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Grassland renovation (grassland renewal or temporary conversion to arable land) is a common practice employed by farmers to reverse declining yields in agricultural grasslands. Renovation practices may lead to substantial nitrate (NO 3) leaching and nitrous oxide (N 2 O) emissions. Farmers lean towards renovating grasslands in autumn instead of spring because of better sward establishment and low weed infestation. Limited nitrogen (N) uptake of renewed grassland during autumn may, however, increase the potential risk of NO 3 leaching and N 2 O emission. Potential options to mitigate these N losses are reduction in tillage intensity and N application rate, or the application of nitrification inhibitors. We assessed the effects of these measures as well as timing of renewal and the conversion to grassland to maize on NO 3 leaching and N 2 O emissions after one growing season. Five replicated field experiments were conducted of one year each at five locations in the Netherlands. We measured crop yield, soil mineral N, NO 3 concentrations in groundwater and N 2 O emission. Averaged over all experiments, the measures had an effect on NO 3 concentration in groundwater, although not significant. The highest NO 3 concentrations in groundwater were observed after autumn renewal (17.2 mg NO 3 -N L−1), autumn renewal combined with mitigation strategies (12.8–19.4 mg NO 3 -N L−1) and conversion to maize (12.5–12.8 mg NO 3 -N L−1), with all of these treatments exceeding the European norm of 11.3 mg NO 3 -N L−1. Concentrations after renewal in spring were lower (3.1 mg NO 3 -N L−1) and did not lead to an increase compared to the control (4.4 mg NO 3 -N L−1). N 2 O emissions increased significantly directly after grassland renewal relative to the reference grassland and N 2 O fluxes were similar in spring and autumn. We conclude that autumn renewal leads to significant increases in the risk of NO 3 leaching which cannot be mitigated by the technical measures in this study. • Five field experiments to assess timing of renewal and technical measures on N losses. • Renewal in autumn leads to an increased risk of soil mineral N and NO 3 leaching. • Conversion to maize results in increased risk of NO 3 leaching. • N 2 O emissions increased significantly directly after grassland renewal. • Increased risk of NO 3 leaching cannot be mitigated by technical measures. [ABSTRACT FROM AUTHOR]

• Rewetting above −20 cm decreased CO 2 emissions, but caused strong N 2 O emissions. • CO 2 from peat oxidation showed different responses to drying and rewetting. • Production of CO 2 and N 2 O occurred mostly in the top soil. • Variable soil moisture at a −20 cm groundwater level creates a risk for N 2 O peaks. • Oxidation was estimated to contribute 66 to 79% to vertical displacement. Peat soils store a large part of the global soil carbon stock, which can potentially be lost when they are drained and taken into cultivation, resulting in CO 2 emission and land subsidence. Groundwater level (GWL) management has been proposed to mitigate peat oxidation, but may lead to increased emissions of nitrous oxide (N 2 O) and methane (CH 4). The aim of this experiment was to study trade-offs between greenhouse gas emissions from peat soils as a function of GWL. We incubated 1 m deep, 24 cm diameter undisturbed bare soil cores, after removal of the grass layer, from three contrasting Dutch grassland peat sites for 370 days at 16 °C. The cores were subjected to drying-wetting cycles, with the GWL varying between near the soil surface to 160 cm below the surface. We measured gas fluxes of CO 2 , N 2 O and CH 4 from the soil surface, extracted pore water for DOC and mineral nitrogen analysis, and measured soil hydraulic and shrinkage characteristics. Emissions of CO 2 increased after lowering the GWL, but showed different GWL-response curves during rewetting of the soil. On average, highest CO 2 emissions of 1.5 g C·m−2 day−1 were found at a GWL of 80 cm below the surface. However, the 0 cm GWL was the only treatment with significantly lower CO 2 emissions than other GWLs. Cumulative CO 2 emissions differed significantly between sampling sites. Emissions of N 2 O showed a different response, peaking at GWL heights above −20 cm, particularly after a recent GWL rise. Though not significantly different, the highest N 2 O emissions were measured at the 0 cm GWL treatment. We confirmed this pattern for N 2 O in un-replicated soil cores with grass sward, although emission values were lower in these cores due to the root uptake of mineral nitrogen. CH 4 emissions or −uptake remained low under any GWL. We conclude that raising the GWL is a successful strategy to reduce CO 2 emissions from peat oxidation. However, raising the GWL close to the soil surface could lead to N 2 O emissions that negate any gains in terms of global warming potential. Our results suggest that raising the GWL in peat grasslands to −20 cm creates such a risk. A constant GWL at the surface (0 cm) would be preferential for mitigating both CO 2 and N 2 O emissions, although such conditions don't allow for agricultural grass production (mowing or grazing). [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]

