations until about 60 mg As kg −1 soil and then decreases. Of the total grain As, 54% is composed of inorganic As. Therefore, when considering the WHO-permissible grain inorganic As concentration, i.e. 0.2 mg As kg −1 , the permissible grain total As concentrations is 0.37 mg total As kg −1 grain. Soil total As concentration when grain total As concentration reaches permissible level is 5.5 mg As kg −1 soil. Therefore, the suitable soil As concentrations for screening rice cultivars in rice agroecosystems for As resistance is 5–60 mg As kg −1 soil. Rice has traits to reduce uptake and translocation of As to grains. Cultivars with higher root porosity, radial oxygen loss, or formation of iron plaques bind more As to iron plaques, reducing As uptake (i.e. As avoidance). Once taken up, glutathione/glutaredoxin-mediated As reduction, and phytochelatin-dependent complexation and sequestration in vacuoles result in less translocation of As to the grain. Moreover, generation of reactive oxygen species and the production of antioxidant enzymes further reduce As toxicity (i.e. As resistance). These resistance mechanisms in rice agroecosystems are further enhanced when adequate concentrations of silicon and sulfur are present in soils and tissues, and when plants are associated with arbuscular mycorrhizal fungi, particularly under aerobic or intermittent-aerobic soil condition. Therefore, As concentrations in rice ecosystems decrease in the order of: roots > leaves > grains, and in grains: hull > bran polish > brown rice > raw rice> polished rice > cooked rice. Within the grain, As speciation is affected by the location in the grain, forms of As species, the grain-filling stage, geographic origin, ecosystem management and cultivars used. Indica type accumulates more As in their grains than japonica type. Rice grain production, within safe limits of As, requires the consideration of soil As dynamics including soil management, cultivar responses including uptake and translocation, and post-harvest processing techniques. [ABSTRACT FROM AUTHOR]

Abstract: Anaerobic digestates (AD) from biogas production are applied to agricultural land as organic fertilizers, but pose an ammonia (NH3) emission source. However, data about NH3 emissions of cold season AD land spreading is still lacking. Therefore, in the present study NH3 emissions of AD application under winter conditions were determined. AD was applied via trail hoses to a field plot of 27 ha in Northern Germany during the winter with temperatures around the freezing point and partly frozen soil. NH4 + N application rate was, including a preceding urea application, 123 kg NH4 + and urea N ha−1. The NH3 volatilization was monitored using Open Path Fourier Transform Infrared spectroscopy in combination with a micrometeorological transport model. Cumulative NH3 volatilization during the six day measurements was 17.5 kg NH3 N ha−1 which corresponds to 33.1% of the NH4 + N in applied AD. This NH3 loss is relatively high for low temperature conditions and was most likely caused by the frozen soil restricting AD infiltration. [Copyright &y& Elsevier]

Background: In a long‐term rotation experiment (2016–2022) with different nitrogen (N) fertilizer levels in subtropical South‐India, crop yields of low N plots were unexpectedly high. We therefore hypothesized that in the absence of mineral N application, these yields are largely due to N inputs by N2 fixation in the component crops. To assess the diazotrophic N2‐fixation of lablab (Lablab purpureus L. Sweet) and possible associative N2‐fixation of finger millet (Eleusine coracana L. Gaertn), a controlled experiment was conducted during the 2021 monsoon season within the above‐mentioned long‐term field study. Two approaches were used to estimate the quantity of N derived from the atmosphere (Ndfa): the dilution method using a 15N‐labeled fertilizer and the natural abundance method. Method: For the 15N dilution method irrigated maize (Zea maize L.), finger millet and lablab were labeled with two split applications of 10% 15N fertilizer (50:50 15N‐urea and 15N‐ammonium sulfate) amounting to a total of 15 kg N ha−1. Maize was selected as the non‐fixing reference plant to estimate diazotrophic N2‐fixation. The entire aboveground biomass of the labeled plants was harvested at maturity and analyzed for total DM, N concentration, and the 15N isotope ratio. Results: N2 fixation efficiency for lablab was 52%–69% depending on the calculation method, corresponding to 40–53 kg N ha−1. For finger millet, the natural abundance method resulted in an estimated N2‐fixation of 5 kg N ha−1, which was suggested by the results of the dilution method whereby the reference plant maize was only poorly labeled. Conclusion: Labeling of maize might have been diluted due to unexpected associative N2‐fixation or N‐uptake from unlabeled deep soil N pools. The data underline the importance of symbiotic N2‐fixation in crop rotation systems of South‐India. [ABSTRACT FROM AUTHOR]

There is a lack of affordable and effective strategies to mitigate multiple heavy metal contamination in lowland rice production. We tested phosphorus (P) or silicon (Si) addition to a soil contaminated with both arsenic (As) and cadmium (Cd) for their mitigation potential in rice seedlings. Rice variety IR64 was grown in pots with either P (0, 2.5, 5, 10, 20 (recommended rate), 40 and 80 mg P kg−1 soil), or Si (0, 0.15, 0.3, 0.6, 1.2 and 2.4 g Si kg−1 soil) addition until the end of tillering phase (nine weeks). Over this period, growth of rice was enhanced by up to 35% with the addition of P until the recommended rate, whereas growth was reduced beyond 0.3 g Si kg−1 application. Photosynthetic rate and maximum PSII quantum yield of the youngest fully expanded leaves were similar among P and Si treatments. Phosphorus addition increased shoot [As] and decreased shoot [Cd] by up to 59% and 63%, respectively, and both effects were visible only after plants reached P sufficiency. Silicon addition from 0 to 2.4 mg Si kg−1 soil increased shoot [As] by up to 28% and decreased [Cd] by up to 25%. Accumulation of As and Cd in the leaves from top to bottom of the canopy increased by factors of 10 and 7.6, respectively. Therefore, P or Si application cannot generally be recommended as remedy for rice production on multiple heavy metal contaminated soil. However, rice plants have efficient mechanisms to translocate As and Cd to mature leaves. [ABSTRACT FROM AUTHOR]

Background: In practical farming, there is often a need for short‐term availability of information on the soil nutrient status. Aims: To develop a new express method for the extraction of major plant‐available nutrients and measurement of soil nutrients. In future, this method shall serve for in‐field measurements of soil samples with an ion‐sensitive field‐effect transistor (ISFET). Methods: Various extraction conditions such as type of extractant, soil‐to‐solution ratio, time, and intensity were investigated on a broad selection of dried soil samples in the laboratory. Based on 83 field‐moist soil samples with varying clay contents, these conditions were compared to standard laboratory methods. Results: With increasing extraction time, the nutrient concentrations increased. When the soil‐to‐solution ratio was reduced, a greater share of nutrients was extracted, independent of soil type. H2O and 0.01 M CaCl2 and standard calcium‐acetate‐lactate (CAL) solution proved to be too weak in the short period to reach the ISFET sensor measurement range. Higher concentrated CAL solutions performed much better. Finally, a 5‐min CaCl2 extraction followed by the removal of an aliquot for the determination of soil pH and NO3− was found to be effective. The remaining solution was then mixed with 0.20 M CAL solution for the analysis of H2PO4− and K+ at 10 min of extra extraction time. This extraction method showed very good correlations with the values based on the German laboratory reference methods for pH (R2 = 0.91) and for nitrate (R2 = 0.95). For phosphorus and potassium, we obtained an R2 of 0.70 and 0.81, respectively, for all soils. When soils were grouped according to clay content higher correlations were found. Conclusions: A new express method based on a wet‐chemical approach with a soil preparation procedure was successfully developed and validated. This seems to be a valuable basis for future in‐field measurements via ISFET. [ABSTRACT FROM AUTHOR]

Concentration of mineral elements in rice seeds may vary depending on the soil fertility status and seed polishing (milling). In order to test this, 54 rice varieties were grown in the fields used to cultivate rice either with or without applying fertilizers for over 40 years. Part of the seeds collected from those varieties was polished before using for element analyses using ICP-MS while the rest considered as unpolished seeds. When comparing unpolished rice seeds produced at fertilized and unfertilized sites, the concentrations of Mg, Ca, K, Mn, Co, Cu, As and Cd were decreased, and the concentrations of Na, Al, Fe, Zn, and Mo were unchanged from fertilized to unfertilized sites. When comparing unpolished and polished rice seeds produced at fertilized and unfertilized sites, seed Mg, K, Mn, Fe, Co and Zn concentrations were higher in unpolished seeds than those in polished seeds, and the concentrations of Na, Al, Mo, Cd, As and Pb were similar between polished and unpolished seeds. Estimated values revealed that rice contributed 4–15% of the daily requirement of Fe, Cu, Zn, and 50–60% Mo irrespective of the soil fertility and polishing. Seed concentrations of As, Pb and Cd were lower than the maximum allowable levels. However, rice consumption contributed to the daily intake of 4–8 µg as, 2–6.5 µg Cd, and 19–25 µg Pb. Improving soil fertility is required to enrich rice seeds with most of the micronutrients and the milling should be done with much care to minimize nutrient loss in human diet. [ABSTRACT FROM AUTHOR]

