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While soil physico-chemical and solute properties mainly affect N diffusion directly, root and microbial properties have a strong indirect effect on diffusion by critically controlling N production and consumption rates and thus N pool sizes. Keywords: amino acid speciation; diffusion; microdialysis; soil nitrogen (N) dynamics; soil organic N fluxes EN amino acid speciation diffusion microdialysis soil nitrogen (N) dynamics soil organic N fluxes 2104 2106 3 08/19/21 20210915 NES 210915 Soils typically contain a large variety of nitrogen (N) forms, including inorganic N and a range of organic N compounds of varying molecular size (Warren, 2013). [Extracted from the article]

Vegetated filter strips (VFS) act as buffer zones between fields and water bodies that are supposed to retain incoming runoff, sediment, and nutrients. The factors that govern nutrient retention and cycling in VFS are complex and act in all three dimensions. A key element that determines VFS effectivity is flow type, e.g., sheet vs. concentrated flow. These aspects are, however, often insufficiently accounted for in VFS research and design recommendations. In this study, we attempt to tackle these shortcomings by examining the nutrient distribution in detail at two field-VFS transitions, applying a three-dimensional sampling array together with extensive laboratory analyses. Concentrated runoff was the dominant type we found and we argue that flow convergence is the norm rather than the exception. Further complicating this issue is that entry locations of runoff may vary, calling for more sophisticated sampling designs. Overall trends were similar across the analyzed nutrient fractions (different K- and P-pools) and there were distinct trends of decreasing nutrients along the longitudinal (from the field to the VFS) and vertical planes. The horizontal plane (from outside to inside the area of concentrated flow) showed mostly inconclusive or U-shaped gradients. Both sites were similar and close to each other, nevertheless, there were significant differences that affected nutrient retention in the VFS which were linked to site-specific factors. The spatial extent (i.e., width) is often considered the main variable in VFS designs. However, other VFS traits such as vegetation type and structure, as well as external factors such as field topography and the severity of erosive events are equally important and should be attributed more significance. • Vegetated filter strips (VFS) protect surface waters from sediment and nutrient input. • VFS design and research is often oversimplified, e.g., not viewed in 3D. • Nutrient distribution in the soil shows distinct gradients along all spatial planes. • Differences in nutrient retention are caused by site-specific factors. • An elaborate 3D approach is indispensable for a holistic VFS assessment. [ABSTRACT FROM AUTHOR]

Large differences in productivity and species composition are characteristic for the boreal forest and nitrogen (N) availability has been deemed the proximate cause of this variation. We used a modified microdialysis technique to assess N availability through monitoring in situ inorganic and organic soil N fluxes in the presence and absence of mass flow in two forest ecosystems of contrasting fertility, a nutrient rich Norway spruce forest and a nutrient poor Scots pine forest. This was enabled by using solutions of different osmotic potentials as perfusates. In the absence of mass flow, amino acids dominated soil N fluxes of both ecosystems representing 62 and 82% of total flux in the nutrient rich and the nutrient poor ecosystem respectively. In the presence of mass flow, N flux increased by nine times in the nutrient rich and four times in the nutrient poor soil and nitrate comprised a greater share of total N flux. Our results suggest that mass flow may be a strong driver for plant N acquisition in boreal forests through delivering higher amounts of amino acids and NO 3 − to plant roots and mycorrhizas. These results points to a strong interaction between water and N availabilities, the former enhancing the supply of the latter through enabling high rates of transpiration. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is the most important limiting nutrient for plants and soil microorganisms in almost all ecosystems. Global environmental changes significantly affect the terrestrial N cycle but implications for plant available N in-situ remain unclear. Here, we investigated the simultaneous effect of elevated atmospheric CO 2 , an increase in temperature and a drying-rewetting event on diffusive N fluxes in soil of a managed permanent grassland at the multifactor climate manipulation experimental site ClimGrass (Austria) using microdialysis. Rewetting caused a significant, but short-lived increase in diffusive NH 4 + and NO 3 − fluxes, which subsequently dropped until the end of the experiment. Harvesting induced a significant increase in diffusive NH 4 + fluxes in the drying-rewetting treatments. However, elevated CO 2 and soil warming had little effect on diffusive N fluxes. Our study suggests that more frequent soil drying-rewetting cycles associated with increased extreme weather events are uppermost among the climate change drivers affecting soil N availability. • Rewetting leads to significant, short-lived increases in NH 4 + and NO 3 − fluxes. • Harvesting promotes distinct changes in diffusive N fluxes. • Extreme events prevail over elevated CO 2 and warming on N dynamics in soil. [ABSTRACT FROM AUTHOR]

The plant-parasitic cyst nematode Heterodera schachtii induces syncytial feeding structures in the roots of host plants. These syncytia provide all required nutrients, water and solutes to the parasites. Previous studies on the composition of primary metabolites in syncytia revealed significantly increased amino acid levels. However, mainly due to technical limitations, little is known about the role of arginine in plant-nematode interactions. This free amino acid plays a central role in the plant primary metabolism and serves as substrate for metabolites involved in plant stress responses. Thus, in the present work, expression of genes coding for the enzymes of arginine metabolism were studied in nematode-induced syncytia compared to non-infected control roots of Arabidopsis thaliana. Further, amiRNA lines were constructed and T-DNA lines were isolated to test their effects on nematode development. While the silencing of genes involved in arginine synthesis increased nematode development, most T-DNA lines did not show any significant difference from the wild type. Amino acid analyses of syncytia showed that they accumulate high arginine levels. In addition, manipulating arginine cycling had a global effect on the local amino acid composition in syncytia as well as on the systemic amino acid levels in roots and shoots. [ABSTRACT FROM AUTHOR]

The supply of soil nitrogen (N) for plant uptake largely controls plant growth and has a major impact on a wide range of biogeochemical processes in terrestrial ecosystems. The soil solution typically contains a large variety of N forms and recent evidence suggests that the share of amino acids to soil N fluxes dominates over inorganic N in boreal forest soils. In this study we applied a microdialysis technique to investigate in-situ induced diffusive fluxes across microdialysis membranes (FMD) in fertilized and non-fertilized boreal forest sites in early spring, at the onset of plant growth. We studied temporal shifts of FMD at short (minutes to hours) and prolonged time-scales (hours to days). We also estimated N pools in soil water and KCl extracts and critically evaluated the significance of results depending on the method chosen. Our results indicate that FMD of boreal forest soil is dominated by amino acids in early spring and that growing roots should encounter the full range of organic and inorganic N forms in these soils. In contrast, soil water and KCl extracts were dominated by NH4+. Some amino acids displayed rapidly declining FMD (<1 h) possibly due to the rapid formation of a depletion zone near the membrane surface but the FMD of most amino acids remained high and unchanged over extended periods of dialysis indicating that these soils provide a continuous supply of amino acids for root uptake. Forest fertilization with NH4NO3 led to a significant increase in FMD of NO3− and NH4+, with FMD of NH4+ but not of NO3− remaining high for prolonged time. A separate trial with addition of NO3− showed a significantly slower decline of FMD in soils of previously fertilized forests compared to unfertilized forests, suggesting biological immobilization being a major cause of rapid decline of nitrate fluxes. Our results corroborate earlier studies suggesting amino acids to be a significant fraction of plant available N in boreal forests. They also suggest that, besides inorganic N, roots may encounter a wide spectrum of amino acids after intercepting new soil microsites and that most, but not all, amino acids may be constantly replenished at the root surface. Further, from our results we conclude that detailed insights into in-situ N dynamics of soils can be gained through microdialysis. [ABSTRACT FROM AUTHOR]

