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The article "Climate change mitigation through soil carbon sequestration in working lands: A reality check" questions the validity and realism of claims made by Almaraz et al. regarding the potential magnitude of soil carbon sequestration (SCS) in agricultural soils. The authors argue that Almaraz et al. overestimate the availability of organic matter for compost and biochar, inaccurately interpret carbon sequestration rates, and make premature assumptions about the applicability of enhanced rock weathering. They emphasize the need for realistic evaluations of the limitations and trade-offs of SCS practices and caution against overly optimistic estimates that can mislead policymakers. The authors declare no conflicts of interest. [Extracted from the article]

Microbial necromass is a central component of soil organic matter (SOM), whose management may be essential in mitigating atmospheric CO2 concentrations and climate change. Current consensus regards the magnitude of microbial necromass production to be heavily dependent on the carbon use efficiency of microorganisms, which is strongly influenced by the quality of the organic matter inputs these organisms feed on. However, recent concepts neglect agents relevant in many soils: earthworms. We argue that the activity of earthworms accelerates the formation of microbial necromass stabilized in aggregates and organo‐mineral associations and reduces the relevance of the quality of pre‐existing organic matter in this process. Earthworms achieve this through the creation of transient hotspots (casts) characterized by elevated contents of bioavailable substrate and the efficient build‐up and quick turnover of microbial biomass, thus converting SOM not mineralized in this process into a state more resistant against external disturbances, such as climate change. Promoting the abundance of earthworms may, therefore, be considered a central component of management strategies that aim to accelerate the formation of stabilized microbial necromass in wide locations of the soil commonly not considered hotspots of microbial SOM formation. [ABSTRACT FROM AUTHOR]

A number of negative emission technologies (NETs) have been proposed to actively remove CO2 from the atmosphere, with enhanced silicate weathering (ESW) as a relatively new NET with considerable climate change mitigation potential. Models calibrated to ESW rates in lab experiments estimate the global potential for inorganic carbon sequestration by ESW at about 0.5–5 Gt CO2 year−1, suggesting ESW could be an important component of the future NETs mix. In real soils, however, weathering rates may differ strongly from lab conditions. Research on natural weathering has shown that biota such as plants, microbes, and macro‐invertebrates can strongly affect weathering rates, but biotic effects were excluded from most ESW lab assessments. Moreover, ESW may alter soil organic carbon sequestration and greenhouse gas emissions by influencing physicochemical and biological processes, which holds the potential to perpetuate even larger negative emissions. Here, we argue that it is likely that the climate change mitigation effect of ESW will be governed by biological processes, emphasizing the need to put these processes on the agenda of this emerging research field. [ABSTRACT FROM AUTHOR]

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Minimizing nitrogen (N) losses and increasing plant N uptake in agroecosystems is a major global challenge. Ecological concepts from (semi)natural grasslands suggest that manipulating plant community composition using plant species with different traits may represent a promising opportunity to face this challenge. Here, we translate these trait‐based concepts to agricultural systems in a field experiment, aiming to reveal the main determinants of how plant community composition regulates N‐cycling in intensively managed grasslands.We focused on key N pools (plant N from soil and from biological N‐fixation, soil mineral N and N2O emissions) as well as on biological drivers of N‐cycling in soil (abundance of N‐cycling microbial communities, earthworm populations and arbuscular mycorrhizal fungi), using three common grass and one legume species in monoculture, two‐ and four‐species mixtures. We hypothesized that: (a) plant species mixtures increase plant N uptake, reduce soil mineral N concentrations and N2O emissions and promote the abundance of biological N‐cyclers; (b) legume presence stimulates N pools, fluxes and biological N‐cycling activity, (c) but in combination with a grass with acquisitive traits, more N is retained in the plant community, while N2O emissions are reduced.We found that mixtures increased plant N and lowered the soil mineral N pool compared to monocultures. However, plant species identity played an overarching role: Legume presence increased N2O emissions, plant N pools, soil mineral N and the abundance of N‐cycling microbes and earthworms. Combining the legume with a grass with low leaf dry matter content and high root length density (and with high root biomass) reduced the higher soil mineral N and N2O emissions induced by the legume, while harnessing positive effects on plant N pools and biological N‐fixation.Synthesis and applications. Our results show the potential of plant community composition to steer N‐cycling in fertilized agroecosystems, paving the way for a more biologically based agriculture. Legumes will play a crucial role, but selecting an optimum companion species is key for the sustainability of the agroecosystem. [ABSTRACT FROM AUTHOR]