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]

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]

[Display omitted] • Struvite effectiveness was tested on two contrasting soils of low agronomic P status. • In our experiment, struvite resulted in poorer maize growth than DAP. • In soil containing struvite, P-calcium chloride and P-ammonium lactate have less diagnostic value in short-term experiments. • Roots grow preferentially towards both DAP and struvite, despite their different dissolution. Struvite is emerging as a circular, slow-release phosphorus (P) fertiliser. However, its effectiveness for crops and its impact on soil P tests still remain largely unclear. We analysed the effectiveness of struvite as a row fertiliser in a pot experiment with maize in a sandy soil and a loamy soil, both exhibiting a low agronomic soil P status. Maize was seeded in both soils, amended with either granular struvite or water-soluble di-ammonium phosphate (DAP) as row fertilisers. A positive control received an excess of sodium phosphate, mixed homogeneously through both soils; a negative control did not receive additional P. We measured the agronomic maize performance at the end of the vegetative growth stage (V10) and assessed the suitability of two established agronomic soil P tests (i.e. P-CaCl 2 and P-AL) to predict plant-available P in the soils amended with struvite. Furthermore, we performed an additional batch experiment to better understand the dissolution dynamics of struvite during a prolonged period of extraction of soil in 0.01 M CaCl 2. Placed struvite application led to a lower performance of maize in comparison to DAP in terms of biomass production as well as P uptake (53 and 71 % lower, respectively). Yet the agronomic performance of struvite in general was higher than the performance of the negative control, confirming the potential of struvite as an emerging P fertiliser. Surprisingly, both soils fertilised with struvite showed a significantly higher soil P test than any other P treatment in our pot experiment. This was an artefact due to the dissolution of residual struvite granules during the soil extraction procedures. These results call for a reconsideration of how to interpret P-CaCl 2 and P-AL as a basis for P fertiliser recommendations for soils receiving struvite as a P fertiliser. [ABSTRACT FROM AUTHOR]

Because phosphorus (P) is one of the most limiting nutrients in agricultural systems, P fertilisation is essential to feed the world. However, declining P reserves demand far more effective use of this crucial resource. Here, we use meta-analysis to synthesize yield responses to P fertilisation in grasslands, the most common type of agricultural land, to identify under which conditions P fertilisation is most effective. Yield responses to P fertilisation were 40–100% higher in (a) tropical vs temperate regions; (b) grass/legume mixtures vs grass monocultures; and (c) soil pH of 5–6 vs other pHs. The agronomic efficiency of P fertilisation decreased for greater P application rates. Moreover, soils with low P availability reacted disproportionately strong to fertilisation. Hence, low fertiliser application rates to P-deficient soils result in stronger absolute yield benefits than high rates applied to soils with a higher P status. Overall, our results suggest that optimising P fertiliser use is key to sustainable intensification of agricultural systems. [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]

• Phosphomonoesterase (PME) activity among earthworm species and soil spheres assayed. • A downscaled method to measure PME activity in small samples is developed. • PME activity is strongly elevated by earthworms in the order cast > burrows > bulk soil. • The oft-assumed optimum of pH=6.5 overestimates the potential PME activity in casts. • Elevated P contents of grass shoots could result from increased PME activity. Earthworms can increase the solubility of phosphorus (P) in soil, an effect which to a large extent is controlled by mineralisation of organic P. Phosphatases, a class of hydrolytic enzymes, catalyse this mineralisation process. However, a consistent comparison of their activity among earthworm species and for different soil spheres is still missing. Here we aim to better understand the activity of phosphatases in relation to earthworm-enhanced P-availability, and specifically that of phosphomonoesterases (PME) which directly liberate orthophosphate. We conducted a greenhouse pot experiment with five different earthworm species. The PME activity was assessed in earthworm casts, burrows and bulk soil for both single species and mixed communities. Analyses were performed at both the pH-H 2 O of the bulk soil (6.5) and of the casts (7.5). The PME activity measured at both pH values was highly correlated (R2 = 0.98; p = 2.2x10-16) and was strongly elevated by earthworm activity in the order cast > burrows > bulk soil. The PME activity was particularly high in the casts of A. longa relative to casts of other species. Large variation in PME activity in the drilosphere was observed among earthworm species, but this variation was not related to earthworm ecological categories. Our data also indicate that an elevated P concentration in grass shoots could result from increased PME activity via greater hydrolysis of organic P. [ABSTRACT FROM AUTHOR]

