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Little is known about the relationship between soil microbial communities and soil properties in southern boreal forests. To further our knowledge about that relationship, we compared the soil samples in southern boreal forests of the Greater Khingan Mountains—the southernmost boreal forest biome in the world. The forests can be divided into boardleaf forests dominated by birch (Betula platyphylla) or aspen (Populus davidiana) and coniferous forests dominated by larch (Larix gmelinii) or pine (Pinus sylvestris var. mongolica). Results suggested different soil microbial communities and soil properties between these southern boreal forests. Soil protease activity strongly associated with soil fungal communities in broadleaf and coniferous forests (p < 0.05), but not with soil bacterial communities (p > 0.05). Soil ammonium nitrogen and total phosphorus contents strongly associated with soil fungal and bacterial communities in broadleaf forests (p < 0.05), but not in coniferous forests (p > 0.05). Soil potassium content demonstrated strong correlations with both soil fungal and bacterial communities in broadleaf and coniferous forests (p < 0.05). These results provide evidence for different soil communities and soil properties in southern boreal forest, and further elucidate the explicit correlation between soil microbial communities and soil properties in southern boreal forests. [ABSTRACT FROM AUTHOR]

We conducted this study to understand the dynamics of revegetation by examining the levels of mineral nitrogen (NO3− and NH4+) in soil and the capacity of nitrogen use by the principal species growing in secondary forest in central Amazonia. For this, we measured the nitrate and ammonium content of soil, leaves and xylem sap, nitrate reductase activity of the leaves and free amino acid contents of the xylem sap in five tree species (Vismia cayennensis, Vismia japurensis, Bellucia dichotoma, Laetia procera and Goupia glabra) over a chronosequence during recovery after pasture abandonment at two seasons. Soil ammonium was higher in the dry season and nitrate higher in the wet season and increased these with pasture abandonment age. V. japurensis, B. dichotoma and G. glabra decreased foliar ammonium due to pasture abandonment in the dry season and foliar ammonium increased in L. procera in the wet season. V. japurensis and V. cayennensis showed a decrease in nitrate reductase activity, while B. dichotoma and L. procera showed an increase. Xylem nitrate decreased in L. procera and B. dichotoma in the dry and wet seasons, respectively, and ammonium increased only in V. japurensis and V. cayennensis in the wet. Xylem arginine increased in all plant species after a period of pasture abandonment (except B. dichotoma). Tree species growing in pasture abandoned areas didn't show the same pattern of use of nitrogen, and this can be important in order to understand the nitrogen metabolism of trees in the Amazon region during forest restoration. [ABSTRACT FROM AUTHOR]

Unlike most horticultural crops, blueberry (Vaccinium spp. section cyanococcus) prefers low-pH (4.2-5.5) soils. Other plants can acidify their rhizosphere to create a hospitable microenvironment. Southern highbush blueberry (SHB; Vaccinium corymbosum interspecific hybrids) plants do not acidify their rhizosphere in response to Fe deficiency, but other factors that affect rhizosphere pH have not been elucidated. We report results from two hydroponic experiments exploring N uptake effects on the rhizosphere pH of 'Emerald' SHB. Ammonium (NH4+) uptake led to rhizosphere acidification, whereas nitrate (NO3-) uptake led to rhizosphere alkalization. When grown in a split-root hydroponic system, roots that took up NH4+ acidified the rhizosphere to a greater extent that roots not exposed to NH4+. Rhizosphere acidification was observed even in a nontreated control. These results suggest that NH4+ uptake is the main driver of rhizosphere pH in SHB. N form effects suggest that fertilization with NO3- might lead to undesirable rhizosphere alkalization. [ABSTRACT FROM AUTHOR]

Abstract Economic and nature-friendly amendments are needed to increase fertilizer efficiency and crop yield for sustainable agriculture in coarse-textured soil. In this investigation, the effects of different application rates (2%, 4% and 8%) of vermiculite, nanoclay and zeolite amendments on adsorption and transport of ammonium (NH 4 -N) in sandy loam soil were studied using batch and column experiments. The results of batch experiment indicated that the maximum distribution coefficient (K d) was obtained in soil amended with zeolite 8%, while minimum K d was achieved in the control soil. Column experiments revealed that the application of inorganic amendments decreased the leaching of NH 4 -N from 89.4% in control soil to 77.6%, 70.2%, 52.5%, 85.3%, 60.8%, 38.5%, 12.6% and 2.1% in soil columns amended with vermiculite 2%, 4%, 8%, nanoclay 2%, 4% and zeolite 2%, 4%, 8%, respectively. On the basis of the application rate of inorganic amendments required to retain 40% of injected NH 4 -N, the effectiveness of zeolite is about 2 and 4 times that of nanoclay and vermiculite, respectively. The HYDRUS-1D program was used to simulate the leaching of NH 4 -N using the advection–dispersion equation model (ADE) along with batch and fitted K d. The simulation results indicated that the ADE model with fitted K d described the behavior of NH 4 -N transport more suitable than the ADE model using batch K d (root-mean-square error, RMSE < 0.06 and RMSE < 0.13, respectively). Results of our study indicated that amending sandy loam soil with inorganic amendments can be an effective technique for decreasing NH 4 -N transport and improving the efficiency of N fertilizer in sustainable agriculture systems. Highlights • Sorption of NH 4 -N were increased and varied by amendment and application rate. • NH 4 -N leaching from sandy loam soil was reduced using inorganic amendments. • Zeolite was the most effective amendment in reducing NH 4 -N leaching. • Leaching of NH 4 -N was best simulated by HYDRUS-1D program. [ABSTRACT FROM AUTHOR]

Abstract Dissimilatory nitrate reduction to ammonium (DNRA) and diazotrophic N 2 fixation contribute to nitrogen (N) supply in rice paddies, whereas denitrification contributes to N loss. Continuous N fertilization in rice paddies is known to increase denitrification and reduce N 2 fixation, however little is known about its effect on DNRA and the NO 3 − partitioning between DNRA and denitrification. Here, we investigated the rates of DNRA, denitrification and N 2 fixation, and their relevant microbial gene abundances, in long-term high and low N fertilized rice paddies using a 15NO 3 − tracer, an acetylene reduction assay and quantitative PCR analysis, in laboratory incubation studies. We observed that DNRA exceeded denitrification by a factor of eight in low N fertilized rice paddies, while DNRA was almost half of the denitrification rate in high N fertilized rice paddies. The nrfA gene abundance, related to DNRA, was significantly higher in the low N fertilized rice paddies and was positively correlated with DNRA rates. However, no clear difference in denitrifying gene (narG, nirK and nosZ) abundances was observed between the N fertilization regimes. The proportion of total NO 3 − reduced by DNRA had a significantly positive correlation with the soil organic carbon-to-NO 3 - ratio and negative correlation with the soil NO 3 − concentration. N 2 fixation added ten times more N in the low N input than in the high N input paddies. Our findings highlight the self-regulated microbial N cycling in low N input paddy systems which maintain long-term paddy soil N nutrition. Highlights • N 2 fixation added ten times more N to low N input than to high N input rice paddies. • DNRA retained the majority of nitrate in long-term low N input rice paddies. • Denitrification dominated nitrate reduction in long-term high N input rice paddies. • High SOC:NO 3 − ratio favours NO 3 − partitioning to DNRA in NO 3 − limited paddy soils. [ABSTRACT FROM AUTHOR]

Abstract Microbial oxidation of ammonia controls the rate of nitrification in the majority of soils. Both nitrification rate and the composition of communities of ammonia oxidising archaea (AOA) and ammonia oxidising bacteria (AOB) are influenced by drought, with evidence that AOA are more sensitive to periods of drought than AOB. This has been explained by greater sensitivity of AOA to ammonia concentration, which will increase in soil solution during drought, but an alternative, previously unexplored explanation, is greater sensitivity of AOA to matric and/or osmotic stress. A soil microcosm experiment was designed to distinguish these different explanations in which AOA and AOB abundances (amoA abundance) and nitrification rate were measured over 28 days in nine treatments corresponding to all combinations of three soil matric potentials and three initial ammonia concentrations. Comparison of amoA abundance dynamics suggested that AOA were more susceptible to reduced matric potential than AOB, irrespective of soil ammonia concentration. The greater sensitivity of soil AOA to osmotic stress was also tested in 10-day cultures of representative strains of AOA and AOB in liquid medium containing different concentrations of NaCl and sorbitol as osmo-inducer. AOA were significantly more sensitive to osmotic stress than AOB. These results provide evidence for greater sensitivity of AOA than AOB to both components of water stress, matric and osmotic potential, representing an additional niche differentiation between these two essential groups of ammonia oxidisers. Highlights • NH 3 -oxidising archaea (AOA) more sensitive to drought than NH 3 -oxidising bacteria (AOB). • Water stress more important than NH 3 conc. in explaining effects of drought. • Pure culture studies supported greater sensitivity of AOA to water stress. • Evidence that desiccation tolerance is niche differentiation factor for AOA and AOB. • Findings challenge proposals that NH 3 conc. explains AO responses to drought. [ABSTRACT FROM AUTHOR]

