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Plant communities comprising species with different growth strategies and belonging to different functional groups can ensure stable productivity under variable climatic conditions. However, how plant communities can influence the response of nitrogen (N) cycling, in particular, soil microbial N cycling communities, N leaching and N 2 O fluxes under flooding, and their capacity to suppress flooding-induced N 2 O fluxes, remains unresolved. The aim of this study was to examine the effect of different plant communities composed of grasses and/or legumes on N cycling soil microorganisms and N 2 O fluxes, and how these effects are influenced by flooding. Our field experiment consisted of monocultures and two- and four-species mixtures of two grass and two legume species with different growth strategies (slow- and fast-growing species), grown in a fertilised sandy soil in the Netherlands. One year after plant establishment, we imposed paired control and flooding treatments for three weeks. We found that flooding significantly reduced plant N uptake and increased N 2 O fluxes. This increase was associated with higher abundances of N cycling microbial communities (except for ammonia-oxidising bacteria). Legume presence increased N 2 O fluxes, irrespective of the legume growth strategy or flooding, but this was not driven by changes in N cycling microbial communities; instead, it was related to an increase in soil nitrate availability. Mixing grasses with legumes promoted high plant N uptake and reduced N losses under control and flooded conditions, in particular when combining slow-growing species, and in the four-species mixture. Our results show that flooding exerted a strong influence on N cycling by increasing N leaching, N 2 O fluxes, microbial community abundances and decreasing plant N uptake. However, plant communities with slow-growing strategy had lowest relative abundance of nos ZII bacteria and ameliorated flooding effects by both reducing N losses and enhancing plant N uptake. • Flooding increased N 2 O fluxes and the abundance of N cycling microbial communities. • Legumes increased N 2 O fluxes due to an increase in soil nitrate availability. • Grass-legume mixtures promoted high plant N uptake and reduced N losses. • Combination with slow-growing species can potentially reduce N losses. [ABSTRACT FROM AUTHOR]

Earthworm activity can strongly influence soil structure and organic matter (OM) dynamics of agricultural soils. Several short-term studies (≤90 days) have shown that earthworms can increase incorporation of residue carbon (C) into soil aggregates, suggesting reduced decomposition in the longer term. In contrast, another body of short-term studies reported that earthworms can increase carbon dioxide (CO 2 ) emission from soils, thus suggesting increased decomposition in the longer term instead. To solve this controversy, we measured the effect of the epigeic Lumbricus rubellus (Hoffmeister) and the endogeic Aporrectodea caliginosa (Savigny) on the soil C balance in a unique 750-day mesocosm experiment, where loess soil was surface-applied with maize ( Zea mays L.) residues every six months. Carbon inputs and outputs were strictly controlled: no soil C input through growing plants and no leaching of soil organic C. Flux measurements of CO 2 were taken regularly and aggregate size distribution and total C and residue-derived C in the aggregate fractions (using the natural δ 13 C signature of maize) were measured after 185, 565 and 750 days. Both earthworm species increased cumulative CO 2 emissions by at least 25%, indicating a higher C loss compared to the no-earthworm control. Yet, both earthworm species also increased the amount of soil C associated with the macroaggregate fraction after 750 days. L. rubellus increased the amount of residue-derived C in the macroaggregate fraction after 565 and after 750 days, whereas A. caliginosa increased residue-derived C in all the measured soil fractions after 750 days. Our results show that earthworms can simultaneously enhance CO 2 emissions and C incorporation in aggregate fractions. However, over 750 days the presence of earthworms resulted in a lower total C content due to a higher overall OM decomposition rate. We therefore propose that under the most realistic incubation conditions so far (longer term and multiple residue applications), earthworms stimulate the mineralization of freshly added and older OM to a greater extent than that they stabilize residue-derived C inside biogenic aggregates. Future studies should focus on the balance between these processes in the presence of growing plants. [ABSTRACT FROM AUTHOR]

Agricultural soils are a large source of nitrous oxide (N 2 O) emissions. Soil mesofaunal species can accelerate, delay, increase or decrease N 2 O emissions. However, it is still unknown whether the soil fauna affect N 2 O emissions through trophic interactions or through their effect on soil structure. We explored the role of these two pathways in a 70 day microcosm experiment with a sandy loam subsoil with hay mixed in. Enchytraeids, fungivorous mites and predatory mites were added to the soil in a full factorial design to test for both single species effects as well as interactions between species. We measured N 2 O and CO 2 fluxes and we analysed soil structural parameters using X-ray micro tomography. After 35 days of incubation, enchytraeid presence significantly increased the volumetric air content of the soil (0.049–0.067 cm 3 cm −3 , P = 0.010) as well as the abundance of pores with sizes similar to their body width. At the same time N 2 O emissions, NO 3 − and DOC concentrations were significantly higher when enchytraeids were present (3.6 to 8.4 mg N 2 O-N m −2 , P < 0.001; 6.0 to 17.1 mg NO 3 − - N kg −1 soil, P < 0.001; 121.1 to 135.6 mg C kg −1 soil, and P = 0.017, respectively). Neither fungivorous mites nor predatory mites nor their interactions had a significant effect on soil structure or N 2 O emissions. Enchytraeids accelerated peak N 2 O emissions ( P = 0.001), but did not increase cumulative N 2 O emissions on day 70. Structural equation modelling confirmed that enchytraeids enhanced nitrogen mineralisation directly and also indirectly by creating a higher volumetric air content, and thereby increased N 2 O emissions. We conclude that the soil structure pathway was important in driving N 2 O emissions, and that soil ecosystem engineers such as enchytraeids disproportionately affected N 2 O emissions as compared to other soil fauna. [ABSTRACT FROM AUTHOR]