Plant biomass production and species composition is largely regulated by the availability of soil nitrogen (N). Detailed knowledge about the concentrations and composition of soil N pools are crucial for better understanding plant N nutrition. One commonly applied method to characterize soil N pools is the extraction of soil with water or salt solutions. The apparent problem with this sampling technique is the disruption of the soil matrix and the natural equilibrium of soil N pools during sampling and sample handling. Sieving and homogenizing soils as well as the extraction procedure itself are known to alter soil N composition through transformations, losses and contamination. Until now, however, information on the impact of soil extraction on individual N forms is scarce. This study therefore aimed at estimating the effect of sieving and extraction with water or KCl on NH4+, NO3− and individual amino acids. Nine different soils including boreal forest, agricultural and grassland soils were used for a series of experiments. In detail, after initial estimation of inorganic N and amino acid pools in extracts, in two separate experiments a small amount of standard solution containing NH4+, NO3− and amino acids was added either directly to the extractant or to the soils before sieving and extraction and, subsequently, the recovery of standard added was determined. I found that a significant proportion of amino acids were not recovered in any of the treatments and, conversely, there was a significant increase of inorganic N. Sieving and extracting generally led to a lower recovery of amino acids and a stronger increase of inorganic N than extraction only. The recovery of individual N forms strongly depended on soil type, extractant and N form, indicating that a comparison of results from soil extractions between different soils should be done with care. Still, soil extraction can provide valuable information on total plant-available N, as the sum of N added could largely be recovered in all soils and treatments. Future studies investigating the availability of individual N forms in soil for plant uptake should be aware of possible errors introduced during sample handling to avoid misinterpretation. [ABSTRACT FROM AUTHOR]

Plant nutrient uptake from soil is mainly governed by diffusion and transpirationally induced mass flow, but the current methods for assessing the relative importance of these processes are indirect., We developed a microdialysis method using solutions of different osmotic potentials as perfusates to simulate diffusion and mass flow processes, and assessed how induced mass flow affected fluxes of nitrogen (N) compounds in solution and in boreal forest soil., Varying the osmotic potential of perfusates induced vertical fluxes in the direction of the dialysis membranes at rates of between 1 × 10−8 and 3 × 10−7 m s−1, thus covering the estimated range of water velocities perpendicular to root surfaces and induced by transpiration., Mass flow increased N fluxes in solution but even more so in soil. This effect was explained by an indirect effect of mass flow on rates of diffusive fluxes, possibly caused by the formation of steeper gradients in concentrations of N compounds from membrane surfaces out in the soil. Our results suggest that transpiration may be an essential driver of plant N acquisition. [ABSTRACT FROM AUTHOR]

Abstract: One major challenge in agriculture is improving the nitrogen (N) use efficiency of crop plants and at the same time reducing the losses of fertilizer N to the environment. The use of 15N tracer studies in combination with process-based models has been proven to be a powerful tool for increasing our understanding of the dynamic interactions between soil, microbes and plants. Here we present a novel approach that includes plant uptake of fertilizer NH4 + and NO3 −. We developed, evaluated and applied an analytical model allowing the simultaneous estimation of 14 processes within the N cycle using results from a previously published 15N tracer study (Inselsbacher, E., Hinko-Najera Umana, N., Stange, F.C., Gorfer, M., Schüller, E., Ripka, K., Zechmeister-Boltenstern, S., Hood-Novotny, R., Strauss, J., Wanek, W., 2010. Short-term competition between crop plants and soil microbes for inorganic N fertilizer. Soil Biology & Biochemistry 42, 360–372]. The model revealed that plant NO3 − uptake governed the overall N cycle during the 8-days greenhouse study. Nitrification was the main fate of NH4 + but its kinetics differed significantly between soils. The model-based calculations proved to be a major advancement compared to the commonly used calculations based on the pool dilution technique, due to the number of estimated parameters, their respective kinetic shifts over prolonged time periods and their explanatory power. In future 15N tracer studies this analytical tool will allow accounting for the effect of plant N uptake on soil N transformations. [Copyright &y& Elsevier]

Nitrogen (N) availability has a major impact on a wide range of biogeochemical processes in terrestrial ecosystems. Changes in N availability modify the capacity of plants to sequester carbon (C), but despite the crucial importance for our understanding of terrestrial ecosystems, the relative contribution of different N forms to plant N nutrition in the field is not known. Until now, reliably assessing the highly dynamic pool of plant-available N in soil microsites was virtually impossible, because of the lack of adequate sampling techniques., For the first time we have applied a novel microdialysis technique for disturbance-free monitoring of diffusive fluxes of inorganic and organic N in 15 contrasting boreal forest soils in situ., We found that amino acids accounted for 80% of the soil N supply, while ammonium and nitrate contributed only 10% each. In contrast to common soil extractions, microdialysis revealed that the majority of amino acids are available for plant and mycorrhizal uptake., Our results suggest that the N supply of boreal forest soils is dominated by organic N as a major component of plant-available N and thus as a regulator of growth and C sequestration. [ABSTRACT FROM AUTHOR]

Background and aims: Soil temperature influences nitrogen (N) diffusion in soil but until now, such effects have been difficult to quantify. This study aimed at estimating the effect of temperature on the diffusive fluxes of plant-available N forms in two contrasting soils. Methods: Using a novel technique based on micro-dialysis, we established relationships between diffusive fluxes and temperature in aqueous solutions and in soil samples. Results: Averaged for all compounds, the decreases of diffusive fluxes from the soil to the microdialysis sampler were 3.8 and 4.7% per degree Celsius in an agricultural and a boreal forest soil, respectively. The temperature-related shift of diffusive flux was, however, significantly dependent on molecular weight of the N compound. In accordance with established functions for temperature effects on diffusive fluxes, the non-linearity of this relationship resulted in a greater temperature response for small N compounds compared to larger compounds. Conclusions: Our results show that, all other factors being equal, the relative contribution of smaller N compounds to the diffusive flux of total plant-available N increases with increasing soil temperatures. [ABSTRACT FROM AUTHOR]