Emissions of nitrous oxide (N2O), a potent greenhouse gas, from agricultural soil have been recognized to be affected by nitrogen (N) application and temperature. Most of the previous studies were carried out to determine effects of temperature on N2O emissions at a fixed N application rate or those of N application rates at a specific temperature. Knowledge about the effects of different ammonium (NH4+) application rates and temperatures on N2O emissions from tropical agricultural soil and their interactions is limited. Five grams of air-dried sandy loam soil, collected in Central Vietnam, were adjusted to 0, 400, 800 and 1200 mg NH4-N kg-1 soil (abbreviated as 0 N, 400 N, 800 N and 1200 N, respectively) at 60% water holding capacity were aerobically incubated at 20°C, 25°C, 30°C or 35°C for 28 days. Mineral N contents and N2O emission rates were determined on days 1, 3, 5, 7, 14, 21 and 28. Cumulative N2O emissions for 28 days increased with increasing NH4+ application rates from 0 to 800 mg N kg-1 and then declined to 1200 mg N kg-1. Cumulative N2O emissions increased in the order of 35°C, 20°C, 30°C and 25°C. This lowest emission at 35°C occurred because N2O production was derived only from autotrophic nitrification while other N2O production processes, e.g., nitrifier denitrification and coupled nitrification-denitrification occurred at lower temperatures. More specifically, cumulative N2O emissions peaked at 800 N and 25°C, and the lowest emissions occurred at 1200 N and 35°C. In conclusion, N2O emissions were not exponentially correlated with NH4+ application rates or temperatures. Higher NH4+ application rates at higher temperatures suppressed N2O emissions. [ABSTRACT FROM AUTHOR]

In plants, nutrient provision of shoots depends on the uptake and transport of nutrients across the root tissue to the vascular system. Nutrient delivery to the vasculature is mediated via the apoplastic transport pathway (ATP), which uses the free space in the cell walls and is controlled by apoplastic barriers and nutrient transporters at the endodermis, or via the symplastic transport pathway (STP). However, the relative importance of these transport routes remains elusive. Here, we show that the STP, mediated by the epidermal ammonium transporter 1;3 (AMT1;3), dominates the radial movement of ammonium across the root tissue when external ammonium is low, whereas apoplastic transport controlled by AMT1;2 at the endodermis prevails at high external ammonium. Then, AMT1;2 favors nitrogen (N) allocation to the shoot, revealing a major importance of the ATP for nutrient partitioning to shoots. When an endodermal bypass was introduced by abolishing Casparian strip (CS) formation, apoplastic ammonium transport decreased. By contrast, symplastic transport was increased, indicating synergism between the STP and the endodermal bypass. We further establish that the formation of apoplastic barriers alters the cell type–specific localization of AMTs and determines STP and ATP contributions. These results show how radial transport pathways vary along the longitudinal gradient of the root axis and contribute to nutrient partitioning between roots and shoots. [ABSTRACT FROM AUTHOR]

The nitrous oxide and molecular N emissions from 5-cm length subsamples taken from 20-cm length sample corers containing eutric Cambisol soil fertilised either with urea, ammonium or nitrate for 1 year have been examined using gas chromatography. At the beginning of the incubation, the same N rate (260 kg N/ha) was added to the soil and kept constant during the experiment. The total abundance of the soil Bacteria and Archaea and that of nitrifiers and denitrifiers was estimated by quantitative PCR of the corresponding biotic variables 16S rRNA, amoA and napA, narG, nirK, nirS, norB, nosZI and nosZII genes. The abiotic variables dissolved oxygen, pH, exchangeable NH4+-N and NO3−-N contents and total C and total N were also analysed. None of the three fertilisers affected the total abundance of Bacteria and Archaea and nitrification was the main driver of nitrous oxide production in the 0- to 5-cm and 5- to 10-cm soil layers while denitrification was in the 10- to 15-cm and 15- to 20-cm soil horizons. Parallel to the reduction in the content of dissolved oxygen along the soil profile, there was a decrease in the total and relative abundance of the bacterial and archaeal amoA gene and an increase in the abundances of the denitrification genes, mainly in the 10- to 15-cm and 15- to 20-cm soil layers. A non-metric multidimensional scaling plot comparing the biotic and abiotic variables examined in each of the four 5-cm soil subsamples and the whole 20-cm sample showed a disparate effect of N fertilisation on N gas emissions and abundance of nitrifiers and denitrifiers bacterial and archaeal communities. [ABSTRACT FROM AUTHOR]

Ammonia-oxidizing archaea (AOA) and bacteria (AOB), which convert NH3 to NO2− in soils, are important for agricultural production. It is well known that N addition can strongly affect soil ammonia oxidizers, but little is known about P addition. Based on microcosm experiments, this study assessed the responses of ammonia oxidizers to chemical P addition in a typically high P agricultural soil with or without N supply. Six treatments examined were neither N nor P, P alone (0.15, 0.45, and 0.75 g P2O5 kg−1 soil, respectively), N alone (0.25 g N kg−1 soil), and N plus P (0.25 g N and 0.15 g P2O5 kg−1 soil). Quantitative real-time PCR for the abundance and high-throughput sequencing for community structure were applied. The results revealed that P addition did not affect the abundances and community structures of AOA and AOB, but N addition significantly increased AOB abundance and alter its community structure. Without N supply, continuously increasing soil P availability did not affect these two groups of ammonia oxidizers. This study highlights the relationship between soil P availability and ammonia oxidizers and suggests that soil P availability could be as a potential indicator for predicting N-related ecosystem functions in agricultural production. [ABSTRACT FROM AUTHOR]

Although soil microbes are responsible for important ecosystem functions, and soils are under increasing environmental pressure, little is known about their resistance and resilience to multiple stressors. Here, we test resistance and recovery of soil methane-oxidizing communities to two different, repeated, perturbations: soil drying, ammonium addition and their combination. In replicated soil microcosms we measured methane oxidation before and after perturbations, while monitoring microbial abundance and community composition using quantitative PCR assays for the bacterial 16S rRNA and pmoA gene, and sequencing of the bacterial 16S rRNA gene. Although microbial community composition changed after soil drying, methane oxidation rates recovered, even after four desiccation events. Moreover, microcosms subjected to soil drying recovered significantly better from ammonium addition compared to microcosms not subjected to soil drying. Our results show the flexibility of microbial communities, even if abundances of dominant populations drop, ecosystem functions can recover. In addition, a history of stress may induce changes in community composition and functioning, which may in turn affect its future tolerance to different stressors. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) dynamics have considerable effects on the terrestrial carbon cycle, and land use changes could affect soil N through impacts on soil aggregate stability. This study selected nine sites including apple orchards of different ages and restored sites to explore the effects of soil aggregate stability on soil total N (STN), ammonium N (AN), and nitrate N (NN) following different stages of apple orchard on the Loess Plateau of China. The results showed that when compared with apple orchard sites, the restored sites had higher contents of STN and lower contents of AN and NN, but when compared with restored sites, inorganic-N storage played a more important role in the apple orchards. Following different stages of apple orchards, the STN, AN and NN contents and mean weight diameter (MWD) of soil aggregates were all improved. MWD had a negative effect on inorganic-N content at 10–30 cm soil depths but had a positive relationship with soil N at 0–10 cm soil depth. In addition, planting apple orchards destroyed soil aggregate stability in this erodible environment, but it could be restored soon after abandonment. Therefore, considering soil N and soil aggregate stability, it is feasible to plant apple trees in this area and we propose that the effects of trade-offs between soil aggregate stability and soil erosion on soil N in terraced agroecosystems should be a focus of future research. [ABSTRACT FROM AUTHOR]