Increased human‐derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N‐induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N‐induced P limitation. Here we show, using a meta‐analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short‐term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long‐term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short‐ or long‐term studies. Together, these results suggest that N‐induced P limitation in ecosystems is alleviated in the long‐term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought. [ABSTRACT FROM AUTHOR]

The increasing frequency of extreme weather events, such as floods, requires management strategies that promote resilience of grassland productivity. Mixtures of plant species may better resist and recover from flooding than monocultures, as they could combine species with stress‐coping and resource acquisition traits. This has not yet been tested in intensively managed grasslands despite its relevance for enhancing agroecosystem resilience.Using intact soil cores from an 18‐month‐old field experiment, we tested how 11 plant communities (Festuca arundinacea, Lolium perenne, Poa trivialis and Trifolium repens in monoculture, two‐ and four‐species mixtures) resist and recover from repeated flooding in a 4‐month greenhouse experiment.We found that plant community composition, not whether the community was a mixture or monoculture, influenced the community's resistance to flooding, although most communities were able to resist and recover from both floods.The plant community's position on the leaf economic spectrum in flooded conditions was related to its resistance to and recovery from flooding. Resistance to and recovery from a severe flood were related to flood‐induced intraspecific trait variation, causing a shift in the community's position on the leaf resource economic spectrum. In flooded conditions, resource‐conservative communities (characterized by low specific leaf area, low leaf nitrogen content and high leaf dry matter content) better resisted and recovered from flooding. The community's position on the root resource economic spectrum was less connected to the community's resistance and recovery.Synthesis and applications. Our study shows that in flooded conditions, resource‐conservative plant communities are more resilient to flooding than resource‐acquisitive communities in an intensively managed grassland. This suggests that plant community position on the leaf economic spectrum, as well as species' flood‐induced intraspecific variation, should be considered when designing grasslands to withstand increasing flood frequency and severity. [ABSTRACT FROM AUTHOR]

Soil faunal activity can be a major control of greenhouse gas (GHG) emissions from soil. Effects of single faunal species, genera or families have been investigated, but it is unknown how soil fauna diversity may influence emissions of both carbon dioxide (CO2, end product of decomposition of organic matter) and nitrous oxide (N2O, an intermediate product of N transformation processes, in particular denitrification). Here, we studied how CO2 and N2O emissions are affected by species and species mixtures of up to eight species of detritivorous/fungivorous soil fauna from four different taxonomic groups (earthworms, potworms, mites, springtails) using a microcosm set‐up. We found that higher species richness and increased functional dissimilarity of species mixtures led to increased faunal‐induced CO2 emission (up to 10%), but decreased N2O emission (up to 62%). Large ecosystem engineers such as earthworms were key drivers of both CO2 and N2O emissions. Interestingly, increased biodiversity of other soil fauna in the presence of earthworms decreased faunal‐induced N2O emission despite enhanced C cycling. We conclude that higher soil fauna functional diversity enhanced the intensity of belowground processes, leading to more complete litter decomposition and increased CO2 emission, but concurrently also resulting in more complete denitrification and reduced N2O emission. Our results suggest that increased soil fauna species diversity has the potential to mitigate emissions of N2O from soil ecosystems. Given the loss of soil biodiversity in managed soils, our findings call for adoption of management practices that enhance soil biodiversity and stimulate a functionally diverse faunal community to reduce N2O emissions from managed soils. [ABSTRACT FROM AUTHOR]