The application of inorganic nitrogen (N) fertilizers strongly influences the contribution of agriculture to the greenhouse effect, especially by potentially increasing emissions of nitrous oxide (NO), carbon dioxide (CO) and methane (CH) from soils. The present microcosm-study investigates the effect of different forms of inorganic N fertilizers on greenhouse gas (GHG) emissions from two different agricultural soils. The relationship between greenhouse gas emissions and soil microbial communities, N transformation rates and plant ( Hordeum vulgare L. cv. Morex) growth were investigated. Repeated N fertilization led to increased NO emissions. In a parallel survey of functional microbial population dynamics we observed a stimulation of bacterial and archaeal ammonia oxidisers accompanied with these NO emissions. The ratio of archaeal to bacterial ammonium monooxygenase subunit A ( amoA) gene copies (data obtained from Inselsbacher et al., 2010) correlated positively with NO fluxes, which suggests a direct or indirect involvement of archaea in NO fluxes. Repeated N fertilization also stimulated methane oxidation, which may also be related to a stimulation of ammonia oxidizers. The fertilizer effects differed between soil types: In the more organic Niederschleinz soil N-turnover rates increased more strongly after fertilization, while in the sandy Purkersdorf soil plant growth and soil respiration were accelerated depending on fertilizer N type. Compared to addition of NH and NO, addition of NHNO fertilizer resulted in the largest increase in global warming potential as a summary indicator of all GHG related effects. This effect resulted from the strongest increase of both NO and CO emission while plant growth was not equally stimulated, compared to e.g. KNO fertilization. In order to decrease N losses from agricultural ecosystems and in order to minimize soil derived global warming potential, this study points to the need for interdisciplinary investigations of the highly complex interactions within plant-soil-microbe-atmosphere systems. By understanding the microbial processes underlying fertilizer effects on GHG emissions the N use efficiency of crops could be refined. [ABSTRACT FROM AUTHOR]

Abstract: Plant nitrogen (N) acquisition is strongly controlled by the concentration of available inorganic and organic N in the soil solution and by biogeochemical processes in the rhizosphere. However, until now it was hardly possible to reliably estimate plant-available N in soil microsites. Here, a novel microdialysis approach based on passive diffusion sampling is presented and compared qualitatively and quantitatively with lysimeter and soil extraction techniques when analyzing two contrasting boreal soils. Further, preliminary dialysis membrane calibration issues for sampling plant-available N compounds are discussed. Due to its miniaturized design microdialysis was shown to be a suitable tool for continuous sampling of ammonium, nitrate and free amino acids from the soil solution with only minimal disturbance of the soil structure. Microdialysis proved to be outstanding regarding the possible spatial (<0.5mm) and temporal (<30min) resolution of soil solution N chemistry. The different methods for soil N sampling resulted in significantly different results. In lysimeter and soil extraction samples, nitrate and ammonium were found at the highest concentrations, while results from microdialysis revealed that the pool of plant-available amino acids was contributing most to the total N pool tested. Application of a standard N solution to the tested soils led to an immediate peak of recovery via the microdialysis probes followed by a rapid decrease due to the formation of a depletion zone at the probe surfaces. Therefore, this relatively new technique will not only provide essential data on diffusion rates of a variety of N compounds in the soil but might be used for monitoring quantitative and qualitative changes in plant-available N in soil microsites such as the rhizosphere. [Copyright &y& Elsevier]

Abstract: Soil respiration is the largest terrestrial source of CO2 to the atmosphere. In forests, roughly half of the soil respiration is autotrophic (mainly root respiration) while the remainder is heterotrophic, originating from decomposition of soil organic matter. Decomposition is an important process for cycling of nutrients in forest ecosystems. Hence, tree species induced changes may have a great impact on atmospheric CO2 concentrations. Since studies on the combined effects of beech–spruce mixtures are very rare, we firstly measured CO2 emission rates in three adjacent stands of pure spruce (Picea abies), mixed spruce–beech and pure beech (Fagus sylvatica) on three base-rich sites (Flysch) and three base-poor sites (Molasse; yielding a total of 18 stands) during two summer periods using the closed chamber method. CO2 emissions were higher on the well-aerated sandy soils on Molasse than on the clayey soils on Flysch, characterized by frequent water logging. Mean CO2 effluxes increased from spruce (41) over the mixed (55) to the beech (59) stands on Molasse, while tree species effects were lower on Flysch (30–35, mixed > beech = spruce; all data in mg CO2–C m−2 h−1). Secondly, we studied decomposition after fourfold litter manipulations at the 6 mixed species stands: the Oi – and Oe horizons were removed and replaced by additions of beech –, spruce – and mixed litter of the adjacent pure stands of known chemical quality and one zero addition (blank) in open rings (20 cm inner diameter), which were covered with meshes to exclude fresh litter fall. Mass loss within two years amounted to 61–68% on Flysch and 36–44% on Molasse, indicating non-additive mixed species effects (mixed litter showed highest mass loss). However, base cation release showed a linear response, increasing from the spruce – over the mixed – to the beech litter. The differences in N release (immobilization) resulted in a characteristic converging trend in C/N ratios for all litter compositions on both bedrocks during decomposition. In the summers 2006 and 2007 we measured CO2 efflux from these manipulated areas (a closed chamber fits exactly over such a ring) as field indicator of the microbial activity. Net fluxes (subtracting the so-called blank values) are considered an indicator of litter induced changes only and increased on both bedrocks from the spruce – over the mixed – to the beech litter. According to these measurements, decomposing litter contributed between 22–32% (Flysch) and 11–28% (Molasse) to total soil respiration, strengthening its role within the global carbon cycle. [Copyright &y& Elsevier]

Abstract: Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH4 +), nitrate (NO3 −) or a combination thereof are expected to differ in soil N transformation rates and fates of NH4 + and NO3 −. Using 15N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH4 +, NO3 − or NH4NO3. Within each fertilizer treatment traces of 15NH4 + and 15NO3 − were added separately. During 8 days of fertilization the fate of fertilizer 15N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45–80% of initially applied 15N was recovered in crop plants compared to only 1–10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO3 − than NH4 + independent of initially applied N form. Surprisingly, no inhibitory effect of NH4 + on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH4 + uptake rates, gross nitrification rates were 3–75-fold higher, indicating that nitrifiers were the strongest competitors for NH4 + in both soils. The rapid conversion of NH4 + to NO3 − and preferential use of NO3 − by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO3 − and shift towards enhanced reliance on NO3 − for their N supply. [Copyright &y& Elsevier]

Globally, peatlands have been recognized as important carbon sinks while only covering approximately 3% of the earth's land surface. Root exudates are known key drivers of C cycling in soils and rhizosphere priming effects have been studied extensively in terrestrial ecosystems. Their role for decomposition of peat still remains unclear, as little research about their fate and potential priming effects in peat exists. In this study, we aimed to evaluate pathways of root exudates and their short-term priming effects by daily determination of stable carbon isotope fluxes of C O 2 and C H 4 . As the drainage of peatlands strongly alters processes of decomposition, we included measurements after drainage as well. Results revealed the immediate respiration of root exudates in peat, mainly as C O 2 , while C H 4 release was associated with a lag time of several days. However, the largest proportion of added root exudates remained in the solid and liquid phase of peat. In conclusion, our findings suggest that no priming occurred as added substrates remained immobile in peat. [ABSTRACT FROM AUTHOR]