Field experiments were conducted to determine the effect of nitrogen (N) fertilizer forms and doses on wheat (Triticum aestivum L.) on three soils differing in their ammonium (NH4) fixation capacity [high = 161 mg fixed NH4-N kg−1 soil, medium = 31.5 mg fixed NH4-N kg−1 soil and no = nearly no fixed NH4-N kg−1 soil]. On high NH4+ fixing soil, 80 kg N ha−1 Urea+ ammonium nitrate [NH4NO3] or 240 kg N ha−1 ammonium sulfate [(NH4)2SO4]+(NH4)2SO4, was required to obtain the maximum yield. Urea + NH4NO3 generally showed the highest significance in respect to the agronomic efficiency of N fertilizers. In the non NH4+ fixing soil, 80 kg N ha−1 urea+NH4NO3 was enough to obtain high grain yield. The agronomic efficiency of N fertilizers was generally higher in the non NH4+ fixing soil than in the others. Grain protein was highly affected by NH4+ fixation capacities and N doses. Harvest index was affected by the NH4+ fixation capacity at the 1% significance level. [ABSTRACT FROM AUTHOR]

Despite the importance of plant nitrogen (N) acquisition strategies for plant growth and the structure and functioning of ecosystems, few studies have investigated the N acquisition preference of plants and their reliance on various N sources (nitrate (NO 3 − ), ammonium (NH 4 + ), and organic N (DON) in soil) during different stages of plantation restoration caused by land-use change. In this study, we used the isotopic mass-balance methods to quantitatively estimate the contributions of N sources (NO 3 − vs. NH 4 + vs. DON) and plant N preferences in spruce ( Picea asperata Mast.) plantations of different ages (i.e., 20, 30, 40, 50, and 70 years old) on the eastern Tibetan Plateau, China. Across all restoration stages, while plants in spruce plantations preferred soil inorganic N over soil DON, soil DON was a notable N contributor to plant N nutrition, with 23%–44% of plant N derived from DON. Moreover, with the development of plantation restoration, the N source preferences of plants switched from NO 3 − (i.e., 20- and 30-yr-old plantations) to NH 4 + (age of plantations ≥ 40 years old). Our results suggest that soil DON can be an important N source in alpine coniferous forests. In addition, the shift in the dominant N source during plantation restoration may manifest a strategic adjustment of plant N acquisition in response to changes in soil N availability and/or physiological traits. [ABSTRACT FROM AUTHOR]

Anaerobic ammonium oxidation (anammox process) widely occurs in paddy soil and may substantially contribute to permanent N removal; however, little is known about the factors controlling this process. Here, effects of temperature, pH, organic C, and substrates on potential rate of anammox and the relative contribution of anammox to total N2 production in a paddy soil were investigated via slurry incubation combined with 15N tracer technique. Anammox occurred over a temperature range from 5 to 35 °C with an optimum rate at 25 °C (1.7 nmol N g−1 h−1) and a pH range from 4.8 to 10.1 with an optimum rate at pH 7.3 (1.7 nmol N g−1 h−1). The presence of glucose and acetate (5-100 mg C L−1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. The response of potential rates of anammox to ammonium concentrations fitted well with Michaelis-Menten relationship showing a maximum rate (Vmax) of 4.4 nmol N g−1 h−1 and an affinity constant (Km) of 6.3 mg NH4+-N L−1. Whereas, nitrate addition (5-15 mg 15NO3−-N L−1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. Our results provide useful information on factors controlling anammox process and its contribution to N loss in the paddy soil. [ABSTRACT FROM AUTHOR]

Abstract: Citrus production in Florida accounts for ≈ 60% of national production in USA. The sandy soil characteristic (> 95% sand) makes water and nutrient management extremely difficult, raising concerns about environmental sustainability as a result of nutrient inputs in citrus producing regions where sandy soils dominate. Thus, laboratory column and field experiments were conducted to better understand the leaching patterns of NH 4 + and NO 3 - ions in Florida's sandy soils. The soil columns were first saturated from the bottom with two pore volumes of simulated Florida rain followed by pumping a pulse of fertilizer mixture at a steady Darcy flux of 14 cm h−1. Nitrate and Cl− appeared earlier in the effluent than NH 4 + in the A and Bh horizons, due to cation exchange of NH 4 + . Essentially identical breakthrough curves (BTCs) for NH 4 + and NO 3 - were observed in the E‐horizon, due to very low sorption of NH 4 + . The convective and dispersive equilibrium (CDE) model simulations were in good agreement with measured breakthrough curves (BTCs) for NH 4 + , NO 3 - , and Cl−. However, the sorption coefficient (KD) values used in the CDE model to simulate the BTCs for NH 4 + were about 10 times less than the batch isotherm KD values. This was attributed to differences in pH, cation composition, and ionic strength between batch (static) and dynamic (leaching) systems. The field experiment showed that under unsaturated flow, improved short‐pulse fertigation systems (drip and microsprinkler) limited NH 4 + and NO 3 - transport beyond the root zone (top 30 cm), which might have promoted nutrient and water uptake in citrus. The column study revealed that under extreme weather events such as hurricanes or storm surge in Florida, saturated soil conditions can trigger N mobility below the root zone to surficial or groundwater aquifers. In the field experiment, the use of judicious, minimal and split applications and accurate placement of N‐fertilizers reduced leaching of N especially during heavy storms in the summer rainy months of Florida. The field experiment demonstrated that it is possible to manage inorganic N forms for optimal residence time for uptake and minimal leaching concerns. [ABSTRACT FROM AUTHOR]

Mineral nitrogen (Nmin = N-NH4+ + N-NO3-) in oven-dried and field-moist samples was investigated in 2011- 2014 in five areas of Lithuania with soils of different texture (sand, sandy loam, loam and clay) and different mineral nitrogen (Nmin) content. The samples were taken in spring (from late March to early April) and autumn (late October - early November), from the 0-30, 30-60 and 60-90 cm as well as from 0-60 cm soil layers. The aim of the research was to determine the influence of different methods of soil sample preparation on the concentrations of ammonium (N-NH4+) and nitrate (N-NO3-) nitrogen in oven-dried and field-moist samples and to assess the suitability of the obtained results for the development of agricultural plant fertilisation plans. According to the research results, drying of soil samples at a temperature of 35°C for 12 hours resulted in significantly higher levels of N-NH4+ compared to those of field-moist samples. The N-NO3- concentrations in the field-moist samples, when stored at a 4°C temperature for 3 days, increased marginally compared to the oven-dried soil samples. Weak or moderate, however, statistically significant correlations were observed between N-NH4+ concentrations in the oven-dried and field-moist samples. In order to perform faster chemical analyses of soil Nmin for making agricultural plant fertilisation plans, it is necessary to carry out analyses on oven-dried soil samples, and using the linear correlation equations, to convert the results of N-NH4+ analyses into field-moisture data. [ABSTRACT FROM AUTHOR]

Coated urea fertilizers are assumed to enhance crop yield and reducing the environmental pollution. Nevertheless, many of the coated urea fertilizers are expensive, thus not readily available for most farmers. In addition, many of these fertilizers release N not in tandem with the plant’s need, thus retard growth. Therefore, a laboratory study was conducted to evaluate effects of coated urea fertilizers on N losses via volatilization. Measurement of ammonia volatilization was carried out using the closed-dynamic air flow system. The study for ammonia volatilization was conducted using different rates of fertilizer (50, 100, and 200 kg N ha−1) with different types of fertilizer (Urea, Sulfur-coated urea; SCU and Gypsum sulfur coated urea using rotating drum; GSCUD) in 37 days of incubation. The results indicate that SCU represents the best fertilizer which decreases the amount of ammonia volatilization at each rate of fertilizer. Besides, the rate of 50 kg N ha−1 has the lowest percentage of ammonia volatilization. Moreover, the result proved the effectiveness of coating urea fertilizer may reduce the ammonia loss to the environment and new product which GSCUD can be comparable to the commercial product. [ABSTRACT FROM AUTHOR]

Potassium (K+) and nitrogen [N, as the form of ammonium (NH4+)] are major nutrients for plant growth. Although there have been a number of studies on the kinetic fixation of potassium and ammonium ions in soils and clays, however, investigations on the kinetics of competitive fixation of these ions have been few, if any, especially by taking into account silt component of the soils. In this study, the kinetics of potassium and ammonium fixation were examined in the silt components of several soil samples. The results revealed that considerable amounts of K+ and in lesser amounts, NH4+ ions were fixed by silt components of the soils. Potassium fixation was strongly preferred over ammonium. To describe the fixation kinetics, seven mathematically models were evaluated. A comparison of the models showed that pseudo-second-order equation properly described the fixation of these ions by the silt components. [ABSTRACT FROM AUTHOR]