Intensive agriculture is dominated by monocultures of high‐yielding plants that receive large applications of nitrogen (N) fertilizers to boost plant productivity. However, these systems have low N use efficiency (NUE) as fertilized plants generally take up less than half of the N applied. A large fraction of the remainder N is susceptible to be lost from the agroecosystem generating a cascade of environmental and socio‐economic problems. Climate change and projected global increases in fertilizer use pose further risks to N losses and yield stability.We review and translate concepts from ecology in natural systems to demonstrate that NUE in intensive agroecosystems can be strongly increased by fine‐tuning the traits of the plant communities to the levels of N fertilization intensity.We present key plant traits of importance for N‐cycling (architectural, morphological and physiological traits, as well as symbiotic associations and exudation patterns); discuss ecological (with soil fauna and N‐cycling microbial communities) and agronomic interactions of this approach; propose interdisciplinary methodologies for future research ranging from pot to global scales; and highlight possible solutions leading to an optimal balance between N fertilizer use and productivity.Synthesis and applications. By showing the strong links between plant traits and nitrogen (N) cycling, our work opens possibilities to test ecologically informed hypotheses across gradients of soil fertility and N fertilizer management intensity, setting a research agenda for the coming years. Accordingly, the choice of plant species based on their functional traits will play a central role for the development of modern and productive agroecosystems that retain and use N more efficiently. [ABSTRACT FROM AUTHOR]

Abstract: Grasslands are a major form of agricultural land use worldwide. Current and future declines of phosphorus (P) inputs into production grasslands necessitate a shift towards selecting grass species based on high efficiency under suboptimal, rather than optimal P conditions. It is therefore imperative to identify key root traits that determine P acquisition of grasses in soils with a low P status. In a 9‐month greenhouse experiment, we grew eight common grass species and cultivars on a soil with a low P status and related root morphological traits to their performance under P‐limiting conditions. We applied (P1) or withheld (P0) P fertilization while providing adequate amounts of all other nutrients. Omitting P fertilization greatly reduced yield and nutrient acquisition for the various grass species. Biomass production differed significantly (P < 0.1%) among species and P fertilization treatments, varying from 17.1 to 72.1 g pot−1 in the P0 treatment and from 33.4 to 85.8 g pot−1 in the P1 treatment. Root traits were species‐specific and unresponsive to P fertilization, but overall we observed a trade‐off between root biomass and specific root length. Structural equation modeling identified total root length as key factor with respect to resistance to P deficiency, especially when roots explored the subsoil. Optimizing root length and subsoil exploration could be the key to maintaining high productivity of production grasslands with decreasing P availability. This is relevant for both plant breeding programs and for composing seed mixtures. [ABSTRACT FROM AUTHOR]

Plant species exert a dominant control over the nitrogen (N) cycle of natural and managed grasslands. Although in intensively managed systems that receive large external N inputs the emission of the potent greenhouse gas nitrous oxide (N2O) is a crucial component of this cycle, a mechanistic relationship between plant species and N2O emissions has not yet been established. Here we use a plant functional trait approach to study the relation between plant species strategies and N2O emissions from soils. Compared to species with conservative strategies, species with acquisitive strategies have higher N uptake when there is ample N in the soil, but also trigger N mineralization when soil N is limiting. Therefore, we hypothesized that (1) compared to conservative species, species with acquisitive traits reduce N2O emissions after a high N addition; and (2) species with conservative traits have lower N2O emissions than acquisitive plants if there is no high N addition. This was tested in a greenhouse experiment using monocultures of six grass species with differing above- and below-ground traits, growing across a gradient of soil N availability. We found that acquisitive species reduced N2O emissions at all levels of N availability, produced higher biomass and showed larger N uptake. As such, acquisitive species had 87% lower N2O emissions per unit of N uptake than conservative species ( p < .05). Structural equation modelling revealed that specific leaf area and root length density were key traits regulating the effects of plants on N2O emission and biomass productivity. These results provide the first framework to understand the mechanisms through which plants modulate N2O emissions, pointing the way to develop productive grasslands that contribute optimally to climate change mitigation. [ABSTRACT FROM AUTHOR]