Intensive agricultural crop production largely depends on the input of nitrogen (N) fertilizers, at the cost of environmental integrity. Nitrogen pollution derived from inefficient use of fertilizers by agricultural crops is ranked as a foremost global concern. Synchronizing soil N supply and crop N demand addresses this problem, but assessing how much and in which form N arrives at the root surfaces remains a major challenge. Microdialysis can be used to accurately quantify in situ N fluxes, but is of limited suitability for studying field-scale N dynamics due to its miniature design. The objective of this study was to test the possibility of upscaling results from small-scale microdialysis sampling by using soil N concentrations estimated by soil water extractions. We estimated diffusive fluxes and concentrations of ammonium, nitrate and amino acids in an agricultural field used for corn production. Results from soil extraction and microdialysis differed significantly regarding the relative contribution of each N form to total N. Nitrate was the dominant N form (~80%) in water extracts while diffusive fluxes of nitrate, ammonium and amino acids were similar (38, 34 and 28%, respectively). Surprisingly, diffusive N fluxes were decoupled from N concentrations, i.e. no clear correlation between these two parameters could be found. We propose that other soil physical and biological factors have a stronger influence on diffusive N fluxes than N concentrations alone. Future efforts should be directed into including such factors in more complex modelling approaches in order to assess soil N supply at field-scales. [ABSTRACT FROM AUTHOR]

Soils are the foundation of agricultural production, ecosystem functioning and human well‐being. Bridging soil knowledge gaps and improving the knowledge system is crucial to meet the growing EU soil policy ambitions in the face of climate change and the ongoing trend in soil degradation. The objective of this article is to assess the current state of knowledge, knowledge use and knowledge gaps concerning sustainable soil management in Europe. This study is based on interviews with 791 stakeholders and 254 researchers and on a comprehensive review of >1800 documents carried out under the European Joint Programme on agricultural soils. Despite differences in stakeholder groups, the conclusions are rather consistent and complementary. We identified major knowledge gaps with respect to (1) soil carbon stocks, (2) soil degradation and fertility and (3) strategies for improved soil management. Transcending these three areas, particularly the loss of soil organic carbon, peatland degradation and soil compaction, are most critical, thus, we stress the urgency of developing more models and monitoring programmes on soils. Stakeholders further report that insufficient transfer of existing soil research findings to practitioners is a hindrance to the adoption of sustainable soil management practices. In addition to knowledge production, soil knowledge gaps may be addressed by considering seven recommendations from the stakeholders: (1) raising awareness, (2) strengthening knowledge brokers, (3) improving relevance of research activities and resource allocation for land users, (4) peer‐to‐peer communication, (5) targeting advice and information, (6) improving knowledge access, and (7) providing incentives. We argue that filling and bridging knowledge gaps should be a priority for policymakers and the insights provided in the article may help prioritise research and dissemination needs enabling a transition to more sustainable soil management in Europe. [ABSTRACT FROM AUTHOR]

Nitrogen (N) availability to plants in dry soil is limited by diffusive flux of N compounds through the soil solution towards the root surface. Conventional soil extraction procedures only provide information about bulk soil N concentrations, which can be distorted during soil sampling, transport, storage and extraction, and hence are of limited use to detect short-term N dynamics. Soil microdialysis is a new tool to monitor diffusive flux of mineral and organic N compounds in situ in high temporal and spatial resolution with minimal disturbance, and is therefore well-suited to determine dynamic fractions of plant-available N in soil microsites. We investigated N availability and mobilization during a drying–rewetting event in a temperate beech forest using soil microdialysis and soil extractions with water. While water extracts mainly revealed mineral N in the form of NH 4 + and NO 3 − , diffusive N fluxes in situ were dominated by amino acids. Microdialysis showed that rewetting of dry soil led to a fast but short-lived mobilization of NO 3 − and some neutral hydrophilic amino acids (lysine, glutamine, cysteine, glycine), which was not detected in water extracts, and the rewetting N flush was larger with increasing drought duration. Our results suggest that at our temperate forest site plant-available N was dominated by amino acids, a fraction of N that might be missed using conventional soil extraction methods. Considering expected increases in the frequency of extreme climatic events, the observed release of mobile N forms bears the potential of N loss from soil if severe drought is followed by a heavy rain event. [ABSTRACT FROM AUTHOR]

Increasing global temperatures have been reported to accelerate soil carbon (C) cycling, but also to promote nitrogen (N) and phosphorus (P) dynamics in terrestrial ecosystems. However, warming can differentially affect ecosystem C, N and P dynamics, potentially intensifying elemental imbalances between soil resources, plants and soil microorganisms. Here, we investigated the effect of long‐term soil warming on microbial resource limitation, based on measurements of microbial growth (18O incorporation into DNA) and respiration after C, N and P amendments. Soil samples were taken from two soil depths (0–10, 10–20 cm) in control and warmed (>14 years warming, +4°C) plots in the Achenkirch soil warming experiment. Soils were amended with combinations of glucose‐C, inorganic/organic N and inorganic/organic P in a full factorial design, followed by incubation at their respective mean field temperatures for 24 h. Soil microbes were generally C‐limited, exhibiting 1.8‐fold to 8.8‐fold increases in microbial growth upon C addition. Warming consistently caused soil microorganisms to shift from being predominately C limited to become C‐P co‐limited. This P limitation possibly was due to increased abiotic P immobilization in warmed soils. Microbes further showed stronger growth stimulation under combined glucose and inorganic nutrient amendments compared to organic nutrient additions. This may be related to a prolonged lag phase in organic N (glucosamine) mineralization and utilization compared to glucose. Soil respiration strongly positively responded to all kinds of glucose‐C amendments, while responses of microbial growth were less pronounced in many of these treatments. This highlights that respiration–though easy and cheap to measure—is not a good substitute of growth when assessing microbial element limitation. Overall, we demonstrate a significant shift in microbial element limitation in warmed soils, from C to C‐P co‐limitation, with strong repercussions on the linkage between soil C, N and P cycles under long‐term warming. [ABSTRACT FROM AUTHOR]