Biochar application to soil is currently being widely posited as a means to improve soil quality and thereby increase crop yield. Next to beneficial effects on soil nutrient availability and retention, biochar is assumed to improve soil water retention. However, evidence for such an effect in the primary literature remains elusive. Therefore, we studied the effect of biochar on soil hydrological characteristics in two separate field experiments on a sandy soil in The Netherlands. In Experiment I, biochar produced through slow pyrolysis of herbaceous feedstock at two temperatures (400 °C and 600 °C) was applied to soil at a rate of 10 t ha − 1 . In Experiment II, the 400 °C biochar was applied at rates of 1, 5, 20 and 50 t ha − 1 . Soils were analysed for soil water retention, aggregate stability and other soil physical parameters after three growing seasons and one growing season for Experiment I and Experiment II, respectively. We characterised the pore structure of the biochar using X-ray computed micro-tomography (XRT) and hydrophobicity using contact angle measurements. We found no significant effects of biochar application on soil water retention in either experiment. Aggregate stability was also not significantly affected, nor was field saturated hydraulic conductivity. XRT analysis of the biochars showed that they were highly porous, with 48% and 57% porosity for the 400 °C and 600 °C biochar respectively. More than 99% of internal pores of the biochar particles were connected to the surface, suggesting a potential role for biochars in improving soil water retention. However, the biochars were highly hydrophobic. We postulate that this strong hydrophobicity prevented water from infiltrating into the biochar particles, prohibiting an effect on soil water retention. Our results suggest that, in addition to characterising pore space, biochars should be analysed for hydrophobicity when assessing their potential for improving soil physical properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • We measured and modelled earthworm-enhanced PO 4 solubility across several soils. • Stimulated mineralisation of organic matter contributed strongly to PO 4 solubility. • Competition between NOM and PO 4 decreased in casts compared to the bulk soil. • Reactive surface area decreased in casts from Fe-(hydr)oxide-dominated soils. • Earthworm impact on PO 4 solubility was largest in Fe-(hydr)oxide-dominated soils. Sustainability of agricultural systems is at stake, as phosphorus (P) is a non-renewable resource while its global reserves are limited. Stimulating earthworm activity can be a technology to increase the level of readily plant-available phosphate (PO 4). However, conclusive evidence on the mechanisms underlying an earthworm-enhanced PO 4 solubility is yet missing. This study aimed to reveal possibly overlooked pathways by which earthworms affect PO 4 solubility, and quantify the relative importance of all contributing mechanisms. Therefore, we set up a greenhouse pot experiment in which we investigated the large increase in water-extractable PO 4 in casts of three earthworm species (Lumbricus rubellus, Aporrectodea caliginosa, Lumbricus terrestris) in soils with either predominantly Fe- or Al-(hydr)oxides. Oxalate-extractable PO 4 was increased in earthworm casts compared to bulk soil which can be attributed to the mineralisation of natural organic matter (NOM). Surface complexation modelling was used to elucidate the mechanisms that control earthworm-enhanced PO 4 solubility. The results of our modelling showed that the increase in pH in earthworm casts relative to bulk soil affects PO 4 solubility only to a minor extent. Besides NOM mineralisation, two major mechanisms contributing to earthworm-enhanced PO 4 solubility are (i) a decrease in the reactive surface area (RSA) of the metal-(hydr)oxide fraction; and (ii) a decrease in the competition between NOM and PO 4 for binding sites of the metal-(hydr)oxides. As the newly discovered decrease of the RSA was only found for Fe-(hydr)oxide-dominated soils, earthworms have the largest potential to enhance PO 4 solubility in those soils. [ABSTRACT FROM AUTHOR]