Biochar application to soil is currently widely advocated for a variety of reasons related to sustainability. Typically, soil amelioration with biochar is presented as a multiple-'win' strategy, although it is also associated with potential risks such as environmental contamination. The most often claimed benefits of biochar (i.e. the 'wins') include (i) carbon sequestration; (ii) soil fertility enhancement; (iii) biofuel/bioenergy production; (iv) pollutant immobilization; and (v) waste disposal. However, the vast majority of studies ignore possible trade-offs between them. For example, there is an obvious trade-off between maximizing biofuel production and maximizing biochar production. Also, relatively little attention has been paid to mechanisms, as opposed to systems impacts, behind observed biochar effects, often leaving open the question as to whether they reflect truly unique properties of biochar as opposed to being simply the short-term consequences of a fertilization or liming effect. Here, we provide an outline for the future of soil biochar research. We first identify possible trade-offs between the potential benefits. Second, to be able to better understand and quantify these trade-offs, we propose guidelines for robust experimental design and selection of appropriate controls that allow both mechanistic and systems assessment of biochar effects and trade-offs between the wins. Third, we offer a conceptual framework to guide future experiments and suggest guidelines for the standardized reporting of biochar experiments to allow effective between-site comparisons to quantify trade-offs. Such a mechanistic and systems framework is required to allow effective comparisons between experiments, across scales and locations, to guide policy and recommendations concerning biochar application to soil. [ABSTRACT FROM AUTHOR]

Removing agricultural cellulosic residues from fields for the production of 'second generation biofuels'has the potential to profoundly alter C and N cycling in soil, increasing the risk of soil organic matter depletion and favoring soil-atmosphere gaseous exchanges. However, these negative impacts could potentially be offset by amending the soil with the solid by-product which is generated during bioethanol production. In a 100 days laboratory study, we investigated the fate of C and N after soil amendment with doubly labeled (13C, 15N) wheat residue ( WR) and the corresponding bioethanol by-product (i.e. nonfermentable wheat residue NFWR) with and without extra N addition. Substituting WR with the corresponding amount of recovered bioethanol by-product partially compensated the C losses of full crop residue removal. When the equivalent amount of C was added as WR and NFWR, NFWR-derived C was found in significantly higher proportion in macroaggregates in soil (17.0 vs. 8.9%) after 100 days. Addition of both WR and NFWR reduced soil organic C ( SOC) mineralization, i.e. it caused a negative priming effect in soil. However, this pattern was reversed when extra N was added. Both WR and NFWR increased the proportion of soil water-stable macroaggregates from 16% (in control) to 20-24% (in the different treatments). The results suggest that the more recalcitrant compounds derived from bioethanol production may stabilize more strongly and persist within the protected fractions of SOM pools. Our study demonstrates that NFWR, compared with WR application, neither increased N2O emissions nor had a negative impact on aggregate formation in the midterm. This demonstrates that NFWR has potential for replenishing SOC stocks. [ABSTRACT FROM AUTHOR]

Biochar (pyrolyzed biomass) amendment to soils has been shown to have a multitude of positive effects, e.g., on crop yield, soil quality, nutrient cycling, and carbon sequestration. So far the majority of studies have focused on agricultural systems, typically with relatively low species diversity and annual cropping schemes. How biochar amendment affects plant communities in more complex and diverse ecosystems that can evolve over time is largely unknown. We investigated such effects in a field experiment at a Dutch nature restoration area. In April 2011, we set up an experiment using biochar produced from cuttings collected from a local natural grassland. The material was pyrolyzed at 400°C or at 600°C. After biochar or residue (non-pyrolyzed cuttings) application (10 Mg/ha), all plots, including control (0 Mg/ha) plots, were sown with an 18-species grassland mixture. In August 2011, we determined characteristics of the developed plant community, as well as soil nutrient status. Biochar amendment did not alter total plant productivity, but it had a strong and significant effect on plant community composition. Legumes were three times as abundant and individual legume plants increased four times in biomass in plots that received biochar as compared to the control treatment. Biomass of the most abundant forb (Plantago lanceolata) was not affected by biochar addition. Available phosphorous, potassium, and pH were significantly higher in soils that received biochar than in Control soils. The rate of biological nitrogen fixation and seed germination were not altered by biochar amendment, but the total amount of biological N fixed per Trifolium pratense (red clover) plant was more than four times greater in biochar-amended soil. This study demonstrates that biochar amendment has a strong and rapid effect on plant communities and soil nutrients. Over time these changes may cascade up to other trophic groups, including above- and belowground organisms. Our results emphasize the need for long-term studies that examine not only the short-term effects of biochar amendment, but also follow how these effects evolve over time and affect ecosystem functioning. [ABSTRACT FROM AUTHOR]