Plant nutrient acquisition largely depends on the availability and constant replenishment of nutrients at root surfaces. The main process providing nutrients for root uptake is diffusion which is critically affected by soil physicochemical and biological factors. The effective diffusion path length increases with decreasing soil water content thus limiting diffusive nutrient supply in dry soils. Microdialysis, an increasingly popular sampling tool for monitoring nutrient fluxes in soil in situ , could be used to measure such a limitation provided it operates reliably under dry soil conditions. However, due to its mode of operation, the performance of microdialysis could be impeded in dry soils since a significant amount of perfusate might be lost via leaching to the surrounding soil. Until now, however, no data is available on the applicability of microdialysis in dry soils. In this project, we estimated leaching losses of perfusate to eight soils with different texture maintained at 10, 20, 30, 50 and 80% (w/w) of maximum water-holding capacity in dependence of perfusion flow rate. We found that microdialysis operates reliably even in dry soils when using a high perfusion flow rate (10 μl min−1). At lower perfusion flow rates (1 or 5 μl min−1), however, significant amounts of perfusate were leaching out to soils. Diffusive fluxes of nitrate and ammonium were strongly affected by soil water content, but not fluxes of phosphate. Even at the highest flow rate (where no losses of perfusate where observed) concentrations of both inorganic N forms in dialysates decreased with decreasing soil water content. Overall, we conclude that microdialysis can be applied in dry soils without high risk of perfusate leaching when a high perfusion flow rate (10 μl min−1) is used. Further, our results highlight that microdialysis can be used in much dryer soils than previously suggested (70% MWHC) and allows monitoring the effect of soil water content on nutrient supply for root uptake via diffusion. • Microdialysis operates reliably in dry soils at high perfusion flow rates. • Perfusate leaching depends on perfusion flow rate rather than soil water content. • Effect of soil water content on nutrient diffusion depends on nutrient mobility. • Change in soil water content when using microdialysis must be regarded. [ABSTRACT FROM AUTHOR]

The central role that ectomycorrhizal ( EM) symbioses play in the structure and function of boreal forests pivots around the common assumption that carbon (C) and nitrogen (N) are exchanged at rates favorable for plant growth. However, this may not always be the case. It has been hypothesized that the benefits mycorrhizal fungi convey to their host plants strongly depends upon the availability of C and N, both of which are rapidly changing as a result of intensified human land use and climate change. Using large-scale shading and N addition treatments, we assessed the independent and interactive effects of changes in C and N supply on the transfer of N in intact EM associations with ~15 yr. old Scots pine trees. To assess the dynamics of N transfer in EM symbioses, we added trace amounts of highly enriched 15 NO3- label to the EM-dominated mor-layer and followed the fate of the 15N label in tree foliage, fungal chitin on EM root tips, and EM sporocarps. Despite no change in leaf biomass, shading resulted in reduced tree C uptake, ca. 40% lower fungal biomass on EM root tips, and greater 15N label in tree foliage compared to unshaded control plots, where more 15N label was found in fungal biomass on EM colonized root tips. Short-term addition of N shifted the incorporation of 15N label from EM fungi to tree foliage, despite no significant changes in below-ground tree C allocation to EM fungi. Contrary to the common assumption that C and N are exchanged at rates favorable for plant growth, our results show for the first time that under N-limited conditions greater C allocation to EM fungi in the field results in reduced, not increased, N transfer to host trees. Moreover, given the ubiquitous nature of mycorrhizal symbioses, our results stress the need to incorporate mycorrhizal dynamics into process-based ecosystem models to better predict forest C and N cycles in light of global climate change. [ABSTRACT FROM AUTHOR]

The plant-parasitic cyst nematode Heterodera schachtii induces syncytial feeding structures in the roots of host plants. These syncytia provide all required nutrients, water and solutes to the parasites. Previous studies on the composition of primary metabolites in syncytia revealed significantly increased amino acid levels. However, mainly due to technical limitations, little is known about the role of arginine in plant-nematode interactions. This free amino acid plays a central role in the plant primary metabolism and serves as substrate for metabolites involved in plant stress responses. Thus, in the present work, expression of genes coding for the enzymes of arginine metabolism were studied in nematode-induced syncytia compared to non-infected control roots of Arabidopsis thaliana. Further, amiRNA lines were constructed and T-DNA lines were isolated to test their effects on nematode development. While the silencing of genes involved in arginine synthesis increased nematode development, most T-DNA lines did not show any significant difference from the wild type. Amino acid analyses of syncytia showed that they accumulate high arginine levels. In addition, manipulating arginine cycling had a global effect on the local amino acid composition in syncytia as well as on the systemic amino acid levels in roots and shoots. [ABSTRACT FROM AUTHOR]

Studying plant-aphid interactions is challenging as aphid feeding is a complex process hidden in the plant tissue. Here we propose a combination of two well established methods to study nutrient acquisition by aphids focusing on the uptake of isotopically labelled nitrogen (15N). We combined the Electrical Penetration Graph (EPG) technique that allows detailed recording of aphid feeding behaviour and stable isotope ratio mass spectrometry (IRMS) to precisely measure the uptake of nitrogen. Bird cherry-oat aphids Rhopalosiphum padi L. (Hemiptera, Aphididae) fed for 24 h on barley plants (Hordeum vulgare L., cultivar Lina, Poaceae) that were cultivated with a 15N enriched nutrient solution. The time aphids fed in the phloem was strongly positive correlated with their 15N uptake. All other single behavioural phases were not correlated with 15N enrichment in the aphids, which corroborates their classification as non-feeding EPG phases. In addition, phloem-feeding and 15N enrichment of aphids was divided into two groups. One group spent only short time in the phloem phase and was unsuccessful in nitrogen acquisition, while the other group displayed longer phloem-feeding phases and was successful in nitrogen acquisition. This suggests that several factors such as the right feeding site, time span of feeding and individual conditions play a role for the aphids to acquire nutrients successfully. The power of this combination of methods for studying plant-aphid interactions is discussed. [ABSTRACT FROM AUTHOR]

Oxidation of ammonia by nitrifying microorganisms is a major pathway that fertilizer nitrogen (N) may take upon application to agricultural soils, but the relative roles of bacterial (AOB) vs. archaeal (AOA) ammonia oxidizers are controversial. We explored the effects of various forms of mineral N fertilizer on the AOB and AOA community dynamics in two different soils planted with barley. Ammonia oxidizers were monitored via real-time PCR and terminal restriction fragment length polymorphism analysis of bacterial and archaeal amoA genes following the addition of either [NH]SO, NHNO or KNO. AOB and AOA communities were also studied specifically in the rhizospheres of two different barley varieties upon [NH]SO vs. KNO addition. AOB changed in community composition and increased in abundance upon ammonium amendment in bulk soil and rhizosphere, with changes in bacterial amoA copy numbers lagging behind relative to changes in soil ammonium. In both soils, only T-RFs corresponding to phylotypes related to Nitrosospira clade 3a underwent significant community changes. Increases in AOB abundance were generally stronger in the bulk soil than in the rhizosphere, implying significant ammonia uptake by plant roots. AOA underwent shifts in the community composition over time and fluctuated in abundance in all treatments irrespective of ammonia availability. AOB were thus considered as the main agents responsible for fertilizer ammonium oxidation, while the functions of AOA in soil N cycling remain unresolved. [ABSTRACT FROM AUTHOR]