Background and aims: Sensitivity and uncertainty analyses are critical for identifying the most sensitive parameters of the crop simulation models. These analyses will help the users to concentrate more on calibrating those sensitive parameters. The soil parameters are important input parameters as the soils regulate the supply of nutrients and water required for the growth and yield of crops. The aim of this investigation is to perform the sensitivity and uncertainty analyses of the soil parameter inputs of the Web InfoCrop Wheat model, especially under the water deficit- and high temperature stress conditions for different agro-climatic conditions of India.Methods: Sensitivities of soil input parameters (15) were analysed by crop simulations under the no stress (potential), water deficit- (limited irrigation), high temperature- (+3 °C above ambient), and their combined stress conditions for a period of 10 years across four sites in the wheat-growing regions of India. The partial rank correlation coefficients (PRCC) were computed, followed by the top-down concordance coefficients.Results: Amongst the soil input parameters (15) and outputs (10) considered, the nutrient parameters such as NITRATE, ORG_CARB (Organic carbon), AMMONIUM were the most sensitive input parameters under the no stress (potential) and high temperature stress conditions. On the contrary, the parameters related to the soil moisture such as CLAY (Clay %), FLD_CAP (Field capacity), SATU (Saturation) and MOISTURE (initial soil moisture contents) were the most sensitive input parameters under water deficit stress conditions.Conclusions: Both the sensitivities and uncertainty of soil parameters under different crop production and environmental conditions influenced the outputs of the Web InfoCrop model significantly. Identification of the most sensitive soil parameters that need to be measured accurately improved the simulation efficiencies of this crop model. [ABSTRACT FROM AUTHOR]

Tropical acid soils are highly weathered as they exist under tropical environment with high rainfall and temperature throughout the year, which affects nitrogen availability. Soil organic nitrogen is important in estimating soil nitrogen availability. The combined use of urea and compost in this study was carried out to decrease sole dependence on urea, buffer soil acidification, and reduce nitrogen losses through leaching. Thus, soil buffering capacity, incubation, and organic nitrogen fractionation studies were conducted to determine soil buffering capacity, availability of total nitrogen, organic fractions nitrogen, and inorganic nitrogen in soil after 90 days of incubation following compost. Soil pH, buffering capacity, total nitrogen, organic nitrogen fractions, exchangeable ammonium, and available nitrate were higher in all treatments with compost and combined use of urea and compost. Total hydrolyzable nitrogen, ammonium-nitrogen, (ammonium + amino sugar)-nitrogen, amino sugar-nitrogen, and amino acid-nitrogen were higher in soils with urea and compost suggesting that decomposition of soil organic fractions nitrogen into inorganic nitrogen (ammonium and available nitrate was affected by the addition of urea and compost. Urea can be amended with compost to regulate availability nitrogen in soil for crop use. [ABSTRACT FROM AUTHOR]

Ammonia oxidation, the primary and rate-limiting step of nitrification, is mediated by both ammonia-oxidizing archaea (AOA) and bacteria (AOB). However, the dominant environmental driver of the activity, abundance, diversity and composition of ammonia-oxidizers has not been well understood enough, and the relative contribution of AOA and AOB to nitrification is still under debate. Soils treated with different fertilization regimes in an over 35-year field experiment were collected to explore the variation in function and structure of ammonia-oxidizer communities and corresponding driver. The highest nitrification activity (44.49 mg N kg −1 soil d −1 ) was found in only organic fertilizer (O) treated soil, whereas the lowest activity (2.81 mg N kg −1 soil d −1 ) was observed in only mineral fertilizer (NPK) treated soil. Moreover, as 1-octyne was employed to discriminate AOA- and AOB-supported nitrification, AOA dominated (93.53%) the nitrification in the Control soil, while AOB contributed dominantly (84.73–89.10%) in all of the organic amended soils, and NPK-treated soil showed an almost equal contribution to AOA (45.79%) and AOB (54.21%). Compared with the Control soil, AOA abundance increased in soils with organic and low chemical fertilizer but decreased in only chemically treated soil, whereas the AOB abundance in all fertilized soils was greatly enhanced. The AOA activity was linearly dependent on AOA abundance, whereas the AOB activity was exponentially correlated with AOB abundance. The sequences of AOA and AOB in the Control soil were mostly affiliated with group I.1b thaumarchaeota and genus Nitrosospira clusters 3a.1. Soil treated with NPK increased the abundance of AOA that belonged to group I.1a-associated lineage, whereas more abundant AOB was related to Nitrosospira clusters 3a.2 and 8b. In contrast, the O-treated soil showed more abundant AOB that belonged to Nitrosospira clusters 3b and 8b. As revealed by aggregated boosted tree analysis, the soil ammonium (NH 4 + ) content was identified as the dominant driver of activity and diversity of AOA, and soil pH was considered to be the major influencing factor in abundance and composition of AOA; the AOB composition was mainly affected by soil NH 4 + content, the relative activity and diversity by soil pH, and the relative abundance by soil electrical conductivity (EC). Collectively, different fertilization regimes will result in variations in activity, abundance, diversity and composition of ammonia-oxidizers with distinct drivers. Our research could be helpful to identify better strategies for the mitigation of nitrate production in agricultural soils. [ABSTRACT FROM AUTHOR]

The traditional qualitative analysis of the individual factors on the kinetic and thermodynamic parameters cannot sufficiently reveal the mechanism underlying ammonia volatilization in soil. This study aimed to determine the effects of temperature, moisture content, and their interaction on the kinetic and thermodynamic parameters, which revealed the key control mechanism underlying ammonia volatilization, modified the traditional Arrhenius model, and established a quantitative prediction model of cumulative NH3-N loss (CNL). Laboratory culture experiments were conducted under different temperatures (T) (15°C, 20°C, 25°C and 35°C) and moisture contents (θ) (60%, 80%, and 100% field capacities). Soil ammonia volatilization was also measured every 2 d. Results showed that the effects of individual factors and their interaction on the values of reaction rate (KN), Activation free energy (ΔG), and activation entropy (ΔS) followed the descending order of T>θ>T·θ, whereas those of activation enthalpy (ΔH) and activation degree (lgN) followed the descending order of θ>T>T·θ. The interaction showed significant effect on KN value and insignificant effect on all the thermodynamic parameters. The effects of water and temperature were mainly observed during the preparatory stage and the most critical transition state stage of the chemical reaction, respectively. Given that ΔH > 0, ΔG > 0, and ΔS > 0, ammonia volatilization is found to be an endothermic reaction controlled by enthalpy. The new KN(T)-2 model with the determination coefficient (R²) of 0.999 was more accurate than the traditional Arrhenius model with the R² of 0.936. The new NH3(T, θ) model with the mean absolute percentage error (MAPE) of 4.17% was more accurate than the traditional NH3(T) model with the MAPE of 7.11%. These results supplemented the control mechanism underlying ammonia volatilization in soil fertilized with urea and improved the prediction accuracy of CNL. [ABSTRACT FROM AUTHOR]

Application of urea in lowland rice fields leads to ammonia (NH3) volatilization and environmental pollution, and diminishes nitrogen recovery by rice (Oryza sativa L.). Amending urea with biochar could reduce NH3loss from urea as well as improve chemical properties of acid soils. An incubation study was conducted using a closed-dynamic air flow system to determine NH3volatilization from urea and chemical properties of an acid soil (Typic Paleudults). The soil was mixed with three rates of chicken litter biochar (20, 40, and 60 g pot−1) and 1.31 g urea. Mixing an acid soil with biochar (60 g pot−1) in waterlogged to stimulate conditions in paddy condition significantly reduced NH3loss and total titratable acidity. Biochar application also increased soil pH, total nitrogen, available nitrate, organic matter, total organic carbon, total carbon, available phosphorus, and exchangeable cations. Thus, chicken litter biochar can be used to reduce urea-N loss and ameliorate chemical properties of acid soils. This aspect is being embarked on in our on-going field experiments. [ABSTRACT FROM PUBLISHER]