The aim of the study was to investigate the influence of potassium (K) nutrition and/or aeration of flooded soil through drainage on iron (Fe) toxicity of rice during the period of establishment. A hydroponic experiment with three Fe (0.1, optimal; 1, high; 2.5 mM, very high) and K levels (0.3, low; 2, optimal; 5 mM, high), and a soil experiment with two moisture levels [continuously flooded; alternate-wetting and drying (AWD)] and three Fe levels (16, low; 163, optimal; 490 mg Fe(II) kg−1 soil, high) was conducted. In the hydroponic experiment, high Fe supply increased tissue concentrations of Fe, phosphorus (P) and magnesium (Mg), and reduced manganese (Mn) concentrations. Particularly Fe, K and P were maintained at root surfaces in larger amounts than taken up by the roots. Growth of rice, photosynthesis, maximum quantum yield, effective quantum yield and electron transport rate were decreased at high and very high Fe concentrations while K application had no positive effect. In the soil experiment, growth and nutrient relations under increased Fe supply were similar to those observed in the hydroponic experiment. However, comparing both moisture levels showed that soil solution Fe concentration and plant growth were reduced and tissue P, Mg and Mn concentrations were increased in the AWD system compared to flooding. Therefore, the application of K or aeration of flooded soil cannot be generalized as promising strategies to alleviate Fe toxicity in rice at the initial crop establishment stage. [ABSTRACT FROM AUTHOR]

Under the hot and moist conditions of irrigated agriculture in the arid subtropics, turnover of organic matter is high, which can lead to considerable carbon (C) and nitrogen (N) losses. Therefore, sustainable use of these soils requires regular manure application at high rates. To investigate the contribution of consecutive manure applications to an arid sandy soil to various soil N pools, goat manure was isotopically labeled by feeding 15N‐enriched Rhodes grass hay and applied to the soil during a two‐year field experiment. In the first year, soils received 15N‐labeled manure to distinguish between soil‐derived and manure‐derived N. In the second year, these plots were split for the application of either 15N‐labeled or unlabeled manure to discriminate N derived from previous (first year) and recent (second year) manure application. Soil samples (of control and 15N‐manured soil) were collected at the end of the first and the second year, and incubated in two laboratory experiments with labeled or unlabeled manure. At the beginning of Experiment 1, 7% of total N, 11% of K2SO4 extractable N, and 16% of microbial biomass N were derived from previously field‐applied manure. While the application of manure during incubation increased microbial biomass N by 225% and 410% in the control soil and the previously field‐manured soil, respectively, N2O emissions were more affected on the control soil, releasing considerable amounts of the soil N‐pool (80% of total emissions). In Experiment 2, 4% of total N, 7% of K2SO4 extractable N, and 7% of microbial biomass N derived from previously applied manure, and 4%, 8%, and 3% from recently applied manure, respectively. Microbial biomass N and N2O‐N derived from manure declined with time after manure application, whereas in Experiment 1 this tendency was only observed for microbial biomass N. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen (N) surpluses from fertilizer application can cause major environmental harm including pollution of surface water, groundwater, and air. To assess such negative externalities, N balances are a complex but useful tool to predict surpluses and to measure effects of nutrient optimization strategies in agriculture. The Yaqui Valley in north‐western Mexico is representative for thousands of square kilometres of intensive, irrigated wheat production under arid conditions worldwide and has been targeted for conservation agriculture in recent years. For these cropping systems, detailed N balances are scarce and often incomplete. To help fill this knowledge gap, data from a long‐term experiment were collected in 2013/14 on a Vertisol to examine the impact of three tillage‐straw management practices (CTB: conventionally tilled beds; PB‐straw: permanent raised beds with residue retention; PB‐burn: permanent raised beds with residue burning) on N dynamics. Tillage had significant effects on soil NO3‐N, NH4‐N, and total N contents across the cropping period. Soil total N content was at all sampling depths lowest in CTB. Soil NO3‐N in the 0–90 cm profile was highest in PB‐burn over the cropping period and ranged from 77 kg ha−1 in the bed before pre‐planting fertilizer application up to 269 kg ha−1 in the furrow after the second fertilizer application. Annual simple N balances were +59 kg N ha−1 in CTB, +39 kg N ha–1 in PB‐straw, and +46 kg N ha−1 in PB‐burn. Residual mineral soil N was significantly affected by tillage‐straw management and lowest for PB‐straw (+205 kg N ha−1) and highest for CTB, and for PB‐burn (+283 kg N ha−1 each) in the 0–90 cm soil profile. Soil NO3‐N moved out of the effective wheat root zone, as indicated by the high residual NO3‐N content at 30–90 cm depth, which is an important pathway of N leaching. Quantifiable N losses through leaching and volatilization averaged 100 kg N ha−1. Our findings suggest that there is potential for substantial reductions in N inputs in all tillage‐straw systems to decrease N losses and to reduce mineral residual soil N, but care should be taken to avoid reducing grain protein content, which in PB straw was already below the quality standard. A knowledge transfer of the European “Nmin” concept is advisable in this region to regulate N fertilizer over‐application. [ABSTRACT FROM AUTHOR]

Topography has strong effects on microclimates; thus may influence the decomposition of organic matter, a key process determines soil nutrient availability and carbon fluxes in terrestrial ecosystems. Yet, little is known if and how topographic factors influence litter decomposition. We studied the effects of slope aspect (south- vs. north-facing slopes) and position (base and middle positions) on plant shoot litter and root decomposition. We analyzed dynamics of litter mass loss in a 442-day period, and soil and vegetation characteristics in a typical semiarid hilly grassland. Our results showed that the decomposition of roots was faster at south-facing than at north-facing sites, which can be explained by the 2 °C higher soil temperature at south-facing sites. Decomposition rate of shoot litter were not different between slope aspects. North-facing sites had 76% higher aboveground biomass and 80% higher belowground biomass than those at south-facing sites. Accordingly, plant N and C storages at north-facing sites were 67% and 76% higher than those at south-facing sites, respectively. The smaller plant carbon and nitrogen stocks and the faster root decomposition at south-facing slopes suggest higher proportions of plant C and N are lost from the ecosystem than that at north-facing slopes. This work highlights the necessity of taking slope aspect into account in carbon and nitrogen cycling studies in hilly grasslands. [ABSTRACT FROM AUTHOR]

Background: Optimum potassium (K) nutrition is essential for high sugar yield and quality of beet crops. Aims: We quantified the effect of potassium deficiency on sugar beet dry matter and sugar yield formation during the growing season. Methods: Sugar beets were grown on low, medium, and high soil K concentrations in a long‐term K fertilizer field experiment on a silt loam alluvial soil. Plants were harvested at four time points during the growing season, including the final harvest. At each harvest, the dry matter yield and K concentration of leaves and beets (when applicable) were recorded, allowing us to study their growth rates in different growth phases. At the final harvest, sugar concentration and internal beet quality were measured. Results: Low soil K concentrations resulted in an approximately 10% lower sugar yield compared to high soil K supply. Interestingly, retarded growth of low K plants already occurred during the germination phase and in the first weeks thereafter, whereas later on, growth rates were similar. Higher soil K levels led to an increased K uptake rate and thus a higher final total K uptake. High K plants showed rapid youth growth and reached their maximum growth rates earlier than plants grown under low K conditions. The initial differentiation between the low and high K treatments persisted because the plants subsequently grew at similar growth rates at both low and high soil K supplies. Conclusion: The early growth reduction in the low K treatment was probably caused by the low amount of K stored in sugar beet seeds in combination with low K availability in the soil. It is concluded that under overall low soil K conditions, adequate amounts of K fertilizer should be applied to meet the demand of sugar beet seedlings for maximum growth and final sugar yield. Banded K fertilizer application next to the plant rows is supposed to be efficient for this purpose. [ABSTRACT FROM AUTHOR]