Recent research has shown that microbial-feeding invertebrate soil fauna species can significantly contribute to N2O emissions. However, in soil food webs microbial-feeding soil fauna interact with each other and with their predators, which affects microbial activity. To date we lack empirical tests of whether or not these interactions play a significant role in N2O emissions from soil. Therefore we studied how interactions between soil microbes, two groups of microbial-feeding soil fauna (enchytraeids and fungivorous mites) and their predators (predatory mites) affect soil N2O emissions. We hypothesized that: 1) the presence of two microbial-feeding fauna groups (enchytraeids and fungivorous mites) together increase N2O emissions more than when only a single group is present; and 2) the addition of predatory mites further enhances N2O emissions. We assembled soil food webs consisting of soil microbes, enchytraeids, fungivorous and predatory mites in microcosms with sandy loamy soil and sterilised hay as a substrate for the soil microbes. N2O emissions were measured during 56 days. We found no support for our first yet support for our second hypothesis. Addition of predatory mites to microcosms with enchytraeids and fungivorous mites increased N2O emissions significantly from 135.3 to 482.1 mg N m−2, which was also significantly higher than the control without fauna (83 mg N m−2) (P < 0.001). In presence of enchytraeids, fungivorous and predatory mites, we found much higher nitrate availability at the time of the N2O peak on Day 35 (10.9 versus 5.5 mg N per kg soil without soil fauna), indicating that the major increase in N2O emissions in this treatment may be due to increased nitrification. Increased nitrification may be attributed to higher availability of N from the dead tissues of fungivorous mites and increased activity of the enchytraeids that might also have affected soil structure and contributed to increased N2O emissions. This study demonstrates the importance of interactions between microbial-feeding invertebrate soil fauna and their predators in understanding N2O emissions. [ABSTRACT FROM AUTHOR]

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Abstract: Nitrogen losses from agricultural grasslands cause eutrophication of ground- and surface water and contribute to global warming and atmospheric pollution. It is widely assumed that soils with a higher fungal biomass have lower N losses, but this relationship has never been experimentally confirmed. With the increased interest in soil-based ecosystem services and sustainable management of soils, such a relationship would be relevant for agricultural management. Here we present a first attempt to test this relationship experimentally. We used intact soil columns from two plots from a field experiment that had consistent differences in fungal biomass (68 ± 8 vs. 111 ± 9 μg C g−1) as a result of different fertilizer history (80 vs. 40 kg N ha−1 y−1 as farm yard manure), while other soil properties were very similar. We performed two greenhouse experiments: in the main experiment the columns received either mineral fertilizer N or no N (control). We measured N leaching, N2O emission and denitrification from the columns during 4 weeks, after which we analyzed fungal and bacterial biomass and soil N pools. In the additional 15N experiment we traced added N in leachates, soil, plants and microbial biomass. We found that in the main experiment, N2O emission and denitrification were lower in the high fungal biomass soil, irrespective of the addition of fertilizer N. Higher 15N recovery in the high fungal biomass soil also indicated lower N losses through dentrification. In the main experiment, N leaching after fertilizer addition showed a 3-fold increase compared to the control in low fungal biomass soil (11.9 ± 1.0 and 3.9 ± 1.0 kg N ha−1, respectively), but did not increase in high fungal biomass soil (6.4 ± 0.9 after N addition vs. 4.5 ± 0.8 kg N ha−1 in the control). Thus, in the high fungal biomass soil more N was immobilized. However, the 15N experiment did not confirm these results; N leaching was higher in high fungal biomass soil, even though this soil showed higher immobilization of 15N into microbial biomass. However, only 3% of total 15N was found in the microbial biomass 2 weeks after the mineral fertilization. Most of the recovered 15N was found in plants (approximately 25%) and soil organic matter (approximately 15%), and these amounts did not differ between the high and the low fungal biomass soil. Our main experiment confirmed the assumption of lower N losses in a soil with higher fungal biomass. The additional 15N experiment showed that higher fungal biomass is probably not the direct cause of higher N retention, but rather the result of low nitrogen availability. Both experiments confirmed that higher fungal biomass can be considered as an indicator of higher nitrogen retention in soils. [Copyright &y& Elsevier]

Abstract: Urine patches from cattle and sheep on pastures represent considerable, highly localized N applications. Subsequent nitrification and denitrification of the nitrogenous compounds may result in high nitrous oxide (N2O) emissions. Not much is known about the extent of these emissions, or about possible mitigation options. The aims of this study were to experimentally quantify the effects of urine composition, dung addition, compaction and soil moisture on N2O emissions from urine patches. For an incubation study at 16°C, soil was collected from a typic Endoaquoll, and N2O production was monitored during a 103-day period. Emissions for the whole period averaged 0.3 and 0.9% of the applied urine-N for dry and moist soil, respectively. When compacted or when dung was added, emissions from moist soils increased to 4.9 and 7.9%, respectively. Both addition of dung and soil compaction resulted in a delay of the peak N2O emission of approximately 10–15 days. No significant effect of amount of urine-N on emission percentages was detected. Changing the volume of urine with equal amounts of urine-N resulted in highly significant effects, peaking with an emission of 2.3% at a water-filled pore space (WFPS) of 78%. When the soil was water-saturated, N2O production was delayed until evaporation had decreased moisture contents. We concluded that denitrification was the main N2O forming process in the incubation study. Emission factors for urine reported in the literature do not generally include the potentially considerable effects of compaction or combination with dung. We conclude that realistic emission factors should take into account such an effect, together with estimates for the occurrence of camping areas in pastures. From our results, the best mitigation strategies appear to be increasing the volume of urine through feed additives, and avoiding compaction and promoting more homogeneous application of N through a lower cattle stocking rate. Also, research efforts may be targeted at management practices to avoid camping areas in pastures. [Copyright &y& Elsevier]