In the present work, group contribution method (GC) together with the Artificial Neural Network (ANN) and Artificial Neuro-Fuzzy Inference System (ANFIS) models have been used to predict the carbon dioxide sorption in several Poly Ionic Liquids (PILs). A number of PILs based on ammonium and imidazolium by different anion and cation parts have been investigated based on a dataset containing 35 PILs with 350 data points. 70% of data points have been used for training the network, 25% for testing and 5% for validation. The model is a multilayer Feed Forward Artificial Neural Network (FFANN) with Levenberg-Marquardt as a function for optimizing error. The number of optimum neurons in the hidden layer is equal to 20. To distinguish PILs from each other, their structure has been defined by group contribution method. In addition of type and number of the chemical structures, pressure (bar) and temperature (K) have been also fed as input parameters to the network. Amount of carbon dioxide sorption in terms of concentration has been defined as the output of the network. The Absolute Average Deviation (AAD%) indicates that the ANN-GC and ANFIS-GC models are appropriate tools to predict carbon dioxide solubility where ANN-GC model shows sign of some superiority. [ABSTRACT FROM AUTHOR]

Analysis of fixed ammonium (NH4+) in soils by the most common method requires the subsample to be ground to pass through a 100-mesh sieve. We tested a hypothesis that the ball milling pretreatment for the analysis of fixed NH4+may influence the analytical results depending on the content of exchangeable NH4+. Five paddy soils collected from the surface layer in Japan were treated with15N-labeled ammonium sulfate at 0, 100, 500, or 1000 mg N kg−1, incubated aerobically at 30°C for 7 days, and air-dried. Part of each soil was then ground in a vibration ball mill at 1200 rpm for 10 min as a pretreatment (BMP treatment). The samples with and without the BMP treatment were analyzed for exchangeable NH4+and total fixed NH4+. The samples that received 0 or 1000 mg N kg−1were also analyzed for nonexchangeable15N (total fixed15NH4+plus organic15N) and hot KCl-extractable organic N. At 0 mg N kg−1, the BMP treatment significantly decreased total fixed NH4+in all soils and significantly increased exchangeable NH4+in two of the five soils. The fixation of exchangeable NH4+started to occur when its content is greater than around 300 mg N kg−1, and its ratio to total fixed NH4+is greater than around 1.0. At 1000 mg N kg−1, the BMP treatment significantly decreased exchangeable NH4+and significantly increased total fixed NH4+in all soils especially two clayey, vermiculitic soils. On the average of all soils applied 1000 mg N kg−1, the increase in total fixed NH4+explained 55% of the decrease in exchangeable NH4+, and the decrease in exchangeable NH4+was almost equal to the increase in nonexchangeable15N. The rest of the NH4+unaccounted by the fixation was partly due to the transformation of exchangeable NH4+to hot KCl-extractable organic N, because the content of hot KCl-extractable organic N was significantly increased by the BMP treatment. In conclusion, the milling treatment prior to the analysis of fixed NH4+in soils might cause either underestimation or overestimation depending on the content of exchangeable NH4+. [ABSTRACT FROM PUBLISHER]

The question of where and how to carry out reclamation work in coastal areas is still not well addressed in coastal research. To answer the question, it is essential to quantify the impact of reclamation and the associated ecological and/or environmental responses. In this study, ordinary least square (OLS) analysis and geographical weighted regression (GWR) analysis were performed to identify the reclamation variables that affect soil and vegetation characteristics. Reclamation related variables, including residential population (RP), years of reclamation (YR), income per capita (IP), and land use-based human impact index (HII), were used to explain nitrate, ammonium, total phosphorous, and heavy metals in soil, and the height, density, and above-ground biomass of native hydrophytic vegetation. It was found that variables IP, RP, and HII could be used to explain the height of Scirpus and Phragmites australis as well as above-ground biomass with a R 2 value of no > 0.55, and almost all the variables could explain the hydrophytic vegetation characteristics with a higher R 2 value. In comparison to OLS, GWR more reliably reflected the reclamation effects on soil and vegetation characteristics. By GWR analysis, total soil phosphorous, and nitrate and ammonium nitrogen could be explained by RP, YR, and HII, with the highest Ad-R 2 value of 0.496, 0.631 and 0.632, respectively. Both of the GWR and OLS analysis revealed that HII and RP were the better variables for explaining the soil and vegetation characteristics. This work demonstrated that coastal reclamation was highly spatial dependent, which sheds a light on the future development of spatial explicit and process-based models to guide coastal reclamation around the world. [ABSTRACT FROM AUTHOR]

Anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) are two nitrogen-cycle processes that have recently received more attention. However, few studies have been performed on the effects of different fertilizations in upland-cropping soil. Therefore, the objective of this work was to assess the effects of different fertilizations on the distribution and diversity of anammox and n-damo bacteria along the profiles of soil during wheat cropping season of the paddy-upland rotation system. In our study, a five-year fertilization (from 2010 to 2015) was conducted on a paddy-upland rotation field, with four fertilizer treatments: an unfertilized control (CK), chemical fertilizer (CF), pig manure plus 50% chemical fertilizer (PMCF), and rice straw plus 100% chemical fertilizer (SRCF). The quantitative PCR results showed that the total abundance of anammox and n-damo bacteria during the wheat cropping season were lower than those reported previously in paddy soils, and the abundance of anammox bacteria decreased with soil depth, but the abundance of n-damo bacteria remarkably increased. Long-term fertilization reduced the abundance of anammox bacteria in the 0–40 cm layer and that of n-damo bacteria in the 20–60 cm layer. Furthermore, Illumina high-throughput sequencing of the hzs B and pmo A genes was performed, showing that anammox bacteria were affiliated with “ Candidatus Brocadia” and n-damo bacteria were related to “ Candidatus Methylomirabilis oxyfera.” Diversity analysis demonstrated that the diversity of anammox bacteria increased with soil depth. Among the fertilizer treatments evaluated, pig manure plus 50% chemical fertilizer (PMCF) yielded the highest abundance and lowest diversity of anammox bacteria in the 20–60 cm layer. Moreover, fertilization diversified the community structure of anammox bacteria in deep soil. For n-damo bacteria, a higher diversity was found under chemical fertilization than under other treatments. Redundancy analysis showed that the community structure of anammox was mainly related to of total nitrogen and soil organic carbon and that of n-damo bacteria was mainly associated with NO 3 − content. Overall, soil depth and fertilization patterns influenced the distribution and diversity of anammox and n-damo bacteria by changing the soil properties. [ABSTRACT FROM AUTHOR]

Ammonium and nitrate are the major forms of nitrogen (N) present in tropical soils. An experiment was conducted to assess the influence of nitrate and ammonium forms (NO3−, NH4+, and mix of NO3− + NH4+), and levels (1.5–12.0 mM) of N on the growth and nutrition of cacao (Theobroma cacao L). Growth parameters were significantly influenced by N forms, and nitrogen supplied as NH4+ proved better for the growth of cacao compared with NO3− form and mixtures of these two forms. Irrespective of the forms of N, levels of N had no significant effect on plant growth parameters. Nutrient efficiency ratios (NERs) (shoot dry matter produced per unit of nutrient uptake) for macronutrients were sulfur>phosphorus>calcium>magnesium>nitrogen>potassium (S>P>Ca>Mg>N>K) and for micronutrients NERs were in the order of copper>boron>zinc>iron>manganese (Cu>B>Zn>Fe>Mn). [ABSTRACT FROM AUTHOR]

Ammonia oxidation, the conversion of ammonium (NH 4 + -N) to nitrite (NO 2 − -N), is the critical step of nitrification and performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB). However, the effects of land-use conversion and fertilization on the AOA and AOB are not well documented, and the contribution of these two groups to soil ammonia oxidation is still debatable. This study aimed to explore how land-use conversion from rice paddies (RP) to citrus orchards (OR) and fertilization affect the abundance and communities of AOA and AOB, and whether AOA and AOB were correlated to ammonia oxidation. The potential ammonia oxidation (PAO) was measured by chlorate inhibition method, and the abundance and communities of AOA and AOB were quantified using quantitative real-time polymerase chain reaction (qPCR), terminal restriction fragment length polymorphism (T-RFLP) and cloning and sequence analysis. Land-use conversion from RP to OR tended to increase PAO, whereas fertilization increased PAO only in the OR. qPCR results demonstrated that land-use conversion significantly increased AOA abundance, but failed to affect the AOB abundance. The AOA abundance was increased in the OR but decreased in the RP after fertilization, whereas fertilization distinctly stimulated AOB abundance in the OR and RP. T-RFLP analysis showed that the communities of AOA and AOB were significantly changed after land-use conversion. Meanwhile, fertilization significantly affected the communities of AOB both in the OR and RP and only affected the communities of AOA in the OR. Phylogenetic analyses of amo A genes showed that group 1.1 a like lineage predominated archaea ammonia oxidizers in the acidic red soil, while all AOB clones were belonged to the Nitrosospira . Besides, soil ammonia oxidation was positively correlated to AOA and AOB abundance both in the OR and RP. Our findings imply that land-use conversion and fertilization could potentially alter the ammonia oxidation by regulating the growth of AOA and AOB in this acidic red soil. [ABSTRACT FROM AUTHOR]