Grassland ecosystems worldwide not only provide many important ecosystem services but they also function as a major source of the greenhouse gas nitrous oxide ( N2O), especially in response to nitrogen deposition by grazing animals. To explore the role of plants as mediators of these emissions, we tested whether and how N2O emissions are dependent on grass species richness and/or specific grass species composition in the absence and presence of urine deposition. We hypothesized that: (i) N2O emissions relate negatively to plant productivity; (ii) four-species mixtures have lower emissions than monocultures (as they are expected to be more productive); (iii) emissions are lowest in combinations of species with diverging root morphology and high root biomass; and (iv) the identity of the key species that reduce N2O emissions is dependent on urine deposition. We established monocultures and two- and four-species mixtures of common grass species with diverging functional traits: Lolium perenne L. (Lp), Festuca arundinacea Schreb. (Fa), Phleum pratense L. (Php) and Poa trivialis L. (Pt), and quantified N2O emissions for 42 days. We found no relation between plant species richness and N2O emissions. However, N2O emissions were significantly reduced in specific plant species combinations. In the absence of urine, plant communities of Fa+Php acted as a sink for N2O, whereas the monocultures of these species constituted a N2O source. With urine application Lp+Pt plant communities reduced ( P < 0.001) N2O emissions by 44% compared to monocultures of Lp. Reductions in N2O emissions by species mixtures could be explained by total biomass productivity and by complementarity in root morphology. This study shows that plant species composition is a key component underlying N2O emissions from grassland ecosystems. Selection of specific grass species combinations in the context of the expected nitrogen deposition regimes may therefore provide a key for mitigation of N2O emissions. [ABSTRACT FROM AUTHOR]

Nitrous oxide ( N2O) emissions from soils contribute significantly to global warming. Mitigation of N2O emissions is severely hampered by a lack of understanding of its main controls. Fluxes can only partly be predicted from soil abiotic factors and microbial analyses - a possible role for soil fauna has until now largely been overlooked. We studied the effect of six groups of soil invertebrate fauna and tested the hypothesis that all of them increase N2O emissions, although to different extents. We conducted three microcosm experiments with sandy soil and hay residue. Faunal groups included in our experiments were as follows: fungal-feeding nematodes, mites, springtails, potworms, earthworms and isopods. In experiment I, involving all six faunal groups, N2O emissions declined with earthworms and potworms from 78.4 (control) to 37.0 (earthworms) or 53.5 (potworms) mg N2O-N m−2. In experiment II, with a higher soil-to-hay ratio and mites, springtails and potworms as faunal treatments, N2O emissions increased with potworms from 51.9 (control) to 123.5 mg N2O-N m−2. Experiment III studied the effect of potworm density; we found that higher densities of potworms accelerated the peak of the N2O emissions by 5 days ( P < 0.001), but the cumulative N2O emissions remained unaffected. We propose that increased soil aeration by the soil fauna reduced N2O emissions in experiment I, whereas in experiment II N2O emissions were driven by increased nitrogen and carbon availability. In experiment III, higher densities of potworms accelerated nitrogen and carbon availability and N2O emissions, but did not increase them. Overall, our data show that soil fauna can suppress, increase, delay or accelerate N2O emissions from soil and should therefore be an integral part of future N2O studies. [ABSTRACT FROM AUTHOR]

The article presents a study which aims to identify buffer strip effectiveness (BSE) on a 20-meter (m)-deep, permeable sandy oil. Discharge to ditch from soil was temporarily gathered in an in-stream reservoir to measure its quality and quantity, both for a reference (REF) treatment and a BS. The study shows that the impact of BS for nitrate was much larger in upper groundwater compared in reservoirs that also gathered groundwater from greater depths that were not influenced by the treatments. It reveals that measuring alterations in upper groundwater to examine BSE is just valid under particular hydrogeological conditions.