The increasing probability of seasonal droughts and freshwater scarcity emphasizes the importance of crop traits such as water-use efficiency (WUE) and its relation to nutrient management. In an earlier study using soil substrate in a pot experiment, we reported significant positive effects of N supply on biomass WUE of tobacco. However, there was a debate that the latter may be due to indirect effects of N supply (hidden drought), most likely because plants supplied with adequate N generally have greater biomass and hence faster depletion of soil water in the root zone. In pot and in field situation, therefore, it is difficult to relate any variation in leaf or biomass WUE to the direct effect of N supply. In this context, the aim of the current study was to re-examine to what extent N fertilization directly affects biomass WUE and related parameters under non-limiting water supply in hydroponics. About 2 weeks after the transfer of tobacco seedlings into nutrient solution containing 2 mM N, plants were treated with high-N or low-N in the form of NH4NO3. A marked decrease in CO2 assimilation with low-N supply compared to high-N supply was measured already 5 d after onset of treatments (DAO). In contrast, three different experiments clearly showed that stomatal conductance (gs) remained almost constant until 5 DAO resulting in significantly lower leaf WUE under low-N compared to high-N. Leaf WUE decreased gradually (up to 42% lower leaf WUE) at later stages. Surprisingly, biomass WUE and whole-plant δ13C values were not affected by N supply at any harvest date, which is in contrast to our earlier report where we observed clearly positive effects of N supply on biomass WUE of the same tobacco variety in a pot experiment with soil substrate. Night-time respiration and transpiration rates (measured by gas exchange and thermal imaging) were significantly higher with high-N supply than with low-N supply. The data show that 48.6% and 9.8% of the beneficiary effect of N on daytime leaf WUE were lost when nocturnal stomatal conductance and night-time respiration of the same leaves were taken into consideration. Thus, we conclude that earlier reports showing positive effects of N supply on biomass WUE and δ13C values in soil or field experiments may be due to indirect effect of N supply ( e.g., hidden/mild drought). [ABSTRACT FROM AUTHOR]

The objectives of this field trial were to collect reliable measurement data on N2 emissions and N2O/(N2O + N2) ratios in typical German crops in relation to crop development and to provide a dataset to test and improve biogeochemical models. N2O and N2 emissions in winter wheat (WW, Triticum aestivum L.) and sugar beet (SB, Beta vulgaris subsp. vulgaris) were measured using the improved 15N gas flux method with helium–oxygen flushing (80:20) to reduce the atmospheric N2 background to < 2%. To estimate total N2O and N2 production in soil, production-diffusion modelling was applied. Soil samples were taken in regular intervals and analyzed for mineral N (NO3− and NH4+) and water-extractable Corg content. In addition, we monitored soil moisture, crop development, plant N uptake, N transformation processes in soil, and N translocation to deeper soil layers. Our best estimates for cumulative N2O + N2 losses were 860.4 ± 220.9 mg N m−2 and 553.1 ± 96.3 mg N m−2 over the experimental period of 189 and 161 days with total N2O/(N2O + N2) ratios of 0.12 and 0.15 for WW and SB, respectively. Growing plants affected all controlling factors of denitrification, and dynamics clearly differed between crop species. Overall, N2O and N2 emissions were highest when plant N and water uptake were low, i.e., during early growth stages, ripening, and after harvest. We present the first dataset of a plot-scale field study employing the improved 15N gas flux method over a growing season showing that drivers for N2O and N2O + N2 fluxes differ between crop species and change throughout the growing season. [ABSTRACT FROM AUTHOR]

Herbivory and resource availability interactively regulate plant growth, biomass allocation, and production. However, the compensatory growth of plants under grazing intensities and manipulated environmental conditions is not well understood. A 2-year experiment with water (unirrigated and irrigated) and nitrogen fertilizer (0 and 75 kg N ha −1 year −1 ) addition was conducted at sites with 4 grazing intensities (0–7 sheep ha −1 ) in an annually rotational grazing system in Inner Mongolia. In this study, grazing had no significant effect on aboveground net primary production (ANPP) and net primary production (NPP). However, high grazing intensity strongly reduced the fraction of belowground net primary production to NPP. Water and nitrogen additions significantly increased ANPP by 39% and by 23%, respectively, but had no effect on belowground net primary production. ANPP showed lower response to nitrogen addition at high grazing intensity sites than at low grazing intensity sites. We found no evidence for grazing optimization on primary production of semiarid steppe, regardless of resource supplementations. Grazed plants minimized the reduction of ANPP by altering allocation priority and morphological traits. Our study highlights the “whole-plant” perspective when studying plant–herbivore interactions. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) emissions from agriculture demand attention because they are the main source of total global anthropogenic N2O-emissions. High N-fertilization and soil compaction are important factors that increase N2O-emissions. On intensively managed grassland sites both factors occur. Knowledge of the interaction of high N-fertilization and simultaneous soil compaction on N2O-emissions is therefore essential, but previous studies about this scenario are rare. In the presented study, N-fertilized grass swards (G) and unfertilized lucerne-grass mixtures (LG) were compared over a three-year period (2006-2008): N2O-emissions and dry matter yield were measured as a function of N-fertilization (0 (LG), 360 kg N ha-1 yr-1 (G) as CAN) and soil compaction (0 (C0), 321 kPa (C321)) on a loamy stagnic Luvisol derived from glacial till in northern Germany. CO2-equivalents (CO2eq) per hectare and per unit metabolizable energy (GJ ME) were calculated. N2O-emissions were significantly influenced by the interaction N-fertilization x soil compaction; emissions increased significantly when both factors were induced simultaneously (G/C0: 8.74, LG/C0: 2.46, G/C321: 13.31 and LG/C321: 2.22 kg N2O-N ha-1, respectively). Concerning the specific CO2-emissions, expressed in CO2eq (GJ ME)-1, the N-fertilized G swards emitted 67% more CO2eq than LG swards assuming that 50% of the field plots were compacted due to heavy wheel traffic, which are reliable figures from agricultural practice. Neither dry matter (DM) yield nor forage quality (MJ ME (kg DM)-1) differed significantly between fertilized G and unfertilized LG swards. Hence, legume-based instead of fertilizer-based forage production is a promising mitigation option without significant reduction of DM yields. In addition, results regarding soil compaction effects on GHG-emissions emphasize the urgent need to implement controlled traffic systems on intensively managed grassland in order to reduce the area affected by heavy wheel traffic. [ABSTRACT FROM AUTHOR]

RATIONALE N2O isotopomer ratios may provide a useful tool for studying N2O source processes in soils and may also help estimating N2O reduction to N2. However, remaining uncertainties about different processes and their characteristic isotope effects still hamper its application. We conducted two laboratory incubation experiments (i) to compare the denitrification potential and N2O/(N2O+N2) product ratio of denitrification of various soil types from Northern Germany, and (ii) to investigate the effect of N2O reduction on the intramolecular 15N distribution of emitted N2O. METHODS Three contrasting soils (clay, loamy, and sandy soil) were amended with nitrate solution and incubated under N2-free He atmosphere in a fully automated incubation system over 9 or 28 days in two experiments. N2O, N2, and CO2 release was quantified by online gas chromatography. In addition, the N2O isotopomer ratios were determined by isotope-ratio mass spectrometry (IRMS) and the net enrichment factors of the 15N site preference (SP) of the N2O-to-N2 reduction step (ηSP) were estimated using a Rayleigh model. RESULTS The total denitrification rate was highest in clay soil and lowest in sandy soil. Surprisingly, the N2O/(N2O+N2) product ratio in clay and loam soil was identical; however, it was significantly lower in sandy soil. The IRMS measurements revealed highest N2O SP values in clay soil and lowest SP values in sandy soil. The ηSP values of N2O reduction were between -8.2 and -6.1 ‰, and a significant relationship between δ18O and SP values was found. CONCLUSIONS Both experiments showed that the N2O/(N2O+N2) product ratio of denitrification is not solely controlled by the available carbon content of the soil or by the denitrification rate. Differences in N2O SP values could not be explained by variations in N2O reduction between soils, but rather originate from other processes involved in denitrification. The linear δ18O vs SP relationship may be indicative for N2O reduction; however, it deviates significantly from the findings of previous studies. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Iron (Fe) is an essential mineral element of plants. However, the current status of bioavailable Fe in lowland paddy fields in Sri Lanka is not known. This study was conducted to examine the (i) spatial distribution of bioavailable Fe concentration, and (ii) interactive effects of agro-climatic zone (ACZ), soil order, and water source in determining the bioavailable Fe concentration in lowland paddy fields in Sri Lanka. A total of 6003 soil samples representing six ACZs, six soil orders, and three water sources were collected using a stratified random sampling approach. Bioavailable Fe was extracted using 0.01 M CaCl 2 and measured using Inductively Coupled Plasma Mass Spectrophotometry. Bioavailable Fe concentration ranged between 0.0003 and 339 mg kg−1 with a mean of 49.4 mg kg−1. Samples collected from the Wet zone, particularly the Wet zone Low country, had higher Fe concentrations than those reported in other ACZs (p < 0.05). Among the soil orders, Histosols, Inceptisols and Ultisols had higher Fe concentrations while Entisols, Alfisols and Vertisols reported lower values (p < 0.05). Paddy fields receiving water from major irrigation systems in Wet zone Low country had more Fe. Moreover, Fe concentration was not correlated with rice crop productivity (p > 0.05). The spatial maps of bioavailable Fe generated in this study could be useful for making area-specific agronomic and administrative decisions based on ACZs, soil orders and water sources used in paddy cultivation in Sri Lanka. • Bioavailable iron concentration [Av-Fe] in soil varied from 0.0003 to 339 mg kg−1, with a mean of 49.4 mg kg−1 • A large majority of soil samples had Av-Fe in the optimal range for rice cultivation. • Crop productivity was not correlated with [Av-Fe]. • [Av-Fe] varied among agro-climatic zones, soil orders and water sources used. • Histosols, Inceptisols and Ultisols had higher [Av-Fe] than Alfisols and Vertisols. [ABSTRACT FROM AUTHOR]