Background and aim: The diffusion of inorganic nitrogen (N) in soils is strongly affected by their physical characteristics and hydraulic gradients, which enable them to store and transport nutrients over time and space. The objective of this research was to use the microdialysis technique to continuously monitor the diffusive fluxes of inorganic N from the physical N store towards root surfaces in two volcanic soils after soil rewetting. Methods: Undisturbed soil samples were collected in metallic cylinders (230 cm3) at four depths (0–20, 20–40, 40–70, 70–100 cm) in two distinct volcanic soils (Andisol and Ultisol) that are used in agriculture. Diffusive N fluxes were determined during continuous drying and rewetting cycles (0 to 100 kPa) of intact undisturbed soil cores. Soil water content (SWC) at each water potential was calculated. Results and Conclusions: Diffusive NO3- fluxes increased significantly (p < 0.001) after rewetting in both soils: 16‐fold in the Andisol, from 32.7 ± 12.1 to 541.5 ± 306.5 nmol N cm−2 h−1 (34 to 62% SWC) and 66‐fold in the Ultisol, from 65.6 ± 25.8 to 4298.7 ± 1289.9 nmol N cm−2 h−1 (31 to 58% SWC) in the upper 20 cm of the soils. In addition, diffusive NO3- fluxes decreased with depth in both soils. We found that the residence time of available NO3- in these soils strongly depended upon actual SWC and thereby the functionality of the pores in the system. We propose that the ability of the porous system to transport water throughout the soil profile is a relevant parameter within N diffusion, since it allows us to estimate the increase or decrease of diffusive NO3- fluxes as a function of the soil water content during continuous drying and rewetting cycles. [ABSTRACT FROM AUTHOR]

Root exudation increases the concentration of readily available carbon (C) compounds in its immediate environment. This creates 'hotspots' of microbial activity characterized by accelerated soil organic matter turnover with direct implications for nutrient availability for plants. However, our knowledge of the microbial metabolic processes occurring in the immediate vicinity of roots during and after a root exudation event is still limited. Using reverse microdialysis, we simulated root exudation by releasing a13C-labelled mix of low-molecular-weight organic C compounds at mm-sized locations in undisturbed soil. Combined with stable isotope tracing, we investigated the fine-scale temporal and spatial response of microbial metabolism, soil chemistry, and traced microbial respiration and uptake of exuded compounds. Our results show that a 9-h simulated root exudation pulse leads to i) a large local respiration event and ii) alteration of the temporal dynamics of soil metabolites over the following 12 day at the exudation spot. Notably, we observed a threefold increase in ammonium concentrations at 12 h and increased nitrate concentrations five days after the pulse. Moreover, various short-chain fatty acids (acetate, propionate, formate) increased over the following days, indicating altered microbial metabolic pathways and activity. Phospholipid and neutral lipid fatty acids (PLFAs, NLFAs) of all major microbial groups were significantly 13C-enriched within a 5 mm radius around the microdialysis probes, but not beyond. The highest relative 13C enrichment was observed in fungal NLFAs, indicating that a significant proportion of the exuded compounds had been incorporated into fungal storage compounds. Our findings indicate that the punctual release of low-molecular-weight organic C compounds into intact soil significantly changes microbial metabolism and activity in its immediate surroundings, enhancing mineralization of native organic nitrogen. This highlights the versatility of microbial metabolic pathways in response to rapidly changing C availability and their effectiveness in increasing nutrient availability near plant roots. • We simulated passive root exudation (sugar, organic acid) by reverse microdialysis. • Small-chain fatty acids and NH 4 + were produced in response to local substrate input. • Increased SOM mineralization occurred after initial substrate respiration. • In response to substrate input temporal dynamics of soil metabolites were altered. • Root exudation might have caused local oxygen depletion and anaerobe metabolism. [ABSTRACT FROM AUTHOR]

• Addition of labile and N-rich Betula litter does not enhance decomposition of other litter types. • Presence of Sphagnum in litter mixtures suppresses enzymatic activities. • k-means clustering is a useful tool to detect patterns of decomposition for different litter types and mixtures over time. Peat accumulation is the result of an imbalance between the biomass production and the reduced decomposition of organic matter, which has allowed peatlands to accumulate high levels of carbon over time. Sphagnum -dominated peatlands are among the most threatened ecosystems due to increasing anthropogenic pressures and harsher environmental conditions caused by climate change. The resulting changes in vegetation alter litter interactions and change decomposition patterns by altering nutrient cycling, hydrological conditions and carbon sequestration capacity of bogs. The aim of this study was to identify decomposition patterns for different bog litter types and mixtures over time. We sampled litter from an ombrotrophic bog (Sphagnum , Betula , Calluna), prepared mixtures of litter types and incubated our samples under laboratory conditions. We evaluated decomposition proxies and used k-means clustering to detect the formation of litter- and mixture-specific decomposition patterns over time. Sphagnum litter had a consistently low decomposition and its presence in mixtures reduced enzymatic activities. Initially, Betula litter added high amounts of N and labile C compounds to leachates. k-means clustering revealed a typical initial decomposition pattern (i.e. lowest decomposition directly after starting the incubation, followed by highest decomposition rates, and a steady decrease until the end) for most litter types, except for Sphagnum. Litter-specific decomposition patterns emerged after 14 days. Betula litter did not enhance decomposition of other litter types in our short-term incubation, but additional nutrient inputs could alter peat-litter interactions in the long term, increasing the risk of losing the carbon sink function of nutrient-poor bogs. [ABSTRACT FROM AUTHOR]

Global warming impacts biogeochemical cycles in terrestrial ecosystems, but it is still unclear how the simultaneous cycling of carbon (C) and nitrogen (N) in soils could be affected in the longer-term. Here, we evaluated how 14 years of soil warming (+4 °C) affected the soil C and N cycle across different soil depths and seasons in a temperate mountain forest. We used H 2 18O incorporation into DNA and 15N isotope pool dilution techniques to determine gross rates of C and N transformation processes. Our data showed different warming effects on soil C and N cycling, and these were consistent across soil depths and seasons. Warming decreased microbial biomass C (−22%), but at the same time increased microbial biomass-specific growth (+25%) and respiration (+39%), the potential activity of β-glucosidase (+31%), and microbial turnover (+14%). Warming reduced gross rates of protein depolymerization (−19%), but stimulated gross N mineralization (+63%) and the potential activities of N-acetylglucosaminidase (+106%) and leucine-aminopeptidase (+46%), and had no impact on gross nitrification (+1%). Microbial C and N use efficiencies were both lower in the warming treatment (−15% and −17%, respectively). Overall, our results suggest that long-term warming drives soil microbes to incorporate less C and N into their biomass (and necromass), and to release more inorganic C and N to the environment, causing lower soil C and N storage in this forest, as indicated by lower soil C and total N contents. The decreases in microbial CUE and NUE were likely triggered by increasing microbial P constraints in warmed soils, limiting anabolic processes and microbial growth and promoting pervasive losses of C and N from the soil. • Both soil carbon (C) and nitrogen (N) contents decreased after long-term warming. • Lower microbial C and N use efficiencies under warming imply higher C and N losses. • Long-term soil warming had diverse effects on different soil C and N processes. • Long-term soil warming did not cause C–N decoupling. [ABSTRACT FROM AUTHOR]