Earthworms can increase nitrous oxide ( N2O) emissions, particularly in no-tillage systems where earthworms are abundant. Here, we study the effect of residue incorporation depth on earthworm-induced N2O emissions. We hypothesized that cumulative N2O emissions decrease with residue incorporation depth, because (i) increased water filled pore space ( WFPS) in deeper soil layers leads to higher denitrification rates as well as more complete denitrification; and (ii) the longer upward diffusion path increases N2O reduction to N2. Two 84-day laboratory mesocosm experiments were conducted. First, we manually incorporated maize ( Z ea mays L.) residue at different soil depths (incorporation experiment). Second, 13C-enriched maize residue was applied to the soil surface and anecic species L umbricus terrestris ( L.) and epigeic species L umbricus rubellus ( Hoffmeister) were confined to different soil depths (earthworm experiment). Residue incorporation depth affected cumulative N2O emissions in both experiments ( P < 0.001). In the incorporation experiment, N2O emissions decreased from 4.91 mg N2O-N kg−1 soil (surface application) to 2.71 mg N2O-N kg−1 soil (40-50 cm incorporation). In the earthworm experiment, N2O emissions from L . terrestris decreased from 3.87 mg N2O-N kg−1 soil (confined to 0-10 cm) to 2.01 mg N2O-N kg−1 soil (confined to 0-30 cm). Both experimental setups resulted in dissimilar WFPS profiles that affected N2O dynamics. We also found significant differences in residue C recovery in soil organic matter between L . terrestris (28-41%) and L . rubellus (56%). We conclude that (i) N2O emissions decrease with residue incorporation depth, although this effect was complicated by dissimilar WFPS profiles; and (ii) larger residue C incorporation by L . rubellus than L . terrestris indicates that earthworm species differ in their C stabilization potential. Our findings underline the importance of studying earthworm diversity in the context of greenhouse gas emissions from agro-ecosystems. [ABSTRACT FROM AUTHOR]

The article discusses a study investigating the significance of dissolved organic nitrogen (DON) losses from agricultural systems and to what extent the risk, if any, they pose to human and environmental health. A literature survey of peer-reviewed publications in which DON losses were measured under field conditions in agricultural systems was conducted using the ISI-Web of Science research database with the exclusion of research on DON losses from forest ecosystems or other non-agricultural systems. Researchers concluded that based on the extent of DON losses, which increased with increasing precipitation/irrigation, higher total inputs of nitrogen (N), and increasing sand content, DON leaching can be an important N loss pathway from agricultural systems.

The success of variable rate N fertilizer application rests on our ability to predict the contribution of soil N to growing crops. We assessed relationships between soil N availability indices (SNAIs), yield, and total N accumulation of wheat (Triticum aestivum L.) grown in a typical glacial till landscape in Saskatchewan, Canada. Soil samples were collected at 3-m intervals along a 300-m transect comprised of low (LCFS) and high catchment footslopes (HCFS), and low (LCSH) and high catchment shoulders (HCSH). Total soil N and C, organic C, mineral N, depth of A horizon, spring soil moisture, grain yield, and total plant N were measured. Soil N availability indices used in this study included: (ii) cumulative N released during a 2-wk aerobic incubation (N[sub MIN]); (ii) potentially mineralizable N estimated using a 16-wk aerobic incubation (N[sub 0]); (iii) NO[sub 3] sorbed on anion-exchange membranes (NO[sub 3AEM]); (iv) N extracted with hot KCl (N[sub KCl] and (v) N hydrolyzed with hot KCI (N[sub HYDR]). Although all SNAIs were significantly correlated to yield and, with the exception of N[sub 0], total plant N when analyzed across the transect, typically <40% of the yield variability was explained. Forward stepwise regression revealed that most SNAIs failed to explain more variability in crop N accumulation than did basic soil properties or relative elevation. Although these results do not invalidate the use of SNAIs for soil testing purposes, it is clear that SNAIs must be combined with additional information about field scale variability for predicting fertilizer N requirements. Without this information, grid sampling as a means of assessing N requirements remains ill-advised for glacial till semiarid landscapes. [ABSTRACT FROM AUTHOR]

Examines the possible relationship between salinity stress and natural abundance of carbon and nitrogen on a Lethent Clay Loam in California. Collection of soil and crop samples; Correlation of crop stress with soil salinity; Use of the primary enzyme for carbon dioxide fixation.