In fertilized paddy soils, the role of resource availability on ammonium (NH 4 + ) assimilation and immobilization by archaea requires advanced understanding as this may have considerable implications on subsequent catalytic steps in the soil N cycle including archaeal nitrous oxide (N 2 O) reduction. To gain a deeper understanding about these process links, we incubated a temperate paddy soil under submerged conditions with or without straw and fertilized with either 15 N-enriched (99 atom% 15 N) or non-enriched (NH 4 ) 2 SO 4 . Notably, a change in community structure and a higher abundance of archaeal N 2 O reductase ( arc-nosZ ) genes in the no straw treatment was observed. This was attributed to NH 4 + assimilation by N 2 O reducing archaea as was further corroborated by a considerable 15 N-enrichment of archaeal glutamate dehydrogenase ( gdhA ) genes. Moreover, indications were found that denitrifying archaea controlled their chemotrophic and heterotrophic metabolisms in response to different availabilities of inorganic and organic N and C resources. However, in the presence of straw, bacterial nosZ genes may have also contributed to the completion of denitrification. Our results suggest that N assimilation contributes to the last step of archaeal denitrification. Likewise, archaea may play a key role in regulating major N fluxes in fertilized paddy soils, especially in the absence of rice straw. [ABSTRACT FROM AUTHOR]

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH ) and nitrate (NO ). Previous long-term studies indicate efficient turnover of atmogenic NH to NO in weathered, acidic soils of the subtropics, leading to excessive NO leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. NHNO, NHNO and N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (NO) were monitored for nine days. Incorporation of NO into soil organic N was negligible and 80% of the applied label was lost from the top soil (0-15 cm) primarily by leaching within 9 days. In contrast, NH was largely retained in soil organic N. However, instant production of NO in the NH treatment suggested active nitrification. In both the NH and N-glutamic acid treatments, the N enrichment in the NO pool exceeded that in the NH pool one day after N application, suggesting preferential nitrification of added NH with subsequent dilution of the NH pool and/or immobilization of NH followed by heterotrophic nitrification. The cumulative recovery of N in NO after 9 days ranged from 2.5 to 6.0% in the NO treatment, confirming the previously reported significant response of NO emission to N deposition. Source partitioning of NO demonstrated a measurable contribution of nitrification to NO emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO and NH ) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO in the soil and (ii) rapid immobilization-mineralization of NH followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and NO emission. [ABSTRACT FROM AUTHOR]

In mixedwood forest, different types of commercial thinning that generate different gap sizes are being tested as alternatives to clearcutting to create forest stands with an irregular structure that would emulate the pre-industrial forests. The main goal of this study was to investigate the soil nitrogen (N) dynamics in response to two partial harvesting treatments, used alone or in combination: commercial thinning creating tree-size canopy gaps and harvest gaps creating 0.05 ha gaps. In a 30 year old balsam fir ( Abies balsamea (L.) Mill.) stand, both treatments, alone or in combination, as well as unharvested controls were compared on replicated 0.75 ha plots. The most noticeable changes following treatments were observed in gaps, while commercial thinning did not significantly influence any of the parameters assessed. In gaps, increases in N mineralization rates and mineral N concentrations and proportions (NO3−-N and NH4+-N) relative to dissolved organic N were observed. Our results suggest that these changes are caused by the increase in soil temperature and water content. In these forests, the response threshold of the N cycling regarding the size of the intervention would therefore be located between the removal of one or a few trees (one to three stems, 6-12 m2) and a gap of 500 m2. Other studies conducted in different climate and forest types have shown that this threshold could be of equivalent or of a smaller size. These findings will contribute to optimizing our management strategies regarding partial cuts or small-scale clearcuts. [ABSTRACT FROM AUTHOR]

Many farmlands are periodically flooded or ponded by excessive precipitation resulting in changes to soil chemical and biochemical properties. In this study, one set (eight treatments with four replications) of field-moist surface soils (0-15 cm) and their air-dried counterparts obtained from a long-term liming experiment were incubated at 30 °C under waterlogged conditions for 10 days, and the amounts of net NH -N released (soluble and exchangeable) were determined after extraction with 4 M KCl. Another set of three surface soils were used to evaluate the effect of six heavy metals on the NH -N release under waterlogged conditions. Results showed that increasing the liming rate from 0 to 17,930 kg ha effective calcium carbonate equivalent increased the average soil pH from 4.98 to 7.06, averages of the amounts of NH -N released ranged from 1.6 to 5.2 mg N kg field-moist soil, and the corresponding amounts released in air-dried soils ranged from 18.9 to 32.9 mg N kg soil. This increase of the amount NH -N released in air-dried soil samples is presumably due to a slaking effect. At 5 mmol kg soil, all six heavy metals inhibited the NH -N released. The relative effectiveness of the heavy metals in inhibition of the NH -N released varied among the three soils. Lead(II) was the most effective inhibitor of NH -N release in Clarion and Harps soils and Cd(II) in Harps soil. Cobalt(II), Cu(II), and Cd(II) were the least effective inhibitors of NH -N release in Clarion, Harps, and Okoboji soils, respectively. [ABSTRACT FROM AUTHOR]

Glutamine synthetase (GS) catalyzes a reaction that incorporates ammonium into glutamate and yields glutamine in the cytosol and chloroplasts. Although the enzymatic characteristics of the GS1 isozymes are well known, their physiological functions in ammonium assimilation and regulation in roots remain unclear. In this study we show evidence that two cytosolic GS1 isozymes (GLN1;2 and GLN1;3) contribute to ammonium assimilation in Arabidopsis roots. Arabidopsis T-DNA insertion lines for GLN1;2 and GLN1;3 (i.e. gln1;2 and gln1;3 single-mutants), the gln1;2:gln1;3 double-mutant, and the wild-type accession (Col-0) were grown in hydroponic culture with variable concentrations of ammonium to compare their growth, and their content of nitrogen, carbon, ammonium, and amino acids. GLN1;2 and GLN1;3 promoter-dependent green fluorescent protein was observed under conditions with or without ammonium supply. Loss of GLN1;2 caused significant suppression of plant growth and glutamine biosynthesis under ammonium- replete conditions. In contrast, loss of GLN1;3 caused slight defects in growth and Gln biosynthesis that were only visible based on a comparison of the gln1;2 single- and gln1;2:gln1;3 double-mutants. GLN1;2, being the most abundantly expressed GS1 isozyme, markedly increased following ammonium supply and its promoter activity was localized at the cortex and epidermis, while GLN1;3 showed only low expression at the pericycle, suggesting their different physiological contributions to ammonium assimilation in roots. The GLN1;2 promoter-deletion analysis identified regulatory sequences required for controlling ammonium-responsive gene expression of GLN1;2 in Arabidopsis roots. These results shed light on GLN1 isozyme-specific regulatory mechanisms in Arabidopsis that allow adaptation to an ammonium-replete environment. [ABSTRACT FROM AUTHOR]

High concentrations of ammonium (NH 4 + ) in soil have been shown to inhibit nitrification, and fertilizer injection as conducted during CULTAN (controlled uptake long-term ammonium nutrition) management might thus have the potential to reduce N 2 O emission from arable soil. We conducted an incubation experiment with different NH 4 + concentrations in soil that resembled concentrations as expected at and around injection spots (5000, 2250, 1000, 450, 0 mg NH 4 + -N kg −1 soil) directly after fertilization and after dilution due to plant uptake or precipitation. N 2 O emission was measured in dynamic soil mesocosms over a period of 21 days. Acetylene inhibition and 15 N tracer approaches were used to calculate the relative contribution of nitrification and denitrification to N 2 O emission. An isotopomer approach was applied to gain further insight into N 2 O producing processes. We expected lower contribution of nitrification-derived N 2 O to total N 2 O emission and a higher N 2 O/NO 3 − ratio from nitrification with increasing NH 4 + levels. Nitrification indeed declined with increasing NH 4 + level, and no nitrification occurred in the 5000 mg NH 4 + -N kg −1 soil treatment. A pool dilution approach showed that gross nitrification in 450 mg NH 4 + -N kg −1 soil (nitrification rate: 4.96 mg NO 3 − -N kg soil d −1 ) was by a factor of 2.6 and 6 higher than in 1000 and 2250 mg NH 4 + -N kg −1 soil treatments. In the 5000 mg NH 4 + -N kg −1 soil treatment, gross nitrification occurred at very small rates (0.1 mg NO 3 − -N kg soil d −1 ). Similarly, N 2 O emission declined with increasing NH 4 + level. The N 2 O yield of nitrification was between 0.07 and 0.15% of NO 3 − production, but was not affected by increasing NH 4 + level. Nitrification was the dominant source of N 2 O throughout the incubation at all NH 4 + levels, and there was no significant change in the relative contribution of nitrification and denitrification with NH 4 + level or time. This finding indicates that denitrification derived N 2 O emissions were similarly reduced at high NH 4 + levels. Applying the non-equilibrium technique to our 15 N tracer data revealed heterogeneous distribution of denitrification in soil, with at least two distinct NO x − (NO 3 − + NO 2 − ) pools and spatial separation of NO x − formation and consumption. The isotopomer approach provided reasonable results in comparison with the acetylene inhibition and 15 N tracer approaches and indicated substantial contribution of nitrifier denitrification and/or coupled nitrification-denitrification (10–40%) to total N 2 O production. [ABSTRACT FROM AUTHOR]