In grazed semiarid steppe ecosystems, much attention has been paid to aspects of growth limitation by water. So far, potential limitation of primary production by plant nutrients was rarely considered. This knowledge is essential for identification of sustainable land-use practices in these large and important ecosystems on the background of over-exploitation and climate change. In the present study plant nutrient concentrations and ratios were investigated with factorial additions of water and N fertilizer at two sites with contrasting soil nutrient availability. Combined analysis of nutrient concentrations, contents, biomass production, and plant N:P ratios consistently confirmed primary growth limitation by water and a strong N limitation when sufficient amounts of water were supplied. P limitation only occurred at the site with low P availability when in addition to the natural supply, water and N fertilizer were given. According to reported thresholds of N:K and K:P ratios, K was not limiting in any plot. The observed nutritional patterns in the plant community were related to the dynamics of species composition and their specific nutrient status. Stipa grandis had the highest N:P ratio whereas Artemisia frigida showed lowest N:P. These nutrient characteristics were related to growth strategies of dominant species. Accordingly, the relative biomass contribution of S. grandis and A. frigida strongly affected the nutrient status of the plant community. Plant N:P ratios indicate the relative limitation by N or P in the semiarid grasslands under sufficient water supply, but other methods of nutritional diagnosis should be used when plant N:P ratios remain below critical values. [ABSTRACT FROM AUTHOR]

The Fengqiu long-term field experiment was established to examine effects of organic manure and mineral fertilizers on soil total nitrogen (N) and natural 15N abundance. Fertilizer regimes include organic manure (OM), one-half N from organic manure plus one-half N from mineral N fertilizer (1/2OMN), mineral fertilizers [N-phosphorus (P)-potassium (K), NP, NK, PK], and a control. Organic manure (OM and 1/2OMN) significantly increased soil total N and δ15N, which was expected as a great amount of the N applied remained in soils. Mineral NPK fertilizer and mineral NP fertilizer significantly increased total N and slightly increaed δ15N. Phosphorus-deficient fertilization (NK) and N-deficient fertilization (PK) had no effect on soil total N. Significantly greater δ15N was observed in the NK treatment as compared to the control, suggesting that considerable N was lost by ammonia (NH3) voltalization and denitrification in this P-deficiency fertilization regime. [ABSTRACT FROM AUTHOR]

Abstract: A new water-saving ‘Ground Cover Rice Production System’ (GCRPS) was evaluated in 2001 and 2002 near Beijing, North China. Using GCRPS, lowland rice was cultivated without a standing water layer during the entire growth period and plots were irrigated when soil water tension was below 15kPa (approximately 90% water holding capacity). In order to prevent soil evaporation, the soil surface was covered with 14μm thick plastic film (GCRPSPlastic) or mulched with straw (GCRPSStraw). In a third GCRPS treatment the soil was left uncovered (GCRPSBare). These treatments were compared with lowland rice cultivated under traditional paddy conditions (Paddy control). In an additional treatment with bare soil, one aerobic rice variety was cultivated. Compared to Paddy control, only 32–54% of irrigation water was applied in GCRPS treatments. Plants in GCRPS were smaller, developed fewer panicles and had a smaller leaf area index compared to Paddy control. Yield was significantly less in GCRPSBare and GCRPSStraw compared to Paddy control in both years, while yield in GCRPSPlastic was only 8% less than the Paddy control yield in 2002. Water-use efficiency (WUE, gram grain yield per litre water input) in GCRPSPlastic was higher (0.35) than in Paddy control (0.23). Grain yield was highly significantly correlated with maximum leaf area index and leaf area index duration. Among yield components, the number of productive tillers had the greatest positive effect on yield, while the number of grains per panicle, thousand-grain weight and harvest index remained almost unaffected. Under uncovered condition, the aerobic rice variety had a significantly higher harvest index (HI), yield, and WUE compared to the lowland rice variety (GCRPSBare). The experiment demonstrates that GCRPS has potential to save substantial amounts of water at relatively minor yield penalties, if stress factors such as low soil temperature, water deficit, and nutrient deficiencies during the vegetative growth stage are avoided by suitable management practices. [Copyright &y& Elsevier]

Nitrogen (N) fertilization is the major contributor to nitrous oxide (N2O) emissions from agricultural soil, especially in post‐harvest seasons. This study was carried out to investigate whether ryegrass serving as cover crop affects soil N2O emissions and denitrifier community size. A microcosm experiment was conducted with soil planted with perennial ryegrass (Lolium perenne L.) and bare soil, each with four levels of N fertilizer (0, 5, 10 and 20 g N m−2; applied as calcium ammonium nitrate). The closed‐chamber approach was used to measure soil N2O fluxes. Real‐time PCR was used to estimate the biomass of bacteria and fungi and the abundance of genes involved in denitrification in soil. The results showed that the presence of ryegrass decreased the nitrate content in soil. Cumulative N2O emissions of soil with grass were lower than in bare soil at 5 and 10 g N m−2. Fertilization levels did not affect the abundance of soil bacteria and fungi. Soil with grass showed greater abundances of bacteria and fungi, as well as microorganisms carrying narG, napA, nirK, nirS and nosZ clade I genes. It is concluded that ryegrass serving as a cover crop holds the potential to mitigate soil N2O emissions in soils with moderate or high NO3− concentrations. This highlights the importance of cover crops for the reduction of N2O emissions from soil, particularly following N fertilization. Future research should explore the full potential of ryegrass to reduce soil N2O emissions under field conditions as well as in different soils. Highlights: This study was to investigate whether ryegrass serving as cover crop affects soil N2O emissions and denitrifier community size;Plant reduced soil N substrates on one side, but their root exudates stimulated denitrification on the other side;N2O emissions were lower in soil with grass than bare soil at medium fertilizer levels, and growing grass stimulated the proliferation of almost all the denitrifying bacteria except nosZ clade II;Ryegrass serving as a cover crop holds the potential to mitigate soil N2O emissions. [ABSTRACT FROM AUTHOR]

Background and aims: Plant growth affects soil moisture, mineral N and organic C availability in soil, all of which influence denitrification. With increasing plant growth, root exudation may stimulate denitrification, while N uptake restricts nitrate availability. Methods: We conducted a double labeling pot experiment with either maize (Zea mays L.) or cup plant (Silphium perfoliatum L.) of the same age but differing in size of their shoot and root systems. The 15N gas flux method was applied to directly quantify N2O and N2 fluxes in situ. To link denitrification with available C in the rhizosphere, 13CO2 pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil. Results: Plant water and N uptake were the main factors controlling daily N2O + N2 fluxes, cumulative N emissions, and N2O production pathways. Accordingly, pool-derived N2O + N2 emissions were 30–40 times higher in the treatment with highest soil NO3− content and highest soil moisture. CO2 efflux from soil was positively correlated with root dry matter, but we could not detect any relationship between root-derived C and N2O + N2 emissions. Conclusions: Root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth. [ABSTRACT FROM AUTHOR]

Inherent low soil fertility remains a hindrance to potato production in Kenya and continues to pose a threat to food security. A study was conducted in Nyandarua and Meru counties to assess the soil fertility status in smallholder potato farms. Soil and plant tissue samples were collected and analysed for selected nutrients (pH, OC, N, P, K, S, Ca, Mg, Zn, B and Cu) from 198 farms. Critical nutrient levels were used to assess the sufficiency levels of nutrients for potato growth. Soils in the sampled farms were weakly to strongly acidic (pH-CaCl2 3.9–6.6) and had low to high soil organic matter content (1.5–97.5 g Kg−1). The percent of farms in Meru and Nyandarua with nutrient contents below critical levels were 66% and 20% for N, 46% and 85% for P, 67% and 31% for S, 9% and 51% for Cu, and 87% and 80% for B, respectively. Low tissue nutrient concentrations were observed for N, P, K, and S irrespective of the sites. Soil pH correlated strongly with majority of the analyzed soil and tissue nutrients. These results affirm the need to design integrative soil fertility management strategies to correct the impoverished soil fertility status in the study area. [ABSTRACT FROM AUTHOR]