Investigates the spatial correlation of biological nitrogen fixation (BNF) by field-grown chickpea. Comparison of Nitrogen Natural Abundance and Nitrogen Enriched approaches in estimating BNF; Quantification of BNF spatial variability using NA approach; Causes of discrepancy between NE and NA.

Soil bulk density and moisture influence N2O movement and its reduction. This isotope study shows the sensitivity reduction of N2O, a greenhouse gas, to soil physical properties and their effect on gas diffusion. Increasing soil bulk density and water content promoted N2O reduction. Nitrous oxide is a greenhouse gas and contributes to stratospheric ozone depletion. Soil physical conditions may influence N2O reduction and subsequent N2O emissions. We studied how soil water-filled pore space (WFPS) and soil bulk density (ρb) affect N2O reduction and surface fluxes. Columns were repacked with soil and arranged in a factorial design at three levels of WFPS (60, 75, and 90%) and three levels of soil ρb (0.94, 1.00, 1.07 Mg m-3). Over 19 d, 15N-enriched N2O was introduced at the base of the soil columns and N2O fluxes were measured. Relative gas diffusivities (Dp/Do) were also calculated. Soil ρb and WFPS interacted to affect the recovery of N2O-15N and the antecedent inorganic-N contribution to surface fluxes. Reduction rates of N2O-15N ranged from 0.15 to 0.47 mg N2O-N g-1 soil d-1. Calculated Dp/Do values correlated (P < 0.01) with soil NH4+-N (r = -0.73), NO3--N (r = 0.93), cumulative N2O-N flux (r = 0.76), and N2O-N 15N enrichment (r = 0.80) and were affected by a soil WFPS x soil ρb interaction. Soil N transformations and the net surface N2O flux is dependent on the soil's Dp/Do, and WFPS alone does not suffice to discriminate between N2O emission sources. Consequently, the soil surface N2O flux may be comprised of N2O originating from deeper soil layers transported upward and/or from production in the topsoil. [ABSTRACT FROM AUTHOR]

A method to quantify the biochemical conversion of the strong greenhouse gas N2O into the environmentally benign N2 was developed. It is based on N2O gas diffusing through a soil core under specified laboratory conditions. Soil moisture level in the soil significantly influenced the N2O reduction potential of the soil. Nitrous oxide is produced in soil by nitrifying and denitrifying microorganisms. Most of the N2O produced in soil is reduced to N2 in the final step of denitrification, and only a fraction is released into the atmosphere. No reliable methods exist, however, to quantify N2O reduction and its controlling parameters in soil. In this study, we investigated: (i) a novel method to quantify the reduction of N2O in soil as it diffuses upward; and (ii) the effects of soil moisture content on N2O reduction. We developed a setup where a silicone tube coil permeable to N2O linked to an N2O reservoir simulated a subsoil with a known, artificial N2O source. A soil core filled with repacked sandy soil (a Typic Endoaquoll) was placed on top of the silicone coil. Experimental treatments included water-filled pore space (WFPS) levels of 60, 75, and 90%, as well as a quartz sand treatment at 90% WFPS that served as an abiotic control. Soil surface N2O emissions were measured for 21 d, and after the experiment, residual N2O in the system was determined. During the experiment, 41, 39, and 0% of the applied N2O was emitted as soil surface flux for the 60, 75, and 90% WFPS treatments, respectively. In the 90% WFPS treatment, 52% of applied N2O was reduced to N2. We conclude that our method enables us to quantify N2O reduction in soil columns. Our results further show that wet soils can be effective in reducing N2O to N2 during upward diffusion from the subsoil. [ABSTRACT FROM AUTHOR]