The key nutrient phosphorus (P) binds strongly to reactive soil particles, which makes it poorly available for plant uptake. In the search for sustainable ways to overcome a resulting P shortage, it has been shown that earthworms can increase the pool of plant available P and enhance plant P uptake under controlled (greenhouse) conditions. To validate these findings under field conditions and to study the effect of earthworm community composition, we conducted a mesocosm-field experiment on grassland. Mesocosms containing a sandy soil with a low P-status and communities of five earthworm species common to the Netherlands (Lumbricus rubellus , Aporrectodea caliginosa , Allolobophora chlorotica , Lumbricus terrestris and Aporrectodea longa ; monocultures, three- or five-species mixtures and controls without earthworms) were installed in a field. Aboveground biomass production and P uptake of Lolium perenne were monitored for over two years. Earthworm community composition varied between the start and the end of the experiment, but multiple linear regression on the final earthworm communities yielded strong indications that earthworms increased both biomass production (R2 adj = 0.52, p < 0.001, n = 76) and P uptake (R2 adj = 0.48, p < 0.001, n = 76). The species A. longa and L. terrestris were most influential for this earthworm effect. We did not observe any general relations between the number of earthworm species in a community and a P effect. Our results suggest that earthworms can indeed increase grass biomass production and P uptake on a low P soil in the field, and can thereby contribute to making the P nutrition of our agricultural systems more sustainable. • Earthworms increased grass biomass production and P uptake in the field. • A. longa and L. terrestris were most influential for this effect. • A community with more earthworm species does not translate into a larger effect. [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]

Drip irrigation combined with split application of fertilizer nitrogen (N) dissolved in the irrigation water (i.e. drip fertigation) is commonly considered best management practice for water and nutrient efficiency. As a consequence, its use is becoming widespread. Some of the main factors (water-filled pore space, NH 4 + and NO 3 − ) regulating the emissions of greenhouse gases (i.e. N 2 O, CO 2 and CH 4 ) and NO from agroecosystems can easily be manipulated by drip fertigation without yield penalties. In this study, we tested management options to reduce these emissions in a field experiment with a melon ( Cucumis melo L.) crop. Treatments included drip irrigation frequency (weekly/daily) and type of N fertilizer (urea/calcium nitrate) applied by fertigation. Crop yield, environmental parameters, soil mineral N concentrations and fluxes of N 2 O, NO, CH 4 and CO 2 were measured during 85 days. Fertigation with urea instead of calcium nitrate increased N 2 O and NO emissions by a factor of 2.4 and 2.9, respectively ( P < 0.005). Daily irrigation reduced NO emissions by 42% ( P < 0.005) but increased CO 2 emissions by 21% ( P < 0.05) compared with weekly irrigation. We found no relation between irrigation frequency and N 2 O emissions. Based on yield-scaled Global Warming Potential as well as NO cumulative emissions, we conclude that weekly fertigation with a NO 3 − -based fertilizer is the best option to combine agronomic productivity with environmental sustainability. Our study shows that adequate management of drip fertigation, while contributing to the attainment of water and food security, may provide an opportunity for climate change mitigation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Red clover biomass was significantly greater when grown on biochar-amended soil. [•] An increase in K availability increased the competitive ability of red clover to grass and plantain. [•] This effect disappeared when N fertilisation was applied. [•] The liming effect of the biochar did not influence red clover growth or competitive ability. [Copyright &y& Elsevier]

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]

• Estimates for maximum soil C sequestration rates often take soil as starting point. • Our "Fermi approach" suggests instead to take photosynthesis as the starting point. • Our results suggest maximum sequestration rates in global croplands of 0.2 Pg yr−1. • This is an order of magnitude lower than most previous estimates. • Uncertainty analysis suggests that our results, while relatively rough, are robust. How much C can be stored in agricultural soils worldwide to mitigate rising carbon dioxide (CO 2) concentrations, and at what cost? This question, because of its critical relevance to climate policy, has been a focus of soil science for decades. The amount of additional soil organic C (SOC) that could be stored has been estimated in various ways, most of which have taken the soil as the starting point: projecting how much of the SOC previously lost can be restored, for example, or calculating the cumulative effect of multiple soil management strategies. Here, we take a different approach, recognizing that photosynthesis, the source of C input to soil, represents the most fundamental constraint to C sequestration. We follow a simple "Fermi approach" to derive a rough but robust estimate by reducing our problem to a series of approximate relations that can be parameterized using data from the literature. We distinguish two forms of soil C: 'ephemeral C', denoting recently-applied plant-derived C that is quickly decayed to CO 2 , and 'lingering C,' which remains in the soil long enough to serve as a lasting repository for C derived from atmospheric CO 2. First, we estimate global net C inputs into lingering SOC in croplands from net primary production, biomass removal by humans and short-term decomposition. Next, we estimate net additional C storage in cropland soils globally from the estimated C inputs, accounting also for decomposition of lingering SOC already present. Our results suggest a maximum C input rate into the lingering SOC pool of 0.44 Pg C yr−1, and a maximum net sequestration rate of 0.14 Pg C yr−1 – significantly less than most previous estimates, even allowing for acknowledged uncertainties. More importantly, we argue for a re-orientation in emphasis from soil processes towards a wider ecosystem perspective, starting with photosynthesis. [ABSTRACT FROM AUTHOR]