Abstract: Earlier research with endogeic and epigeic earthworm species in loamy arable soil has shown that both earthworm groups can increase nitrous oxide (N2O) emissions, provided that crop residue placement matches the feeding strategy of the earthworm ecological group(s). However, it is not yet clear whether these effects also occur in sandy soils which typically contain less soil organic matter and have low soil aggregation levels. Here, we aimed to quantify N2O emissions as affected by endogeic and/or epigeic earthworm species, and to relate changes in N2O emissions to earthworm-induced changes in soil properties in a sandy soil. A 90 day mesocosm study was conducted with sandy soil and 15N-labeled radish (Raphanus sativus cv. Adagio L.) residue applied on top. Treatments included: (i) no earthworm addition, (ii) addition of the endogeic species Aporrectodea caliginosa (Savigny), (iii) addition of the epigeic species Lumbricus rubellus (Hoffmeister), and (iv) both species combined. An additional treatment was included without earthworms and with residue manually incorporated into the soil. L. rubellus significantly increased cumulative N2O emissions from 228 to 859μg N2O–Nkg−1 (F 1,12 =83.12, P <0.001), whereas A. caliginosa did not affect N2O emissions. In contrast to earlier studies in loamy soil, no positive interaction between both species with regard to N2O emissions was found. This was probably related to high competition for organic resources in the relatively poor soil and a low potential for stable soil aggregate formation (and associated anaerobic microsites) by endogeic worms in sandy soil. 15N isotope analysis revealed that the activity of L. rubellus significantly increased (F 1,12 =6.20, P =0.028) the recovery of 15N in the 250–8000μm size fraction, indicating incorporation of crop residues into the mineral soil. When residues were manually incorporated, N2O emissions were significantly (P <0.008) lower (509μg N2O–Nkg−1) than when incorporated by L. rubellus. The high N2O emissions in the presence of L. rubellus, when compared to manual mixing, suggest a stimulation of microbial activity and/or changes in the microbial community composition. Insights on the earthworm effects on N2O emission from such soils are discussed. [Copyright &y& Elsevier]

To slow the rise in atmospheric carbon dioxide concentrations, Enhanced Silicate Weathering is emerging as a potentially significant Carbon Dioxide Removal technology. However, the biotic controls on rock weathering are not well understood, particularly for key soil faunal groups such as earthworms. Earthworms have shown to possibly enhance weathering, highlighting their potential to be introduced in controlled or engineered settings, such as reactors, to increase carbon sequestration. Here, we determined the potential for earthworms to thrive and to increase weathering rates in an artificial organo-mineral system simulating a bioreactor. We used two earthworm species (Aporrectodea caliginosa [Savigny] and Allolobophora chlorotica [Savigny]) at four densities (10, 20, 25 and 30 earthworms kg−1 organo-mineral mixture), four silicate rock types (two basanites, dunite and diabase) of two to three grain sizes (d50 between 0.026 and 1.536 mm), two sources of organic materials (straw and co-digestate), two amounts of biochar (0 and 100 g kg−1 organo-mineral mixture) and/or enzyme additions (laccase, urease and carbonic anhydrase at 20, 177 and 1955 units kg−1 organo-mineral mixture, respectively), three water irrigation rates (125, 250 and 375 mL day−1 kg−1 organo-mineral mixture) and three watering frequencies (one, two and five times day−1). The experiment was conducted in eight rounds, each one lasting eight weeks, yielding data for a total of 323 experimental units. We measured earthworm survival and activity, as well as several commonly used weathering indicators in the organo-mineral mixture and in the leachate, as total alkalinity, inorganic carbon, pH, electrical conductivity and major cations. Using random forest regression, we found that earthworm survival and activity mainly depended on variables influencing the structure and drainage potential of the organo-mineral mixture, such as the presence of straw and increasing percentages of coarse grain sizes. Furthermore, we concluded that the effect of earthworms on weathering indicators depended on whether they survived or died by the end of the experimental period. Surviving earthworms had a neutral or negative effect on weathering indicators, likely because the experimental duration was too short to detect an increase in inorganic carbon, or because there was an increase in organic rather than inorganic carbon in the organo-mineral mixture. In contrast, dead earthworms enhanced almost all weathering indicators considered, suggesting that microbial processes associated with decomposing earthworm bodies may play a role in enhancing weathering. Our results also emphasize that the role of earthworms in Enhanced Silicate Weathering within bioreactors might be overestimated if weathering indicators exclusively rely on changes in mineralogy and ions release to quantify earthworm effects on carbon sequestration through weathering. • Earthworm's potential to boost silicate weathering was tested in small-scale reactors. • Earthworm's performance was mainly driven by the structure of the artificial system. • Surviving earthworms decreased or had a neutral effect on weathering indicators. • Dead earthworms enhanced weathering indicators through indirect microbial pathways. • Standardised indicators are needed to assess earthworms' role in silicate weathering. [ABSTRACT FROM AUTHOR]