Although organic nitrogen (N) compounds are ubiquitous in soil solutions, their potential role in plant N nutrition has been questioned. We performed a range of experiments on Arabidopsis thaliana genetically modified to enhance or reduce root uptake of amino acids. Plants lacking expression of the Lysine Histidine Transporter 1 (LHT1) displayed significantly lower contents of 13C and 15N label and of U-13C5,15N2 L-glutamine, as determined by liquid chromatography-mass spectrometry when growing in pots and supplied with dually labelled L-glutamine compared to wild type plants and LHT1-overexpressing plants. Slopes of regressions between accumulation of 13C-labelled carbon and 15N-labelled N were higher for LHT1-overexpressing plants than wild type plants, while plants lacking expression of LHT1 did not display a significant regression between the two isotopes. Uptake of labelled organic N from soil tallied with that of labelled ammonium for wild type plants and LHT1-overexpressing plants but was significantly lower for plants lacking expression of LHT1. When grown on agricultural soil plants lacking expression of LHT1 had the lowest, and plants overexpressing LHT1 the highest C/N ratios and natural δ15N abundance suggesting their dependence on different N pools. Our data show that LHT1 expression is crucial for plant uptake of organic N from soil. [ABSTRACT FROM AUTHOR]

Background and aims: In nutrient poor environments, plant nitrogen (N) acquisition is governed by soil diffusive fluxes and root uptake capacities. However, the relationship between these two processes is not well understood. We explored a way of comparing the processes, enabling identification of the limiting factor for tree N acquisition. Methods: The study comprised N-fertilized and N-limited Scots pine stands, and measurements of uptake capacities of detached tree roots and of induced soil diffusive fluxes (through in-situ microdialysis) done at the onset and the end of the growing season. Results: Soil N fluxes were higher at the onset than at the end of the growing season and amino acids comprised a larger fraction of N than inorganic N. N fertilization reduced root uptake capacities of NH, glycine and NO but not of arginine. For all N compounds except NO, diffusive fluxes were significantly lower than root N uptake capacities. Conclusions: Our results suggest that soil N supply in both, N-fertilized and N-limited forest stands, is dominated by amino acids, thus being the major component of plant-available N. Uptake of N appears more constrained by the diffusive fluxes of N compounds rather than root uptake capacity, except for NO. [ABSTRACT FROM AUTHOR]

Plant roots release a variety of low-molecular weight compounds, such as sugars, amino acids or organic acids into the soil, impacting microbial activities and physico-chemical soil processes in their surroundings. These compounds are a source of easily available Carbon (C) and energy for soil microbes, potentially accelerating microbial decomposition of soil organic matter in the immediate vicinity of roots. However, knowledge about processes in root exudation hotspots remains limited due to experimental difficulties in investigating such hotspots in soil. Microdialysis, a passive sampling technique based on diffusion, has been successfully used to collect soil solutes at small spatial scales. Reverse microdialysis, also termed retrodialysis, can be used to introduce solutes into the soil, mimicking passive root exudation. However, little is known about the dynamics of substances released by passive diffusion into intact soil, a crucial prerequisite for applying reverse microdialysis to study root exudation hotspots in undisturbed soils. Here, we used reverse microdialysis to investigate the spatial and temporal dynamics of thirteen different organic compounds passively introduced into two different intact soils. Diffusion of compounds into soils was substantially lower than into water, and was not – as in water – determined by molecular size. Interestingly, butyrate, oxalate and propionate showed the highest diffusive fluxes into soil combined with the lowest rate of back retrieval after input, indicating that they were quickly removed from the soil solution by biotic or abiotic processes. In contrast, glucose and fructose unexpectedly accumulated around the membrane after input without removal. Furthermore, diffusive fluxes of compounds into soils showed a fluctuating temporal pattern, which may be explained by an observed 2-h delay of microbial respiration of added 13C-labelled compounds. During the course of 12 days, approximately one third of 13C-labelled compounds introduced into soil was respired while 8% ended up in microbial biomass. Our results demonstrate that introducing compounds into intact soil triggers complex biotic and abiotic responses at the time scale of hours. Reverse microdialysis proved to be an excellent tool to investigate such responses as well as the dynamics and metabolic consequences of passively released compounds into intact soil, and – in combination with 13C labelled substrate and respiration measurements - to shed light on potential priming effects that may be triggered by them. • We evaluated reverse microdialysis as a tool to simulate passive root exudation. • Release of compounds differs between soils, and is slower into soil than into water. • Diffusion of compounds into soil show a fluctuating temporal pattern. • In contrast to organic acids, sugars accumulated around the membrane in the soil. • Within 11 days–40% of introduced C was respired, and ∼8% ended up in microbial biomass. [ABSTRACT FROM AUTHOR]

The central role that ectomycorrhizal (EM) symbioses play in the structure and function of boreal forests pivots around the common assumption that carbon (C) and nitrogen (N) are exchanged at rates favorable for plant growth. However, this may not always be the case. It has been hypothesized that the benefits mycorrhizal fungi convey to their host plants strongly depends upon the availability of C and N, both of which are rapidly changing as a result of intensified human land use and climate change. Using large‐scale shading and N addition treatments, we assessed the independent and interactive effects of changes in C and N supply on the transfer of N in intact EM associations with ~15 yr. old Scots pine trees. To assess the dynamics of N transfer in EM symbioses, we added trace amounts of highly enriched 15NO3‐ label to the EM‐dominated mor‐layer and followed the fate of the 15N label in tree foliage, fungal chitin on EM root tips, and EM sporocarps. Despite no change in leaf biomass, shading resulted in reduced tree C uptake, ca. 40 % lower fungal biomass on EM root tips, and greater 15N label in tree foliage compared to unshaded control plots, where more 15N label was found in fungal biomass on EM colonized root tips. Short‐term addition of N shifted the incorporation of 15N label from EM fungi to tree foliage, despite no significant changes in below‐ground tree C allocation to EM fungi. Contrary to the common assumption that C and N are exchanged at rates favorable for plant growth, our results show for the first time that under N‐limited conditions greater C allocation to EM fungi in the field results in reduced, not increased, N transfer to host trees. Moreover, given the ubiquitous nature of mycorrhizal symbioses, our results stress the need to incorporate mycorrhizal dynamics into process‐based ecosystem models to better predict forest C and N cycles in light of global climate change.This article is protected by copyright. All rights reserved. [ABSTRACT FROM AUTHOR]