Aims: Water use efficiency (WUE) of crop plants is an important plant trait for maintaining high yield in water limited areas. By influencing osmoregulation of plants, potassium (K) plays a critical role in stress avoidance and adaptation. However, whole plant physiological mechanisms modulated by K supply in respect of plant drought tolerance and water use efficiency are not well understood. In the present study, growth, development and transpiration dynamics of two barley cultivars were evaluated with and without PEG-induced osmotic stress using an automated balance system and image based leaf area determination.Methods: Experiments were conducted to study the effects of varied K supply under different osmotic stress treatments on a wide range of morphological, biochemical and physiological characteristics of barley plants such as leaf area development, daily whole plant transpiration rate (DTR), stomatal conductance (gs), assimilation rate (AN), biomass and leaf water use efficiency (WUE) as well as foliar abscisic acid (ABA) concentrations. Two barley cultivars (cv. Sahin-91 and cv. Milford) were treated with two K supply levels (0.04 and 0.8 mM K) and osmotic stress induced by polyethylene glycol 6000 (PEG) for a period of 9 days (in total 48 days experiment) in the hydroponic plant culture (non-PEG and + 20% PEG ).Results: Without PEG, low-K supply depressed dry matter (DM) by almost 60% averaged across both cultivars. Under osmotic stress (+PEG), total leaf area was reduced by almost 70% in low-K compared to adequate-K plants. Low K concentration under PEG stress was correlated with higher ABA concentration and was correlated with lower leaf- and whole plant transpiration rate. Biomass-WUE under low K supply decreased significantly in both barley cultivars, to a greater extent in cv. Milford under osmotic stress. However, leaf-WUE was not affected by K supply in the absence of osmotic stress.Conclusions: It was suggested that reduced biomass-WUE in low-K treated barley plants was not related to inefficient stomatal control under K deficiency, but instead due to reduced assimilation rate. It was further hypothesized that under low K supply, a number of energy consuming activities reduce biomass-WUE, which are not distinguished by measuring leaf-WUE. This study showed that low K supply under osmotic stress increases foliar ABA concentration thereby decreasing plant transpiration. [ABSTRACT FROM AUTHOR]

The sustainable production of bioenergy from crops like oilseed rape, barley, and maize presents a significant option to mitigate climate change by reducing fossil CO 2 emissions. Greenhouse gas emissions (specifically N 2 O) during the energy crop production need to be quantified precisely for reliable life cycle analysis of bioenergy cropping systems. Energy crops (specifically oilseed rape) have a very high N demand, which results in a higher N-fertilizer application and thus higher risk of N 2 O emissions not only during the vegetation period but also after crop harvest due to i) incorporation of N rich plant residue to soil and/or ii) residual N. An incubation experiment was conducted under conditions favoring denitrification (80% water-holding capacity), to study the drivers of N 2 O emissions specifically during the post-harvest period. Here we compared two different plant residues varying in C/N ratio (oilseed rape (RST) and barley straw (BST)) with or without N supply and measured CO 2 , and N 2 O emissions. Stable isotope labeling ( 15 N) was used to quantify soil- and residue-born N 2 O. Incorporation of both plant residues alone induced significant increases in CO 2 emissions compared to control soil without straw addition (p < .05). However, the increase in CO 2 emissions was less pronounced when straw was incorporated in conjunction with mineral N. There was a clear increase in cumulative N 2 O emissions (p < .05) when soil amended with BST or RST (6- and 9-fold) was compared to control, however, the increase of cumulative N 2 O emissions was drastic when mineral N was added (15- and 23-fold). No significant differences in N 2 O emission were observed when comparing residue types (p > .05). Stable isotope labeling of barley straw clearly showed that the share of residue-born N 2 O was very low (1.35 or 0.4%) in the overall N 2 O fluxes in BST and BST + N. The present study suggests that N fertilization in autumn should be avoided to minimize N 2 O fluxes regardless of type of straw. [ABSTRACT FROM AUTHOR]

The sustainable production of bioenergy from crops like oilseed rape, barley, and maize presents a significant option to mitigate climate change by reducing fossil CO 2 emissions. Greenhouse gas emissions (specifically N 2 O) during the energy crop production need to be quantified precisely for reliable life cycle analysis of bioenergy cropping systems. Energy crops (specifically oilseed rape) have a very high N demand, which results in a higher N-fertilizer application and thus higher risk of N 2 O emissions not only during the vegetation period but also after crop harvest due to i) incorporation of N rich plant residue to soil and/or ii) residual N. An incubation experiment was conducted under conditions favoring denitrification (80% water-holding capacity), to study the drivers of N 2 O emissions specifically during the post-harvest period. Here we compared two different plant residues varying in C/N ratio (oilseed rape (RST) and barley straw (BST)) with or without N supply and measured CO 2 , and N 2 O emissions. Stable isotope labeling ( 15 N) was used to quantify soil- and residue-born N 2 O. Incorporation of both plant residues alone induced significant increases in CO 2 emissions compared to control soil without straw addition (p < .05). However, the increase in CO 2 emissions was less pronounced when straw was incorporated in conjunction with mineral N. There was a clear increase in cumulative N 2 O emissions (p < .05) when soil amended with BST or RST (6- and 9-fold) was compared to control, however, the increase of cumulative N 2 O emissions was drastic when mineral N was added (15- and 23-fold). No significant differences in N 2 O emission were observed when comparing residue types (p > .05). Stable isotope labeling of barley straw clearly showed that the share of residue-born N 2 O was very low (1.35 or 0.4%) in the overall N 2 O fluxes in BST and BST + N. The present study suggests that N fertilization in autumn should be avoided to minimize N 2 O fluxes regardless of type of straw. [ABSTRACT FROM AUTHOR]

Abstract: Question: Climate change interacts with land use and introduces new pressures that trigger growing concerns about increasing vulnerability of the Eurasian steppes. However, it is not well known how increasing precipitation and atmospheric N deposition interact with the land use legacy to affect nutrient availability, plant species composition and therefore vegetation dynamics of the Mongolian Steppe. Location: Steppe in Xilin River Basin, Inner Mongolia, China. The mean annual precipitation is 343 mm with 60%–80% of it occurring during the plant growing season. Methods: We conducted a 6‐years (2005–2010) field experiment to manipulate N and water availability on sites experiencing two different historical stocking rates. Species composition, above‐ground biomass and plant N concentration were determined at both individual and community levels. Soil cumulative inorganic N and N mineralization rates were determined by laboratory incubation. Results: (1) Supplementary irrigation increased soil cumulative inorganic N and N mineralization rate, plant community N uptake, and the abundance of perennial species for the site with high historical stocking rate. In contrast, long‐term water addition decreased soil cumulative inorganic N and N mineralization rate, and did not change the plant community N uptake, but increased the abundance of Cleistogenes squarrosa as a species indicative of degradation for sites with moderate historic stocking rate. (2) Nitrogen addition increased soil cumulative inorganic N and N mineralization rate irrespective of grazing history under ambient precipitation, and resulted in a burst of annuals in moist years at both sites. Under supplementary irrigation, N addition increased soil cumulative inorganic N and N mineralization rate at the site with moderate historic stocking rate, but not at the site with high historical stocking rate, and increased the abundance of taller perennial species at both sites. Conclusions: Legacy effects of grazing greatly affect plant composition responses to increasing water and N availability. The effectiveness of N application in restoring heavily degraded sites in the Mongolian steppe depends on precipitation availability. In the face of increasing precipitation and atmospheric N deposition, resource managers should be prepared to cope with the different vegetation succession and recovery trajectories as a result of historical land‐use difference. [ABSTRACT FROM AUTHOR]

In mountainous regions with high altitude, rice yield is mostly limited by low temperatures and insufficient irrigation facilities. The innovative ground cover rice production system (GCRPS) has a recognised potential to significantly increase rice grain yield where rice production is limited by water scarcity and low temperatures. We hypothesised that yield advantage of GCRPS over traditional Paddy might become larger at higher altitudes. We sampled 14 pairs of adjacent GCRPS and Paddy fields at altitudes of 900 m and 23 pairs at 500 m altitude with 3 replicates in central China. The study revealed that Badano et al. (2005) grain and straw yield were 40% and 35% greater in GCRPS compared to Paddy at 900 m, while the difference was only 10% and 15% at 500 m Bennie et al. (2006). Compared to Paddy, increase in productive tiller numbers, spikelets per square metre and percentage of filled grains were significantly larger in GCRPS at high than at low altitude Bennie et al. (2008). Soil temperature differences between GCRPS and Paddy were significantly higher at 900 m than at 500 m during the first month after transplanting. Our findings demonstrate that GCRPS has a good potential to increase rice yield in mountain regions with high altitudes where rice production is limited by low temperature and seasonal water shortage. [ABSTRACT FROM AUTHOR]