Abstract: Previous laboratory studies using epigeic and anecic earthworms have shown that earthworm activity can considerably increase nitrous oxide (N2O) emissions from crop residues in soils. However, the universality of this effect across earthworm functional groups and its underlying mechanisms remain unclear. The aims of this study were (i) to determine whether earthworms with an endogeic strategy also affect N2O emissions; (ii) to quantify possible interactions with epigeic earthworms; and (iii) to link these effects to earthworm-induced differences in selected soil properties. We initiated a 90-day 15N-tracer mesocosm study with the endogeic earthworm species Aporrectodea caliginosa (Savigny) and the epigeic species Lumbricus rubellus (Hoffmeister). 15N-labeled radish (Raphanus sativus cv. Adagio L.) residue was placed on top or incorporated into the loamy (Fluvaquent) soil. When residue was incorporated, only A. caliginosa significantly (p < 0.01) increased cumulative N2O emissions from 1350 to 2223 μg N2O–N kg−1 soil, with a corresponding increase in the turnover rate of macroaggregates. When residue was applied on top, L. rubellus significantly (p < 0.001) increased emissions from 524 to 929 μg N2O–N kg−1, and a significant (p < 0.05) interaction between the two earthworm species increased emissions to 1397 μg N2O–N kg−1. These effects coincided with an 84% increase in incorporation of residue 15N into the microaggregate fraction by A. caliginosa (p = 0.003) and an 85% increase in incorporation into the macroaggregate fraction by L. rubellus (p = 0.018). Cumulative CO2 fluxes were only significantly increased by earthworm activity (from 473.9 to 593.6 mg CO2–C kg−1 soil; p = 0.037) in the presence of L. rubellus when residue was applied on top. We conclude that earthworm-induced N2O emissions reflect earthworm feeding strategies: epigeic earthworms can increase N2O emissions when residue is applied on top; endogeic earthworms when residue is incorporated into the soil by humans (tillage) or by other earthworm species. The effects of residue placement and earthworm addition are accompanied by changes in aggregate and SOM turnover, possibly controlling carbon, nitrogen and oxygen availability and therefore denitrification. Our results contribute to understanding the important but intricate relations between (functional) soil biodiversity and the soil greenhouse gas balance. Further research should focus on elucidating the links between the observed changes in soil aggregation and controls on denitrification, including the microbial community. [Copyright &y& Elsevier]

Plants can affect N 2 O emissions by enhancing nitrogen (N) uptake and other below-ground interactions. However, the specific effect of the root systems of different plant species on the production and accumulation of N 2 O within the soil profile remain largely unknown. The aim of this study was to investigate how plant species from different functional groups, their productivity and root traits affect N 2 O emissions and N 2 O concentrations within the soil profile in a fertilised grassland. We conducted a field experiment with two grasses (Phleum pratense , Lolium perenne), two legumes (Trifolium repens , Trifolium pratense), two forbs (Cichorium intybus , Plantago lanceolata), and the six-species mixture in a fertilised grassland. The effects of these plant communities on N-cycling processes were then assessed through the measurement of above- and below-ground plant traits, plant productivity, soil nutrient availability, N 2 O emissions and its distribution in the soil profile. We found that C. intybus and P. pratense had the lowest N 2 O emissions from the soil, which was mainly related to higher root biomass. The six-species mixture also showed lower N 2 O emissions compared to L. perenne monoculture which was explained by complementary effects between the different plant species. We did not find a relationship between N 2 O emission and its concentration in the soil profile. Higher specific root length and root length density coincided with higher N 2 O concentrations at 10–20 and 20–30 cm soil depths. Since these two traits have been previously linked to reductions in N 2 O emissions emitted from the soil, our results show that the relationships between root traits and N 2 O emissions may not be reflected down in the soil profile. Overall, this study underscores the often-neglected importance of root traits for N-cycling and emphasises the need to better understand how root traits modify N 2 O consumption within the soil profile to design more sustainable grasslands. • C. intybus and P. pratense decreased N 2 O emissions due to higher root biomass. • Mixture lower N 2 O emissions compared to L. perenne due to complementary effects. • There is no relationship between N 2 O emission and its concentration in the soil profile. • Higher root traits coincided with higher N 2 O concentrations in the soil profile. [ABSTRACT FROM AUTHOR]

Sampling and analysis or visual examination of soil to assess its status and use potential is widely practiced from plot to national scales. However, the choice of relevant soil attributes and interpretation of measurements are not straightforward, because of the complexity and site-specificity of soils, legacy effects of previous land use, and trade-offs between ecosystem services. Here we review soil quality and related concepts, in terms of definition, assessment approaches, and indicator selection and interpretation. We identify the most frequently used soil quality indicators under agricultural land use. We find that explicit evaluation of soil quality with respect to specific soil threats, soil functions and ecosystem services has rarely been implemented, and few approaches provide clear interpretation schemes of measured indicator values. This limits their adoption by land managers as well as policy. We also consider novel indicators that address currently neglected though important soil properties and processes, and we list the crucial steps in the development of a soil quality assessment procedure that is scientifically sound and supports management and policy decisions that account for the multi-functionality of soil. This requires the involvement of the pertinent actors, stakeholders and end-users to a much larger degree than practiced to date. [ABSTRACT FROM AUTHOR]