• We investigated DOC production and losses in agro-managed temperate fens. • High seasonal fluctuation of DOC concentration in groundwater and soils. • Weather conditions are crucial determinants for DOC variability. • Open-pit mining did not affect short-term DOC production. Disturbance to peatlands via climate change and human activity affects their carbon storage potential. This leads to the loss of a significant part of their carbon stock as dissolved organic carbon (DOC) via fluvial pathways. The grassland fens in the Grójec Valley, Poland are dominated by organic soils, which degraded as a consequence of long-term agricultural use. Here, we assessed the seasonality of DOC production in these soils and its release to groundwater. To the best of our knowledge, this the first example of a seasonal DOC production assessment in agricultural fen soils that have also been affected by open-pit mining. All designated sites in the study area were under agricultural use (grasslands) and two of the sites were also located within predicted depression cones, caused by lignite mining operations. Samples were taken during the growing season (March–October) over three consecutive years 2017–2019. The observed DOC concentrations in the soils (1.00–4.99 g kg−1) and water (18.8–73.4 mg l−1) were similar to other studies conducted on agro-managed fens in central Europe. We found that the highest labile carbon concentrations in the soils and water occurred after dry/warm periods in the study area. However, the observed seasonal variability was especially evident in DOC concentrations in the groundwater, which indicates its dependence on weather conditions. Our preliminary assumption that DOC production and release would be greatest in sites located within predicted depression cones was not confirmed in this study. Overall, our results indicate that temperate grassland fens are endangered if future climate warming results in soil carbon losses via soil water deficiency and further acceleration of organic matter decomposition. [ABSTRACT FROM AUTHOR]

Although fungi contribute significantly to the microbial biomass in terrestrial ecosystems, little is known about their contribution to biogeochemical nitrogen cycles. Agricultural soils usually contain comparably high amounts of inorganic nitrogen, mainly in the form of nitrate. Many studies focused on bacterial and archaeal turnover of nitrate by nitrification, denitrification and assimilation, whereas the fungal role remained largely neglected. To enable research on the fungal contribution to the biogeochemical nitrogen cycle tools for monitoring the presence and expression of fungal assimilatory nitrate reductase genes were developed. To the ∼100 currently available fungal full-length gene sequences, another 109 partial sequences were added by amplification from individual culture isolates, representing all major orders occurring in agricultural soils. The extended database led to the discovery of new horizontal gene transfer events within the fungal kingdom. The newly developed PCR primers were used to study gene pools and gene expression of fungal nitrate reductases in agricultural soils. The availability of the extended database allowed affiliation of many sequences to known species, genera or families. Energy supply by a carbon source seems to be the major regulator of nitrate reductase gene expression for fungi in agricultural soils, which is in good agreement with the high energy demand of complete reduction of nitrate to ammonium. [ABSTRACT FROM AUTHOR]

Climate warming affects nitrogen (N) cycling in forest soils, but implications for plant available N have remained unclear. We estimated in situ diffusive fluxes of amino acids and inorganic N in a temperate forest soil after 14 years of soil warming. Results from four sampling campaigns (n = 1152 microdialysis samples) during the growing season showed no effect of warming on diffusive N fluxes. Diffusive NH 4 + fluxes increased from spring towards autumn while NO 3 − fluxes followed an opposite trend. Overall, the proportion of amino acids in the total diffusive N flux was low (13–30%) in this carbonate soil compared to other temperate and boreal forest soils. • Long-term soil warming had no effect on diffusive N fluxes. • Low proportion of amino acid N (<30% of total N flux) in carbonate forest soil. • Individual N fluxes strongly vary between seasons and soil microsites. [ABSTRACT FROM AUTHOR]

The input of labile C into soil is thought to accelerate microbial decomposition of soil organicmatter in the so-called 'rhizosphere priming effect&#x2019;. However, plant roots release labileorganic substances into spatially and temporally constrained volumes of soil, resulting in aheterogeneous distribution of spots with dramatically increased C concentrations in the soil.Subsequent microbial and chemical dynamics, which may lead to a priming effect, arelikely to take place in the immediate vicinity of these exudation spots. So far, welack an understanding of these small-scale dynamics occurring during a primingevent. The aim of this study was to investigate the fine-scale temporal dynamics ofmicrobial activity and soil chemistry in response to simulated root exudation atdistinct spots in undisturbed soil cores. Therefore, we used microdialysis, whichallows the collection or release of organic compounds via diffusion across a very finemembrane. We placed microdialysis membranes (10 mm length, 500 μm outerdiameter, 20 kDa molecular weight cut-off) into undisturbed soil cores collectedfrom a forest site and simulated a pulse of root exudation by releasing a mixture of13C-labelled labile substrates (glucose, fructose, acetate, and succinate) for 8 hours('reverse microdialysis&#x2019;), while simultaneously collecting metabolites from the soilsolution at an hour-scale time resolution. In the 12 days after the pulse, we continuedcollecting metabolites and measured 13C in soil respiration in regular intervals to assesssubstrate-induced respiration, soil organic matter (SOM) mineralization and primingeffects. The pulse of artificial root exudates reduced mineralization of native SOM in the firstweek of the experiment ("negative priming"). At the same time, acetate, formate, propionate,sulphate, nitrate and ammonium exhibited distinct temporal patterns, increasing significantlybetween 20 to 30 hours after the exudation pulse. Rising levels of acetate, formate andpropionate may have originated from anaerobic metabolism, suggesting oxygen depletion atthe microbial active sites. After one week, native SOM mineralization in soils thatreceived a 'root exudate pulse&#x2019; exceeded that of controls ("positive priming effect"),which persisted for the duration of the experiment. The organic acids, sulphate,nitrate and ammonium also displayed elevated concentrations at the end of theexperiment. The combination of isotopic labelling, 'reverse microdialysis&#x2019; and 13C respirationmeasurements allowed us to observe fine-scale temporal dynamics of soil metabolites inthe immediate vicinity of a simulated root exudation spot in an undisturbed soil,and to link these to the temporal dynamics of the rhizosphere priming effect. Ourresults provide new insights into possible mechanisms behind the priming effect. [ABSTRACT FROM AUTHOR]

Globally only ≈50% of applied nitrogen (N) fertilizer is captured by crops, and the remainder can cause pollution via runoff and gaseous emissions. Synchronizing soil N supply and crop demand will address this problem, however current soil analysis methods provide little insight into delivery and acquisition of N forms by roots. We used microdialysis, a novel technique for in situ quantification of soil nutrient fluxes, to measure N fluxes in sugarcane cropping soils receiving different fertilizer regimes, and compare these with N uptake capacities of sugarcane roots. We show that in fertilized sugarcane soils, fluxes of inorganic N exceed the uptake capacities of sugarcane roots by several orders of magnitude. Contrary, fluxes of organic N closely matched roots' uptake capacity. These results indicate root uptake capacity constrains plant acquisition of inorganic N. This mismatch between soil N supply and root N uptake capacity is a likely key driver for low N efficiency in the studied crop system. Our results also suggest that (i) the relative contribution of inorganic N for plant nutrition may be overestimated when relying on soil extracts as indicators for root-available N, and (ii) organic N may contribute more to crop N supply than is currently assumed. [ABSTRACT FROM AUTHOR]