Winter oilseed rape ( Brassica napus L., WOSR) is the major oil crop cultivated in Europe. Rapeseed oil is predominantly used for production of biodiesel. The framework of the European Renewable Energy Directive requires that use of biofuels achieves GHG savings of at least 50% compared to use of fossil fuel starting in 2018. However, N 2 O field emissions are estimated using emission factors that are not specific for the crop and associated with strong uncertainty. N 2 O field emissions are controlled by N fertilization and dominate the GHG balance of WOSR cropping due to the high global warming potential of N 2 O. Thus, field experiments were conducted to increase the data basis and subsequently derive a new WOSR-specific emission factor. N 2 O emissions and crop yields were monitored for three years over a range of N fertilization intensities at five study sites representative of German WOSR production. N 2 O fluxes exhibited the typical high spatial and temporal variability in dependence on soil texture, weather and nitrogen availability. The annual N 2 O emissions ranged between 0.24 kg and 5.48 kg N 2 O-N ha −1 a −1 . N fertilization increased N 2 O emissions, particularly with the highest N treatment (240 kg N ha −1 ). Oil yield increased up to a fertilizer amount of 120 kg N ha −1 , higher N-doses increased grain yield but decreased oil concentrations in the seeds. Consequently oil yield remained constant at higher N fertilization. Since, yield-related emission also increased exponentially with N surpluses, there is potential for reduction of the N fertilizer rate, which offers perspectives for the mitigation of GHG emissions. Our measurements double the published data basis of annual N 2 O flux measurements in WOSR. Based on this extended dataset we modeled the relationship between N 2 O emissions and fertilizer N input using an exponential model. The corresponding new N 2 O emission factor was 0.6% of applied fertilizer N for a common N fertilizer amount under best management practice in WOSR production (200 kg N ha −1 a −1 ). This factor is substantially lower than the linear IPCC Tier 1 factor (EF1) of 1.0% and other models that have been proposed. [ABSTRACT FROM AUTHOR]

Degradation of semiarid ecosystems from overgrazing threatens a variety of ecosystem services. Rainfall and nitrogen commonly co-limit production in semiarid grassland ecosystems; however, few studies have reported how interactive effects of precipitation and nitrogen addition influence the recovery of grasslands degraded by overgrazing. We conducted a 6-year experiment manipulating precipitation (natural precipitation and simulated wet year precipitation) and nitrogen (0, 25 and 50 kg N ha) addition at two sites with different histories of livestock grazing (moderately and heavily grazed) in Inner Mongolian steppe. Our results suggest that recovery of plant community composition and recovery of production can be decoupled. Perennial grasses provide long-term stability of high-quality forage production in this system. Supplemental water combined with exclosures led, in the heavily grazed site, to the strongest recovery of perennial grasses, although widespread irrigation of rangeland is not a feasible management strategy in many semiarid and arid regions. N fertilization combined with exclosures, but without water addition, increased dominance of unpalatable annual species, which in turn retarded growth of perennial species and increased inter-annual variation in primary production at both sites. Alleviation of grazing pressure alone allowed recovery of desired perennial species via successional processes in the heavily grazed site. Our experiments suggest that recovery of primary production and desirable community composition are not necessarily correlated. The use of N fertilization for the management of overgrazed grassland needs careful and systematic evaluation, as it has potential to impede, rather than aid, recovery. [ABSTRACT FROM AUTHOR]

Potassium (K) is crucial for crop growth and is strongly related to stress tolerance and water-use efficiency (WUE). A major physiological effect of K deficiency is the inhibition of net CO 2 assimilation (A N ) during photosynthesis. Whether this reduction originates from limitations either to photochemical energy conversion or biochemical CO 2 fixation or from a limitation to CO 2 diffusion through stomata and the leaf mesophyll is debated. In this study, limitations to photosynthetic carbon gain of sunflower ( Helianthus annuus L.) under K deficiency and PEG- induced water deficit were quantified and their implications on plant- and leaf-scale WUE (WUE P , WUE L ) were evaluated. Results show that neither maximum quantum use efficiency (F v /F m ) nor in-vivo RubisCo activity were directly affected by K deficiency and that the observed impairment of A N was primarily due to decreased CO 2 mesophyll conductance (g m ). K deficiency additionally impaired leaf area development which, together with reduced A N , resulted in inhibition of plant growth and a reduction of WUE P . Contrastingly, WUE L was not affected by K supply which indicated no inhibition of stomatal control. PEG-stress further impeded A N by stomatal closure and resulted in enhanced WUE L and high oxidative stress. It can be concluded from this study that reduction of g m is a major response of leaves to K deficiency, possibly due to changes in leaf anatomy, which negatively affects A N and contributes to the typical symptoms like oxidative stress, growth inhibition and reduced WUE P . [ABSTRACT FROM AUTHOR]

The application of reactive nitrogen (N) in the form of synthetic/organic fertilizers plays a central role in supporting a larger human population, but also contributes to global warming through the emission of nitrous oxide (N 2 O). The use of nitrification inhibitors (NIs) has repeatedly been shown to minimize N 2 O emissions; however, their effectiveness in reducing N 2 O emissions varies greatly under different environmental conditions. A better understanding of how and to what extent NIs can mitigate fertilizer-related soil-borne N 2 O emissions under a range of different conditions is required. In the present study, we carried out a soil incubation experiment in a fully automated continuous-flow incubation system under conditions favoring either nitrification- or denitrification-derived N 2 O emissions. Additionally, the abundance of AOB amoA , and AOA amoA genes was quantified and N 2 O isotopic signatures were analyzed. We mixed a common NI (PIADIN ® ) with mineral fertilizer (ammonium sulfate) and examined the N 2 O mitigation potential of the NI in a fertilized sandy soil (low denitrification potential) and a sandy soil mixed with wheat straw (high denitrification potential) at 70% water holding capacity (WHC). In non-NI treatments, the addition of straw led to a drastic increase of CO 2 and N 2 O emissions compared to the non-straw-amended soils, suggesting stimulated microbial activity and higher denitrification rate. The NI reduced N 2 O emissions in the straw-amended treatment by 41%, whereas in the treatment without straw this was only 17%. With the combination of N 2 O isotopic signatures and functional gene abundances, fungal denitrification was considered to be the major process contributing to the higher N 2 O fluxes specifically in straw-amended soils. Overall, our study indicated that NI can be used as an effective method for mitigating N 2 O emissions in cropland specifically when the denitrification potential is high, e.g. in moist N-fertilized and straw-amended soils. [ABSTRACT FROM AUTHOR]

Enhancing crop water-use efficiency (WUE) is a major research objective in water-scarce agroecosystems. Potassium (K) enhances WUE and plays a crucial role in mitigating plant stress. Here, effects of K supply and PEG-induced water deficit on WUE of spring wheat ( Triticum aestivum L. var. Sonett), grown in nutrient solution, were studied. Plants were treated with three levels of K supply (0.1, 1, 4 mM K+) and two levels of PEG (0, 25%). WUE was determined at leaf level (WUEL), at whole-plant level (WUEP), and via carbon isotope ratio (δ13C). Effects of assimilation and stomatal conductance on WUEL were evaluated and compared with effects of biomass production and whole-plant transpiration (EP) on WUEP. Adequate K supply enhanced WUEP up to 30% and by additional 20% under PEG stress, but had no effect on WUEL. EP was lower with adequate K supply, but this effect may be attributed to canopy microclimate. Shoot δ13C responded linearly to time-integrated WUEL in adequately supplied plants, but not in K-deficient plants, indicating negative effects of K deficiency on mesophyll CO2 diffusion. It is concluded that leaf-scale evaluations of WUE are not reliable in predicting whole-plant WUE of crops such as spring wheat suffering K deficiency. [ABSTRACT FROM AUTHOR]

Aims: In water-scarce agro-environments a clear understanding of how plant nutrients like magnesium (Mg) affect plant traits related to water-use efficiency (WUE) is of great importance. Magnesium plays a crucial role in photosynthesis and is thus a major determinant of biomass formation. This study investigated the effect of Mg deficiency on leaf and whole plant water-use efficiency, δC composition, hydrogen peroxide (HO) production and the activity of key enzymes involved in ROS scavenging in barley. Methods: Barley ( Hordeum vulgare) was grown in hydroponic culture under three different levels of Mg supply (0.01, 0.1, 0.4 mM Mg). WUE was determined on the leaf-level (leaf-WUE), the biomass-level (biomass-WUE) and via carbon isotope discrimination (δC). Additionally, concentrations of Mg, chlorophyll and HO, and the activities of three antioxidative enzymes (ascorbate peroxidase, glutathione reductase and superoxide dismutase) in youngest fully expanded leaves were analyzed. Results: Dry matter production was significantly decreased (by 34 % compared to control) in Mg deficient barley plants. Mg deficiency also markedly reduced leaf Mg concentrations and chlorophyll concentrations, but increased HO concentrations (up to 55 % compared to control) and the activity of antioxidative enzymes. Severe Mg deficiency decreased biomass-WUE by 20 %, which was not reflected regarding leaf-WUE. In line with leaf-WUE data, discrimination against C (indicating time-integrated WUE) was significantly reduced under Mg deficiency. Conclusions: Mg deficiency increased oxidative stress indicating impairment in carbon gain and decreased biomass-WUE. Our study suggests that biomass-WUE was not primarily affected by photosynthesis-related processes, but might be dependent on effects of Mg on night-time transpiration, respiration or root exudation. [ABSTRACT FROM AUTHOR]