Biochar is proposed as an option to sequester carbon (C) in soils and promote other soil-based ecosystem services. However, its impact on soil biota from micro to macroscale remains poorly understood. We investigated biochar effects on the soil biota across the soil food web, on plant community composition and on biomass production. We conducted a field experiment in a nature restoration grassland testing four treatments: two biochar types (herbaceous feedstock pyrolyzed at 400 °C or 600 °C – hereafter B400 and B600), and a positive (i.e. unpyrolysed biochar feedstock, hereafter Hay) and negative (no addition) control. Responses of plants and soil biota were evaluated one and three years after establishing the treatments. Soil pH and K concentrations increased significantly in the B600 treatment. Mite abundances were significantly higher in B400 whereas nematode abundances were highest in Hay (1st year) and lowest in B400 (3rd year). Other soil fauna groups (enchytraeids and earthworms) varied more between years than between treatments. Legume cover increased significantly in the biochar treatments but this effect was transient. Legumes, grasses and primary productivity also showed a statistically significant Treatment x Year interaction due to transitory effects that were no longer present by the 3rd year. Our results suggest that biochar produced from meadow cuttings and applied at the 10 t/ha rate cause transitory impacts on soil biota abundance and plant communities over the 3-year timeframe used for this experiment. Therefore, this type of biochar could potentially be used for soil carbon sequestration, with minimal impacts on soil biota abundance or diversity, within the groups studied here, or plant biodiversity and productivity. Further research is required to investigate the longer-term impacts of this potential soil C storage sink. • Biochar has differential effects on soil organisms by groups. • Legume biomass increases following to biochar application. • The impacts differ for different biochar types. • Effects are transient and do not persist into the third season. • Biochar has potential to sequester C in soils in nutrient rich grasslands. [ABSTRACT FROM AUTHOR]

The largest part of phosphorus (P) in soil is bound by the soil solid phase; its release to the soil solution therefore often does not meet the demand of plants. Since global P fertilizer reserves are declining, it becomes increasingly important to better utilize soil P. We tested whether earthworm activity can increase P availability to grass (Lolium perenne L.) in a 75-day greenhouse pot experiment in a soil with low P availability. The full factorial design included two factors: P fertilization (control without P; phytate; and inorganic P) and earthworm population (control without earthworms; Lumbricus rubellus Hoffmeister, Lr; Aporrectodea caliginosa Savigny, Ac; and Lumbricus terrestris L., Lt). At four times during the experiment, aboveground plant growth and P uptake were determined. In a separate incubation experiment, earthworm casts and bulk soil were analyzed for inorganic and organic P in water extracts. We observed higher levels of dissolved P pools (p < 0.001) in the water extracts of earthworm casts compared to those of the bulk soil. The magnitude of the difference differed between earthworm species, with the largest levels for Lr: from <0.02 to 8.56 mg L−1 for inorganic P (p = 0.007) and from 0.18 to 1.30 mg L−1 for organic P (p = 0.007). After three harvests, presence of Lt significantly increased P uptake by grass to 44.1 mg per pot compared to 41.8 mg per pot for the control (p = 0.010). Plant growth increased from 15.68 to 16.85 g dry biomass per pot (p < 0.001). We conclude that earthworms casts contain higher levels of plant available P than the bulk soil, and that this might translate into increased plant P uptake. It is well-known that maintaining soil faunal biodiversity is important for a variety of ecosystem services; our results show that these ecosystem services may include improving the utilization of soil P in a world with rapidly declining P stocks. [ABSTRACT FROM AUTHOR]

Abstract: Combined oxygen (O) and nitrogen (N) stable isotope analyses are commonly used in the source determination of nitrate . The source and fate of are studied based on distinct O and N isotopic signatures (δ18O and δ15N) of various sources and isotopic effects during transformation processes, which differ between sources like fertilizer, atmospheric deposition, and microbial production (nitrification). Isotopic fractionation during production and consumption of further affects the δ18O and δ15N signal. Regarding the δ18O in particular, biochemical O exchange between O from and H2O is implicitly assumed not to affect the δ18O signature of . This study aimed to test this assumption in soil-based systems. In a short (24 h) incubation experiment, soils were treated with artificially 18O and 15N enriched . Production of from nitrification during the incubation would affect both the 18O and the 15N enrichment. Oxygen exchange could therefore be studied by examining the change in 18O relative to the 15N. In two out of the three soils, we found that the imposed 18O enrichment of the declined relatively more than the imposed enrichment. This implies that O exchange indeed affected the O isotopic signature of , which has important implications for source determination studies. We suggest that O exchange between and H2O should be taken into consideration when interpreting the O isotopic signature to study the origin and fate of in ecosystems. [Copyright &y& Elsevier]