In this study the GAG model, a process-based ammonia (NH3) emission model for urine patches was extended and applied for the field scale. The new model (GAG_field) was tested over two modelling periods, for which micrometeorological NH3 flux data were available. Acknowledging uncertainties in the measurements, the model was able to simulate the main features of the observed fluxes. The temporal evolution of the simulated NH3 exchange flux was found to be dominated by NH3 emission from the urine patches, offset by simultaneous NH3 deposition to areas of the field not affected by urine. The simulations show how NH3 fluxes over a grazed field in a given day can be affected by urine patches deposited several days earlier, linked to the interaction of volatilization processes with soil pH dynamics. Sensitivity analysis showed that GAG_field was more sensitive to soil buffering capacity (β), field capacity (θfc) and permanent wilting point (θpwp) than the patch scale model. This can be explained by the different initial soil pH and physical characteristics which determine the maximum volume of urine that can be stored in the NH3 source layer. It was found that in the case of urine patches with a higher initial soil pH and higher initial soil water content, the sensitivity of NH3 exchange to β was stronger. Also, in the case of a higher initial soil water content, NH3 exchange was more sensitive to the changes in θfc and θpwp. The sensitivity analysis showed that the nitrogen content of urine (cN) is associated with high uncertainty in the simulated fluxes. However, model experiments based on cN values randomized from an estimated statistical distribution indicated that this uncertainty is considerably smaller in practice. Finally, GAG_field was tested with a constant soil pH of 7.5. The variation of NH3 fluxes simulated in this way showed a good agreement with those from the simulations with the original approach, accounting for a dynamically changing soil pH. These results suggest a way for model simplification when GAG_field is applied later for regional scale. [ABSTRACT FROM AUTHOR]

Here, we characterized nitrogen (N) uptake of beech ( Fagus sylvatica) and their associated ectomycorrhizal (EM) communities from NH4+ and NO3−. We hypothesized that a proportional fraction of ectomycorrhizal N uptake is transferred to the host, thereby resulting in the same uptake patterns of plants and their associated mycorrhizal communities. 15N uptake was studied under various field conditions after short-term and long-term exposure to a pulse of equimolar NH4+ and NO3− concentrations, where one compound was replaced by 15N. In native EM assemblages, long-term and short-term 15N uptake from NH4+ was higher than that from NO3−, regardless of season, water availability and site exposure, whereas in beech long-term 15N uptake from NO3− was higher than that from NH4+. The transfer rates from the EM to beech were lower for 15N from NH4+ than from NO3−. 15N content in EM was correlated with 15N uptake of the host for 15NH4+, but not for 15NO3−-derived N. These findings suggest stronger control of the EM assemblage on N provision to the host from NH4+ than from NO3−. Different host and EM accumulation patterns for inorganic N will result in complementary resource use, which might be advantageous in forest ecosystems with limited N availability. [ABSTRACT FROM AUTHOR]

The 15N isotope pairing technique is widely used to quantify anammox, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in sediments. However, the effects of DNRA on anammox and denitrification in slurry incubations and on genuine N2 production in intact core incubations have not been fully explored. We developed a mathematical model describing these effects of DNRA, and tested this model in field and computational studies. Our model calculations revealed that for slurry incubations the presence of DNRA tends to under -and overestimate the actual anammox and denitrification rates, respectively, when calculated according to Thamdrup and Dalsgaard (2002). We found this underestimate of anammox to be proportional to the 15 [ABSTRACT FROM AUTHOR]

Purpose: Although archaea play an important role in nutrients cycling, the archaeal community in a reservoir water-level fluctuation zone (WLFZ) remains unclear. An elucidation of archaeal community responding to the environmental variables is essential to understand the nutrients dynamics in WLFZ. This study focused on the response of the archaeal community structure and abundance to the periodic water flooding along an elevation gradient in the WLFZ of the Three Gorges Reservoir. Materials and methods: Along the elevation gradient (152-175 m) of the study area, soil samples in the beginning and late stages of water flooding were collected to investigate the influence of water flooding on the archaeal community in soil, using quantitative PCR and Illumina high-throughput sequencing approaches. Results and discussion: An increase of archaeal abundance from 3.8 × 10 to 3.8 × 10 copies (g d.w.s) on average was observed after water flooding. The archaeal abundance was positively correlated with the contents of ammonium, organic matter, and moisture in soil and with the accumulated flooding time. Higher diversity was observed in dry samples (non-flooded soil samples) rather than wet samples (flooded soil samples). The Thaumarchaeota were predominant in most of the dry samples. Interestingly, high proportions of Candidatus Nitrososphaera were observed in the transition zone, while euryarchaeotal methanogens dominated the wet samples. The proportion of methanogens decreased dramatically in the dry samples at higher elevations, which was associated with the decrease of the moisture content and the probably increase of available oxygen in soil. Conclusions: Archaeal abundance, diversity, and community composition shifted along an elevation gradient and were influenced by water flooding. The increased archaea abundance after water flooding and elevation related community composition and diversity indicated that water flooding was a key dynamic environmental variable in the WLFZ. [ABSTRACT FROM AUTHOR]

Inorganic nitrogen (N) determination in soil extracts is a useful tool in studying soil–N interactions. The main method used worldwide is the steam distillation, but colorimetric methodologies show advantages of being quick, simple, sensitive, and higher yield. However, they are subject to disruption when the extract has interfering organic N, limiting its application in tropical soil. The aim of this study is to evaluate colorimetric methods for the determination of NH4+(through Berthelot reaction) and NO3−+ NO2−(through Griess-Ilosvay reaction) in saline extracts of tropical soils by addition of activated charcoal during the extraction process. It is recommended to use extraction solution of 2 mol L−1potassium chloride (KCl) and reaction time of 30 min. Validation parameters confirmed the best performance of the colorimetric methods, revealing results favorable to the addition of the activated charcoal in the extraction process to increase the accuracy and precision of the colorimetric methods. [ABSTRACT FROM AUTHOR]

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH immobilization 38.92, NO immobilization 10.74, and DNRA 1.54 µg N g SDW d) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups. [ABSTRACT FROM AUTHOR]

The impact of temperature on bacterial processes is well known; however temperature related data on nitrification rates in aquaculture systems are fragmented and compiled from different studies. We sought to determine ammonium and nitrite removal kinetics over a temperature range from 6 to 36 °C by using moving bed bio-elements from a freshwater RAS in steady state operated at 18 °C. The impact of temperature on ammonium and nitrite oxidation rates was evaluated by transferring the colonized bio elements to six liter batch reactors (triplicated setup). Each reactor was acclimatized for 24 h at each of the six temperatures (6, 12, 18, 24, 30 and 36 °C) and then spiked with ammonium chloride or sodium nitrite under identical conditions. The average surface specific TAN removal (STR) increased a six-fold from 6 to 30 °C (0.04–0.25 g TAN m −2 day −1 ) and dropped significantly at 36 °C–0.14 g TAN m −2 day −1 . The surface specific nitrite removal (SNR) increased linearly from 0.04 g N m −2 d −1 at 6 °C–0.14 g N m −2 day −1 at 30 °C, decreasing to 0.12 g N m −2 day −1 at 36 °C. Throughout the temperature range tested, STR remained significantly larger than SNR. The temperature coefficient, Ɵ (6–30 °C) for ammonium oxidation was 1.079; for nitrite oxidation the temperature coefficient was found to be 1.054. The data provided by this study can be applied dimensioning future RAS that utilizes temperature ranges below 10 and above 30 °C. [ABSTRACT FROM AUTHOR]