Plants dynamically cope with the variability of mineral nutrient distribution in soil by constantly modulating nutrient uptake and shaping root-system architecture. The changes in root morphology in response to major essential elements are largely documented, but little is known about how the root system responds to magnesium (Mg) availability. Thirty-six natural accessions of the model species Arabidopsis thaliana were subjected to an in vitro screen for identifying variation in root system architecture in response to Mg availability. Response of root morphology was observed on 2-dimensional agar plates. Low Mg supply repressed the elongation of the lateral roots more than of the primary root. However, some accessions exhibited higher number and length of lateral roots than the reference Columbia-0. Across all accessions, the root morphological traits did not correlate with tissue Mg concentrations. Interestingly, shoot calcium and root phosphorus concentrations were positively correlated with the number and length of lateral roots, whereas root iron concentration was negatively correlated with the primary root length. The diversity of root phenotypes identified in this report is a useful resource to study the genetic component determining root morphology in response to Mg availability. [ABSTRACT FROM AUTHOR]

Adoption of the innovative water-saving ground cover rice production system (GCRPS) based on transplanting of rice seedlings under high soil moisture conditions, resulted in an overall increase in grain yield compared to previous reports on GCRPS employing direct seeding. However, there is a lack of quantitative information on water and nitrogen use efficiency as affected by water and nitrogen management in GCRPS-transplanting. To close this knowledge gap, we conducted a two-year field experiment with traditional paddy rice (Paddy) and GCRPS-transplanting under two soil moisture conditions (GCRPS sat and GCRPS 80% ), combined with 3 nitrogen fertilizer management regimes (0, 150 kg urea-N/ha as basal fertilizer for Paddy and GCRPS, 150 kg urea-N/ha in 3 splits for Paddy or 75 kg urea-N/ha plus 75 kg N/ha as chicken manure for GCRPS). Grain yield, water and nitrogen use efficiency, stable isotope 13 C of plant shoots and yield components were evaluated. The study showed: (1) compared to Paddy, both GCRPS sat and GCRPS 80% produced significantly more grain yield, while no significant difference in grain yield was found between both GCRPS treatments. (2) Irrigation water use efficiency was increased by 140% in GCRPS sat and >500% in GCRPS 80% , while total water use efficiency was improved by 52–96% as compared to Paddy. (3) δ 13 C of plant shoots was significantly higher in GCRPS than in Paddy, and showed significant positive correlations with total and irrigation water use efficiencies. (4) Compared to Paddy, agronomic N use efficiency was significantly higher in both forms of GCRPS. However, N recovery rates were only significantly higher in GCRPS than in Paddy when all urea nitrogen was applied as basal fertilizer before transplanting. With improved fertilizer N management, i.e., split N application in Paddy or combined application of urea and chicken manure in GCRPS, there were no significant differences. Overall, this quantitative evaluation of water use efficiency highlights that the use of GCRPS involving transplanting of seedlings has a great potential to reduce irrigation water input, increase grain yield and resource use efficiency. [ABSTRACT FROM AUTHOR]

Assessing effects of organic fertilizer applications on N2O emissions is of great interest because they can cause higher N2O emissions compared to inorganic fertilizers for a given amount of added nitrogen (N). But there are also reports about enhanced N2O reduction to climate-neutral elemental N2 after application of organic manures to soils. Factors controlling the N2O/(N2O + N2) product ratio of denitrification are interrelated, and also the ratio is difficult to study because of limitations in N2 flux measurements. In this study, we investigated N2O and N2 emissions from soil treated with organic fertilizers with different C/N ratios. An N2O isotopomer approach combined with conventional N2O and N2 flux measurements was employed to study underlying microbial pathways. A grassland soil was amended with anaerobic digestate (AD) from food waste digestion (low C/N ratio) or cattle slurry (CS; high C/N ratio), respectively, adjusted to 90% WFPS, and incubated for 52 days under helium-oxygen atmosphere (10% O2) using a soil incubation system capable of automated N2O, N2, and CO2 measurements. N2O isotopomer signatures, i.e. the d18O and SP values (site preference between 15N at the central and the peripheral position in the N2O molecule), were determined by Isotope Ratio Mass Spectrometry and used to model and subsequently estimate the contribution of bacterial denitrification and autotrophic nitrification to N2O production. For this approach the direct determination of emitted N2 is essential to take isotope effects during N2O reduction to N2 into account by correcting the measured isotope signatures for isotope effects during N2O reduction using previously determined fractionation factor ranges. The addition of both organic fertilizers to soil drastically increased the rate of gaseous N emissions (N2O + N2), probably due to the effects of concurrent presence of nitrate and labile C on the denitrification rate. In the initial phase of the experiment (day 1 to ~15), gaseous N emissions were dominated by N2 fluxes in soils amended with organic manures; meanwhile, N2O emissions were lower compared to untreated Control soils, but increased after 15-20 days relative to the initial fluxes, especially with CS. Extremely low N2O, but high N2 emissions in the initial phase suggest that reduction of N2O to N2 via denitrification was triggered when the soil was amended with organic fertilizers. In contrast in the untreated Control, N2O release was highest during the initial phase. Total N2O release from AD treated soil was similar to Control, while N2O from CS treated soil was considerably higher, indicating that denitrification was triggered more by the high labile carbon content in CS, while the cumulative N2O/(N2O + N2) product ratio and thus N2O reduction were similar with both organic fertilizers. The results of the N2O source partitioning based on the isotopomer data suggest that about 8-25% (AD) and 33-43% (CS) of the cumulated N2O emission was due to nitrification in organically amended soil, while in the untreated Control nitrification accounted for about 5-16%. The remaining N2O production was attributed mainly to denitrification, while the poor model fit for other source pathways like fungal denitrification suggested their contribution to be of minor importance. The observed rather distinct phases with predominance first of denitrification and later of nitrification may help developing mitigation measures by addressing N2O source processes individually with appropriate management options. The observation of relatively large shares of nitrification-derived N2O is surprising, but may possibly be related to the low soil pH and will require further investigation. The determination of N2 production is essential for this isotopomer-based source partitioning approach, but so far only applicable under laboratory conditions. The results of this study indicate that the combination of N2O d18O and SP values is very useful in obtaining more robust source estimates as compared to using SP values alone. [ABSTRACT FROM AUTHOR]

Symbiotic nitrogen fixation is a process of considerable economic, ecological and scientific interest. The central enzyme nitrogenase reduces H+ alongside N2, and the evolving H2 allows a continuous and non-invasive in vivo measurement of nitrogenase activity. The objective of this study was to show that an elaborated set-up providing such measurements for periods as long as several weeks will produce specific insight into the nodule activity's dependence on environmental conditions and genotype features. A system was developed that allows the air-proof separation of a root/nodule and a shoot compartment. H2 evolution in the root/nodule compartment can be monitored continuously. Nutrient solution composition, temperature, CO2 concentration and humidity around the shoots can concomitantly be maintained and manipulated. Medicago truncatula plants showed vigorous growth in the system when relying on nitrogen fixation. The set-up was able to provide specific insights into nitrogen fixation. For example, nodule activity depended on the temperature in their surroundings, but not on temperature or light around shoots. Increased temperature around the nodules was able to induce higher nodule activity in darkness versus light around shoots for a period of as long as 8 h. Conditions that affected the N demand of the shoots (ammonium application, Mg or P depletion, super numeric nodules) induced consistent and complex daily rhythms in nodule activity. It was shown that long-term continuous measurements of nodule activity could be useful for revealing special features in mutants and could be of importance when synchronizing nodule harvests for complex analysis of their metabolic status. [ABSTRACT FROM AUTHOR]

There is a growing concern that greenhouse gas (GHG) emissions during agricultural energy crop production might negate GHG emission savings which was not intended when promoting the use of renewable energy. Nitrous oxide (NO) is a major GHG, and in addition, it is the most powerful ozone-depleting compound that is emitted by human activity. The use of N fertilizers and animal manures is the main anthropogenic source of NO emissions. In spite of their high relevance, we still have limited understanding of the complex underlying microbial processes that consume or produce NO and their interactions with soil types, fertilizers (rate and types), plants, and other environmental variables. In a 2-year field experiment, we compared two important biogas crops in two different agro-ecological regions of northern Germany for their productivity and GHG emissions, using the closed-chamber technique and high time-resolution sampling. Silage maize, which is currently the most widespread crop grown for biogas fermentation purposes in Germany, was compared with an alternative bioenergy crop at each site. The three forms of nitrogen fertilizers/manures were given: calcium ammonium nitrate, cattle/pig slurry, and biogas residue. The greatest NO flux activity occurred in the period of May-July in all crops and at both sites. Flux patterns indicated pronounced effects of soil moisture-soil mineral-N interactions which were also seen as causation of the higher NO fluxes in the bioenergy crop maize compared to the other tested energy crops. However, the NO emission per unit methane production (specific NO emission) was clearly lower in soils planted with maize due to significantly higher methane hectare yield of maize. Our data suggest a linear relationship between increasing N input and increases in NO emission in both years at site with sandy loam texture where highest NO fluxes were measured. At sandy loam site, the percentage of applied N being emitted as NO was 1.9 and 1.1 % in soils cropped with maize and 0.9 and 0.8 % in soils cropped with wheat during the investigation period 2007-2008 and 2008-2009, respectively. In contrast, at site with sandy soil texture, the percentage of applied N emitted as NO was only 0.6 and 0.7 % in maize soils and 0.4 and 0.3 % in grassland during 2007-2008 and 2008-2009 period, respectively. Higher daily and annual NO emissions at the sandy loam site were attributed to the finer soil texture and higher denitrification activity. The present study provides a very good basis for the assessment of direct emissions of greenhouse gases from relevant biogas crops in North-West Europe. [ABSTRACT FROM AUTHOR]