Abstract: Soils are the major source of the greenhouse gas nitrous oxide (N2O) to our atmosphere. A thorough understanding of terrestrial N2O production is therefore essential. N2O can be produced by nitrifiers, denitrifiers, and by nitrifiers paradoxically denitrifying. The latter pathway, though well-known in pure culture, has only recently been demonstrated in soils. Moreover, nitrifier denitrification appeared to be much less important than classical nitrate-driven denitrification. Here we studied a poor sandy soil, and show that when moisture conditions are sub-optimal for denitrification, nitrifier denitrification can be a major contributor to N2O emission from this soil. We conclude that the relative importance of classical and nitrifier denitrification in N2O emitted from soil is a function of the soil moisture content, and likely of other environmental conditions as well. Accordingly, we suggest that nitrifier denitrification should be routinely considered as a major source of N2O from soil. [ABSTRACT FROM AUTHOR]

Abstract: Interpretation of the oxygen isotopic signature of soil-derived N2O may be flawed when it is based on reaction stoichiometry and fractionation alone. In fact, oxygen (O) exchange between H2O and intermediates of N2O production pathways may largely determine this O isotopic signature. Although in our previous work we conclusively proved the occurrence of O exchange during N2O production by denitrification of NO3 −, its occurrence in N2O production pathways by nitrifiers remains unclear. The aim of this study was to examine the likeliness of O exchange during various stages of N2O production in soil via nitrification, nitrifier denitrification and denitrification. We evaluated a set of scenarios on the presence of such exchange using data from a series of 18O and 15N tracing experiments. The measured actual O incorporation from H2O into N2O (AOI) was compared with the theoretical maximum O incorporation (MOI) from various scenarios that differed in their assumptions on the presence of O exchange. We found that scenarios where O exchange was assumed to occur exclusively during denitrification could not explain the observed AOI, as it exceeded the MOI for 9 out of 10 soils. This demonstrates that additional O exchange must have occurred in N2O production through nitrifier pathways. It remains to be determined in which steps of these pathways O exchange can take place. We conclude that O exchange is likely to be mediated by ammonia oxidizers during NO2 − reduction (nitrifier denitrification), and that it could possibly occur during NO2 − oxidation to NO3 − by nitrite oxidizers as well. [Copyright &y& Elsevier]

Abstract: The treatment of manures may improve their agricultural value and environmental quality, for instance with regards to greenhouse gases mitigation and enhancement of carbon (C) sequestration. The present study verified whether different pig slurry treatments (i.e. solid/liquid separation and anaerobic digestion) changed slurry composition. The effect of the slurry composition on N2O and CO2 emissions, denitrification and soil mineral nitrogen (N), after soil incorporation, was also examined during a 58-day mesocosm study. The treatments included a non-treated pig slurry (NT), the solid fraction (SF), and the liquid fraction (LF) of a pig slurry and the anaerobically digested liquid fraction (DG). Finally, a non-fertilized (N0) and a treatment with urea (UR) were also present. The N2O emissions measured represented 4.8%, 2.6%, 1.8%, 1.0% and 0.9% of N supplied with slurry/fertilizer for NT, LF, DG, SF and UR, respectively. Cumulative CO2 emissions ranged from 0.40gCO2-Ckg−1 soil (0.38MgCO2-Cha−1) to 0.80gCO2-Ckg−1 soil (0.75MgCO2-Cha−1). They were highest for SF (56% of C applied), followed by NT (189% of C applied), LF (337% of C applied) and DG (321% of C applied). Ammonium was detected in the soil for all treatments only at day one, while nitrate concentration increased linearly from day 15 to day 58, at a rate independent of the type of slurry/fertilizer applied. The nitrate recovery at day 58 was 39% of the N applied for NT, 19% for SF, 52% for LF, 67% for DG, and 41% for UR. The solid fraction generally produced higher potential denitrification fluxes (75.3 for SF, 56.7 for NT, 53.6 for LF, 47.7 for DG and 39.7mgN2O+N2-Nkg−1 soil for UR). The high variability of actual denitrification results obfuscated any treatment effect. We conclude that treatment strongly affects slurry composition (mainly its C, fibre and NH4 + content), and hence N2O and CO2 emission patterns as well as denitrification processes and nitrate availability. In particular, the solid fraction obtained after mechanical separation produced the most pronounced difference, while the liquid fraction and the anaerobically digested liquid fraction did not show significant difference with respect to the original slurry for any of the measured parameters. Combining data from the different fractions we showed that separation of slurry leads to reduced N2O emissions, irrespective of whether the liquid fraction is digested or not. Furthermore, our results suggested that the default emission factor for N2O emissions inventory is too low for both the non-treated pig slurry and its liquid fraction (digested or not), and too high for the separated solid fraction and urea. [Copyright &y& Elsevier]