Understanding the nutrient retention and release characteristics of biochar before its application to agricultural soils is crucial to maximize the benefits and minimize the damage that might be posed. In this study, batch sorption experiments were carried out to determine the ability of biochar produced from rice husk to adsorb or release three major nutrient ions: ammonium (NH 4 + ), nitrate (NO 3 − ), and phosphate (PO 4 3 − ). Column leaching experiments were conducted to investigate the effects of rice husk char (RHC) application on soil nutrient leaching. The results showed that RHC could adsorb ammonium 4.7 mg N g − 1 and nitrate 2.1 mg N g − 1 in solutions with an initial concentration of 200 mg N L − 1 . Almost no phosphate sorption was observed using the RHC. Instead, some phosphate was released into aqueous solution at lower initial concentrations (below 60 mg P L − 1 ). Application of RHC into loamy soils at a rate of 4% ( w / w ) reduced total ammonium leaching by 11% and total nitrate leaching by 23%, but increased phosphate leaching by 72%, compared with the control soil columns. Total leached phosphate throughout experimental period was 0.16 mg P from control soil and 0.27 mg P from soil with RHC application. These findings suggest that the application of RHC can be considered a potential strategy for mitigation of nitrogen leaching in loamy soil. The RHC, however, is unsuitable for phosphorus leaching mitigation. Instead, RHC addition is a possible method to increase the availability of phosphorus in a low P index soil. [ABSTRACT FROM AUTHOR]

Changes in the structural composition of fulvic acids were followed during composting of tannery solid waste after neutralization of its acidity by ammonium or by lime. Different techniques (elemental analysis, Fourier transform infrared (FTIR),13C-NMR spectroscopy) were applied. During both trials of composting, a decrease of carbon and increase of oxygen occurred, which originated from strong microbial oxidation and preservation of some structures during composting. In fact, Nuclear Magnetic Resonance (NMR) spectra showed the preservation of anomeric or tannin structures around 105 ppm. The infrared (IR) spectral data showed a decrease in the intensity of COO−group bands (1623 and 1399.5 cm−1); this is correlated with a decrease of fulvic acid levels. The variations observed in NMR spectra could be explained by the involvement of COO−groups of certain structures in polycondensing to form humic acids. The fulvic acids remain composed of less polycondensed structures with a –COOH extremity, as supported by a decrease of the C/H ratio. In contrast, in the trial with addition of ammonia a slight increase in N content occurred, a decrease in absorbance in the 1730–1000 cm−1regions compared to the absorbance around 3400 cm−1, and a strong decrease of carboxyl carbon in the13C-NMR spectra. [ABSTRACT FROM AUTHOR]

This study aims to determine the effect of glucose addition on the fate of urea- 15 N in fixed NH 4 + and soil microbial biomass N (SMBN) pools in a 96-day incubation experiment. The contributions of organic N (including SMBN and soil microbial necromass N) and fixed NH 4 + pools to mineral N were also assessed. Glucose addition significantly improved 15 N recovery but decreased the availability of urea-derived N. A great portion of urea-derived N was immobilized and then transformed into microbial necromass. The effect of fixed NH 4 + on the conservation and supply of urea-derived N were greater than those of organic N pool in the non-glucose treatment, in contrast to that in the glucose treatment. From the sixth day to the end of the incubation, the amount of urea-derived fixed NH 4 + release was 1.8-fold higher than that of urea-derived organic N mineralization in the treatment without glucose. By contrast, the latter was 3.1-fold higher than the former under glucose addition. Moreover, path coefficient analysis showed that the release of fixed NH 4 + was promoted by microbial immobilization and nitrification in the absence of glucose, whereas the release was primarily induced by microbial immobilization in the presence of glucose. The partitioning of the released fixed NH 4 + in the mineral or organic N pools was identified using path coefficients. This study provides very helpful information for quantification of fertilizer N transformation in soil and combines the abiotic and biotic processes in the N cycle. [ABSTRACT FROM AUTHOR]

Green manure crops are a common N fertilizer source in organic agriculture. Producers need guidance on when to incorporate the green manure for efficient N uptake by the following crop. The objective of this study was to describe the temporal dynamics of soil mineral N following green manure plough-down and N available to arugula (Eruca sativa L. sp.) also known as salad rocket, a fast-growing leafy herb used in salads. Green manure was a mixture of field pea (Pisum sativum L.) cv. CDC Meadow and oat (Avena sativa L. sp.) grown on fields with sandy loam and sandy clay loam soils at an organic vegetable farm in southwestern Quebec, Canada. Ion exchange membranes (IEMs) were used to monitor the ammonium (IEM-NH+ 4-N) and nitrate (IEM-NO- 3 -N) released following green manure plough-down. The IEM-NO- 3-N concentration was positively correlated with rainfall (r = 0.57 to 0.95, p < 0.05) and was greater from 2 to 5 weeks after green manure plough-down than in soils without green manure. Arugula N uptake and IEM-NO- 3-N were positively correlated in sandy clay loam (r = 0.86, p < 0.01) and sandy loam (r = 0.71, p < 0.05) soils. IEM-NO- 3-N concentrations were found to be sensitive to weather, green manure decomposition and arugula growth, and could be a tool to monitor soil mineral N available for shortseason vegetable crops. [ABSTRACT FROM AUTHOR]

Biochar (BC) has been shown to increase the potential for N retention in agricultural soils. However, the form of N retained and its strength of retention are poorly understood. Here, we examined if the N retained could be readily extractable by standard methods and if the amount of N retained varied with BC field ageing. We investigated soil and field-aged BC (BCaged) particles of a field experiment (sandy soil amended with BC at 0, 15, and 30 t ha-1) under two watering regimes (irrigated and rain-fed). Throughout the study, greater nitrate than ammonium retention was observed with BC addition in topsoil (0–15 cm). Subsoil (15–30 cm) nitrate concentrations were reduced in BC treatments, indicating reduced nitrate leaching (standard 2 mol L-1 KCl method). The mineral-N release of picked BCaged particles was examined with different methods: standard 2 mol L-1 KCl extraction; repeated (10×) extraction in 2 mol L-1 KCl at 22 ± 2°C and 80°C (M0); electro-ultrafiltration (M1); repeated water + KCl long-term shaking (M2); and M2 plus one repeated shaking at 80°C (M3). Nitrate amounts captured by BCaged particles were severalfold greater than those in the BC-amended soil. Compared with M0, standard 2 mol L-1 KCl or electro-ultrafiltration extractions retrieved only 13 and 30% of the total extractable nitrates, respectively. Our results suggest that "nitrate capture" by BC may reduce nitrate leaching in the field and that the inefficiency of standard extraction methods deserves closer research attention to decipher mechanisms for reactive N management. [ABSTRACT FROM AUTHOR]

Nitrogen (N) cycling of drylands under changing climate is not well understood. Our understanding about N cycling over larger scales to date relies heavily on the measurement of bulk soil N, and the information about soil internal N transformations remains limited. The 15N natural abundance (δ15N) of ammonium and nitrate can serve as a proxy record for the N processes in soils. To better understand the patterns and mechanisms of water availability on soil N cycling in drylands, we collected soils along a 3200 km dryland transect at about 100 km intervals in northern China, with mean annual precipitation (MAP) from 36 mm to 436 mm. We analysed N pools and δ15N of ammonium, dual isotopes (15N and 18O) of nitrate, and the microbial gene abundance associated with soil N transformations. We found that the N status and their driven factors were different on the two sides of MAP = 100 mm. In the arid zone with MAP below 100 mm, soil inorganic N accumulated, with a large fraction being of atmospheric origin. Ammonia volatilization was strong because of the higher soil pH. The abundance of microbial genes associated with soil N transformations was also significantly low. In the semiarid zone with MAP above 100 mm, soil inorganic N concentrations were low and controlled mainly by biological processes, e.g., plant uptake and denitrification. The preference of soil ammonium to nitrate by the dominant plant species may enhance the possibility of soil nitrate loss via denitrification. Overall, our study suggest that the shifting from abiotic to biotic controls on soil N biogeochemistry under global climate changes would greatly affect N losses, soil N availability, and other N transformation processes in these drylands in China. [ABSTRACT FROM AUTHOR]