During a whole-plant P-depletion process in Medicago, formation of new nodules ceases and leaves become P depleted, while existing active nodules maintain high-P levels and display complex molecular acclimation processes.Legume nodules are plant tissues with an exceptionally high concentration of phosphorus (P), which, when there is scarcity of P, is preferentially maintained there rather than being allocated to other plant organs. The hypothesis of this study was that nodules are affected before the P concentration in the organ declines during whole-plant P depletion. Nitrogen (N2) fixation and P concentration in various organs were monitored during a whole-plant P-depletion process in Medicago truncatula. Nodule gene expression was profiled through RNA-seq at day 5 of P depletion. Until that point in time P concentration in leaves reached a lower threshold but was maintained in nodules. N2-fixation activity per plant diverged from that of fully nourished plants beginning at day 5 of the P-depletion process, primarily because fewer nodules were being formed, while the activity of the existing nodules was maintained for as long as two weeks into P depletion. RNA-seq revealed nodule acclimation on a molecular level with a total of 1140 differentially expressed genes. Numerous genes for P remobilization from organic structures were increasingly expressed. Various genes involved in nodule malate formation were upregulated, while genes involved in fermentation were downregulated. The fact that nodule formation was strongly repressed with the onset of P deficiency is reflected in the differential expression of various genes involved in nodulation. It is concluded that plants follow a strategy to maintain N2 fixation and viable leaf tissue as long as possible during whole-plant P depletion to maintain their ability to react to emerging new P sources (e.g. through active P acquisition by roots). [ABSTRACT FROM AUTHOR]

The increasing input of anthropogenically derived nitrogen (N) to ecosystems raises a crucial question: how does available N modify the decomposer community and thus affects the mineralization of soil organic matter ( SOM). Moreover, N input modifies the priming effect ( PE), that is, the effect of fresh organics on the microbial decomposition of SOM. We studied the interactive effects of C and N on SOM mineralization (by natural 13C labelling adding C4-sucrose or C4-maize straw to C3-soil) in relation to microbial growth kinetics and to the activities of five hydrolytic enzymes. This encompasses the groups of parameters governing two mechanisms of priming effects - microbial N mining and stoichiometric decomposition theories. In sole C treatments, positive PE was accompanied by a decrease in specific microbial growth rates, confirming a greater contribution of K-strategists to the decomposition of native SOM. Sucrose addition with N significantly accelerated mineralization of native SOM, whereas mineral N added with plant residues accelerated decomposition of plant residues. This supports the microbial mining theory in terms of N limitation. Sucrose addition with N was accompanied by accelerated microbial growth, increased activities of β-glucosidase and cellobiohydrolase, and decreased activities of xylanase and leucine amino peptidase. This indicated an increased contribution of r-strategists to the PE and to decomposition of cellulose but the decreased hemicellulolytic and proteolytic activities. Thus, the acceleration of the C cycle was primed by exogenous organic C and was controlled by N. This confirms the stoichiometric decomposition theory. Both K- and r-strategists were beneficial for priming effects, with an increasing contribution of K-selected species under N limitation. Thus, the priming phenomenon described in 'microbial N mining' theory can be ascribed to K-strategists. In contrast, 'stoichiometric decomposition' theory, that is, accelerated OM mineralization due to balanced microbial growth, is explained by domination of r-strategists. [ABSTRACT FROM AUTHOR]

Abstract: Quantifying denitrification in arable soils is crucial in predicting nitrogen fertiliser losses and N2O emissions. Stable isotopologue analyses of emitted N2O (δ15N, δ18O and SP= 15N site preference within the linear N2O molecule) may help to distinguish production pathways and to quantify N2O reduction to N2. However, such interpretations are often ambiguous due to insufficient knowledge on isotopic fractionation mechanisms. Here we present a complex experimental approach to determine the net fractionation factors (η) associated with denitrification. This determination is based on three laboratory experiments differing in their experimental set-up and soil properties. Static and dynamic incubation techniques were compared. All available methods for independent determination of N2O reduction contribution were used, namely, N2-free atmosphere incubation, acetylene inhibition technique and 15N gas-flux method. For N2O production: (i) the determined difference in δ18O between soil water and produced N2O vary from +18‰ to +42‰ and show very strict negative correlation with soil water saturation; (ii) the determined η15N of N2O production vary from −55‰ to −38‰ and the fractionation decreases with decreasing substrate availability; (iii) the determined SP of produced N2O vary from −3‰ to +9‰. For N2O reduction: (i) the determined η18O and η15N of N2O reduction vary in very wide ranges from −18‰ to +4‰ and from −11‰ to +12‰, respectively, and depend largely on the differences in experimental setups; whereas (ii) the determined ηSP of N2O reduction shows a very consistent value with all previous studies and varies in a rather narrow range from −2‰ to −8‰. It can be concluded that η values of N2O production determined during laboratory incubations yield only roughly estimates for respective values expectable under field study conditions. η18O and η15N associated with N2O reduction may vary largely, probably depending on spatial and temporal coincidence of N2O production and reduction, and are hence not yet predictable for natural conditions. However, the ηSP of N2O reduction appeared to be relatively robust and a most probable value of about −5‰ can be used to constrain N2O reduction based on SP of soil emitted N2O. [Copyright &y& Elsevier]

Legumes match the nodule number to the N demand of the plant. When a mutation in the regulatory mechanism deprives the plant of that ability, an excessive number of nodules are formed. These mutants show low productivity in the fields, mainly due to the high carbon burden caused through the necessity to supply numerous nodules. The objective of this study was to clarify whether through optimal conditions for growth and CO2 assimilation a higher nodule activity of a supernodulating mutant of Medicago truncatula (M. truncatula) can be induced. Several experimental approaches reveal that under the conditions of our experiments, the nitrogen fixation of the supernodulating mutant, designated as sunn (super numeric nodules), was not limited by photosynthesis. Higher specific nitrogen fixation activity could not be induced through short- or long-term increases in CO2 assimilation around shoots. Furthermore, a whole plant P depletion induced a decline in nitrogen fixation, however this decline did not occur significantly earlier in sunn plants, nor was it more intense compared to the wild-type. However, a distinctly different pattern of nitrogen fixation during the day/night cycles of the experiment indicates that the control of N2 fixing activity of the large number of nodules is an additional problem for the productivity of supernodulating mutants. [ABSTRACT FROM AUTHOR]

The mechanism through which nitrate reduces the activity of legume nodules is controversial. The objective of the study was to follow Medicago truncatula nodule activity after nitrate provision continuously and to identify molecular mechanisms, which down-regulate the activity of the nodules. Nodule H2 evolution started to decline after about 4 h of nitrate application. At that point in time, a strong shift in nodule gene expression (RNA sequencing) had occurred (1,120 differentially expressed genes). The most pronounced effect was the down-regulation of 127 genes for nodule-specific cysteine-rich peptides. Various other nodulins were also strongly down-regulated, in particular all the genes for leghemoglobins. In addition, shifts in the expression of genes involved in cellular iron allocation and mitochondrial ATP synthesis were observed. Furthermore, the expression of numerous genes for the formation of proteins and glycoproteins with no obvious function in nodules (e.g. germins, patatin, and thaumatin) was strongly increased. This occurred in conjunction with an up-regulation of genes for proteinase inhibitors, in particular those containing the Kunitz domain. The additionally formed proteins might possibly be involved in reducing nodule oxygen permeability. Between 4 and 28 h of nitrate exposure, a further reduction in nodule activity occurred, and the number of differentially expressed genes almost tripled. In particular, there was a differential expression of genes connected with emerging senescence. It is concluded that nitrate exerts rapid and manifold effects on nitrogenase activity. A certain degree of nitrate tolerance might be achieved when the down-regulatory effect on late nodulins can be alleviated. [ABSTRACT FROM AUTHOR]