Abstract: Earthworm activity may have an effect on nitrous oxide (N2O) emissions from crop residue. However, the importance of this effect and its main controlling variables are largely unknown. The main objective of this study was to determine under which conditions and to what extent earthworm activity impacts N2O emissions from grass residue. For this purpose we initiated a 90-day (experiment I) and a 50-day (experiment II) laboratory mesocosm experiment using a Typic Fluvaquent pasture soil with silt loam texture. In all treatments, residue was applied, and emissions of N2O and carbon dioxide (CO2) were measured. In experiment I the residue was applied on top of the soil surface and we tested (a) the effects of the anecic earthworm species Aporrectodea longa (Ude) vs. the epigeic species Lumbricus rubellus (Hoffmeister) and (b) interactions between earthworm activity and bulk density (1.06 vs. 1.61gcm−3). In experiment II we tested the effect of L. rubellus after residue was artificially incorporated in the soil. In experiment I, N2O emissions in the presence of earthworms significantly increased from 55.7 to 789.1μgN2O-Nkg−1 soil (L. rubellus; p<0.001) or to 227.2μgN2O-Nkg−1 soil (A. longa; p<0.05). This effect was not dependent on bulk density. However, if the residue was incorporated into the soil (experiment II) the earthworm effect disappeared and emissions were higher (1064.2μgN2O-Nkg−1 soil). At the end of the experiment and after removal of earthworms, a drying/wetting and freezing/thawing cycle resulted in significantly higher emissions of N2O and CO2 from soil with prior presence of L. rubellus. Soil with prior presence of L. rubellus also had higher potential denitrification. We conclude that the main effect of earthworm activity on N2O emissions is through mixing residue into the soil, switching residue decomposition from an aerobic and low denitrification pathway to one with significant denitrification and N2O production. Furthermore, A. longa activity resulted in more stable soil organic matter than L. rubellus. [Copyright &y& Elsevier]

Abstract: Earthworm activity has been reported to lead to increased production of the greenhouse gas nitrous oxide (N2O). This is due to emissions from worms themselves, their casts and drilosphere, as well as to general changes in soil structure. However, it remains to be determined how important this effect is on N2O fluxes from agricultural systems under realistic conditions in terms of earthworm density, soil moisture, tillage activity and residue loads. We quantified the effect of earthworm presence on N2O emissions from a pasture after simulated ploughing of the sod (‘grassland renovation’) for different soil moisture contents during a 62-day mesocosm study. Sod (with associated soil) and topsoil were separately collected from a loamy Typic Fluvaquent. Treatments included low (L), medium (M) and high (H) moisture content, in combination with: only soil (S); soil+incorporated sod (SG); soil+incorporated sod+the anecic earthworm Aporrectodea longa (SGE). Nitrous oxide and carbon dioxide (CO2) fluxes were measured for 62d. At the end of the incubation period, we determined N2O production under water-saturated conditions, potential denitrification and potential mineralization of the soil after removing the earthworms. Cumulative N2O and CO2 fluxes over 62d from incorporated sod were highest for treatment HSGE (973μg N2O-N and 302mg CO2-Ckg−1 soil) and lowest for LSG (64μg N2O-N and 188mg CO2-Ckg−1 soil). Both cumulative fluxes were significantly different for soil moisture (p<0.001), but not for earthworm presence. However, we observed highly significant earthworm effects on N2O fluxes that reversed over time for the H treatments. During the first phase (day 3–day 12), earthworm presence increased N2O emissions with approximately 30%. After a transitional phase, earthworm presence resulted in consistently lower (approximately 50%) emissions from day 44 onwards. Emissions from earthworms themselves were negligible compared to overall soil fluxes. After 62d, original soil moisture significantly affected potential denitrification, with highest fluxes from the L treatments, and no significant earthworm effect. We conclude that after grassland ploughing, anecic earthworm presence may ultimately lead to lower N2O emissions after an initial phase of elevated emissions. However, the earthworm effect was both determined and exceeded by soil moisture conditions. The observed effects of earthworm activity on N2O emissions were due to the effect of earthworms on soil structure rather than to emissions from the worms themselves. [Copyright &y& Elsevier]

Summary: Knowledge of the effects of species diversity within taxonomic groups on nutrient cycling is important for understanding the role of soil biota in sustainable agriculture. We hypothesized that earthworm species specifically affect nitrogen mineralization, characteristically for their ecological group classifications, and that earthworm species interactions would affect mineralization through competition and facilitation effects. A mesocosm experiment was conducted to investigate the effect of three earthworm species, representative of different ecological groups (epigeic: Lumbricus rubellus; endogeic: Aporrectodea caliginosa tuberculata; and anecic: Lumbricus terrestris), and their interactions on the bacterial community, and on nitrogen mineralization from 15N-labelled crop residue and from soil organic matter. Our results indicate that L. rubellus and L. terrestris enhanced mineralization of the applied crop residue whereas A. caliginosa had no effect. On the other hand, L. rubellus and A. caliginosa enhanced mineralization of the soil organic matter, whereas L. terrestris had no effect. The interactions between different earthworm species affected the bacterial community and the net mineralization of soil organic matter. The two-species interactions between L. rubellus and A. caliginosa, and L. rubellus and L. terrestris, resulted in reduced mineral N concentrations derived from soil organic matter, probably through increased immobilization in the bacterial biomass. In contrast, the interaction between A. caliginosa and L. terrestris resulted in increased bacterial growth rate and reduced total soil C. When all three species were combined, the interaction between A. caliginosa and L. terrestris was dominant. We conclude that the effects of earthworms on nitrogen mineralization depend on the ecological traits of the earthworm species present, and can be modified by species interactions. Knowledge of these effects can be made useful in the prevention of nutrient losses and increased soil fertility in agricultural systems, that typically have a low earthworm diversity. [Copyright &y& Elsevier]

Abstract: Artificial urine, an aqueous solution of various nitrogenous compounds and salts, is routinely used in soil incubation studies on nitrous oxide (N2O) emissions and related nitrogen (N) and pH dynamics. There is, however, no consensus on artificial urine composition, and a wide variety of compositions are used. The aim of this study was to test which artificial urine composition is adequate for simulating N2O fluxes, respiration, soil mineral N and pH dynamics of real cattle urine in both short- and long-term incubation studies. Urine solutions of differing compositions were applied to a sandy soil and incubated for 65 days, and results of measurements on N2O fluxes and soil mineral N were analyzed over the first 5 days as well as over the whole incubation period. Results from two real cattle urines with known nitrogenous composition (R1 and R2) were compared with three artificial urine types: (i) urea+glycine (AG), (ii) urea+hippuric acid (AH) and (iii) an artificial urine identical to the nitrogenous composition of real urine R1 (AR). During the first 5 days, only cumulative N2O emissions for AG deviated significantly () from the N2O emissions for real urines, with 0.4% of applied N emitted compared with 0.0% and 0.1% for R1 and R2, respectively. Respiration from R1 was significantly () higher than from R2 and all artificial urines. Over the whole incubation period, no significant differences could be detected for N2O emissions or respiration with urine type. From all artificial urine types, AH yielded N2O emissions closest to those from real urine. AG deviated most from real urine, both in short- and long-term incubations. Over the whole period, soil NH4 + was higher for all artificial urines () and pH-KCl was lower for AG and AR () than for the real urines. AH was not significantly different from real urine R2 with respect to all measured properties except soil NH4 +. We conclude that only AG did not adequately simulate N2O emissions, and that glycine is therefore not an appropriate substitution for hippuric acid in artificial urine. For future studies using artificial urine we recommend therefore a mixture containing at least urea and hippuric acid as sources of N. As no artificial urine composition resembled real urine with respect to all measured variables, even when nitrogenous composition was identical (AR), we recommend the use of real urines whenever possible. [Copyright &y& Elsevier]

Abstract: Urine patches in grazed pastures are a major source of nitrous oxide (N2O) emission. It is well-documented that the relative concentration of the various nitrogenous urine constituents varies significantly with diet. The effect of these variations on N2O emissions from urine patches, however, has never been reported. The aim of this study was to test whether variations in urine composition, consistent with different diets, lead to significant differences in N2O emission. Four varieties of artificial urine, all with similar total N concentrations, but varying in the relative contribution of the nitrogenous constituents, were applied to undisturbed cores from a sandy pasture soil. N2O fluxes were monitored for 65 days at two moisture treatments; 92% WFPS for the entire incubation, and 70% WFPS up to day 41 and 92% afterwards. Extra replicates were included for destructive analysis on mineral N concentrations and pH. Urine composition was a significant (P<0.001) factor determining N2O emissions. An increase in the relative hippuric acid concentration from 3 to 9% of total N resulted in a significant decline in average N2O fluxes, from 16.4 to 8.7μgN2O–Nh−1 kg−1 soil (averaged over all treatments). Cumulative emission decreased from 8.4 to 4.4% of the applied urine-N (P<0.01). Soil mineral N showed a modest but significant decrease with an increase of hippuric acid content. pH did not show any significant relationship with urine composition. Increasing the urea concentration with 12% of applied urinary N did not significantly affect N2O emissions. Moisture content significantly affected N2O emissions (P<0.001), but no interaction between moisture and urine composition was found. As the inhibitory effect of hippuric acid could not be linked directly to mineral N concentrations in the soil, we hypothesize that the breakdown product benzoic acid either inhibits denitrification or decreases the N2O/N2 ratio. We conclude that hippuric acid concentration in urine is an important factor influencing N2O emission, with a potential for reducing emissions with 50%. We suggest alternative rationing leading to higher hippuric acid concentrations in urine as a possible strategy to mitigate N2O emission from grazed pastures. [Copyright &y& Elsevier]

Abstract: The net flux of soil C is determined by the balance between soil C input and microbial decomposition, both of which might be altered under prolonged elevated atmospheric CO2. In this study, we determined the effect of elevated CO2 on decomposition of grass root material (Lolium perenne L.). 14C-labeled root material, produced under ambient (35Pa pCO2) or elevated CO2 (70Pa pCO2) was incubated in soil for 64 days. The soils were taken from a pasture ecosystem which had been exposed to ambient (35Pa pCO2) or elevated CO2 (60Pa pCO2) under FACE-conditions for 10 years and two fertilizer N rates: 140 and 560kg N ha−1 year−1. In soil exposed to elevated CO2, decomposition rates of root material grown at either ambient or elevated CO2 were always lower than in the control soil exposed to ambient CO2, demonstrating a change in microbial activity. In the soil that received the high rate of N fertilizer, decomposition of root material grown at elevated CO2 decreased by approximately 17% after incubation for 64 days compared to root material grown at ambient CO2. The amount of 14CO2 respired per amount of 14C incorporated in the microbial biomass (q 14CO2) was significantly lower when roots were grown under high CO2 compared to roots grown under low CO2. We hypothesize that this decrease is the result of a shift in the microbial community, causing an increase in metabolic efficiency. Soils exposed to elevated CO2 tended to respire more native SOC, both with and without the addition of the root material, probably resulting from a higher C supply to the soil during the 10 years of treatment with elevated CO2. The results show the importance of using soils adapted to elevated CO2 in studies of decomposition of roots grown under elevated CO2. Our results further suggest that negative priming effects may obscure CO2 data in incubation experiments with unlabeled substrates. From the results obtained, we conclude that a slower turnover of root material grown in an ‘elevated-CO2 world’ may result in a limited net increase in C storage in ryegrass swards. [Copyright &y& Elsevier]