Rice production systems are the largest anthropogenic wetlands on earth and feed more than half of the world's population. However, they are also a major source of global anthropogenic greenhouse gas (GHG) emissions. Several agronomic strategies have been proposed to improve water-use efficiency and reduce GHG emissions. The aim of this study was to evaluate the impact of water-saving irrigation (alternate wetting and drying (AWD) vs. soil water potential (SWP)), contrasting land establishment (puddling vs. reduced tillage) and fertiliser application methods (broadcast vs. liquid fertilisation) on water-use efficiency, GHG emissions and rice yield. The experiment was laid out in a randomised complete block design with eight treatments (all combinations of the three factors) and four replicates. AWD combined with broadcasting fertilisation was superior to SWP in terms of maintaining yield. However, seasonal nitrous oxide (N 2 O) emissions were significantly reduced by 64% and 66% in the Broadcast-SWP and Liquid fertiliser-SWP treatments, respectively, compared to corresponding treatments in AWD. The SWP also significantly reduced seasonal methane (CH 4) emissions by 34 and 30% in the broadcast and liquid fertilisation treatments, respectively. Area-scaled GWPs were reduced by 48% and 54% in Broadcast-SWP and Liquid fertiliser-SWP treatments respectively compared to the corresponding treatments in AWD. Compared to AWD, the broadcast and liquid fertilisation in SWP irrigation treatments reduced yield-scaled GWPs by 46% and 37%, respectively. In terms of suitability, based on yield-scaled GWPs, the treatments can be ordered as follows: Broadcast-SWP < Broadcast-AWD = Liquid fertiliser-SWP < Liquid fertiliser-AWD. Growing-season water use was 15% lower in the SWP treatments compared with the water-saving AWD. Reduced tillage reduced additional water use during land preparation. The conclusions of this study are that improved water management and timely coordination of N fertiliser with crop demand can reduce water use, N loss via N 2 O emissions, and CH 4 emissions. Unlabelled Image • Soil water potential (SWP) scheduling was compared with alternate wetting and drying (AWD) • AWD and broadcasting fertilisation perform better in terms of maintaining yield • SWP scheduling reduced N 2 O and CH 4 emissions up to 66 and 34% respectively • Liquid fertilisation/fertigation was only effective in reducing N 2 O emission under SWP • Reduced tillage and SWP irrigation scheduling significantly reduced water use • Global warming potentials were reduced up to 54% in SWP scheduled reduced tillage treatments. [ABSTRACT FROM AUTHOR]

• e-AWD water regime reduced seasonal CH 4 emissions up to 85 %. • DOC was reduced up to 47 % which was linked to the reduction of CH 4 emissions. • Grain As, Pb and Cd levels were reduced up to 65, 72 and 33 % respectively. • Root biomass and length increased up to 72 %, which was linked to the yield increase. • Yield-scaled GWPs were reduced up to 84 % compared to conventional CF practice. Flooded rice production is crucial to global food security, but there are associated environmental concerns. In particular, it is a significant source of methane (CH4) and nitrous oxide (N2O) emissions and a large consumer of water resources, while arsenic levels in the grain are a serious health concern. There is also a tendency to use more organic fertilisers to close nutrient cycles, posing a threat of even higher GHG emissions and grain arsenic levels. It has been shown that alternate wetting and drying (AWD) water management reduces both water use and GHG emissions, but success at maintaining yields varies. This study tested the effect of early AWD (e-AWD) versus continuous flooding (CF) water management practices on grain yields, GHG emissions and grain arsenic levels in a split-plot field experiment with organic fertilisers under organic management. The treatments included: i) farmyard manure, ii) compost, and iii) biogas digestate, alone or in combination with mineral fertiliser. The e-AWD water regime showed no difference in yield for the organic treatments. Yields significantly increased by 5–16 % in the combination treatments. Root biomass and length increased in the e-AWD treatments up to 72 and 41 %, respectively. The e-AWD water regime reduced seasonal CH4 emissions by 71–85 % for organic treatments and by 51–76 % for combination treatments; this was linked to a 15–47 % reduction in dissolved organic carbon (DOC), thereby reducing methanogenesis. N2O emissions increased by 23–305 % but accounted for <20 % of global warming potential (GWP). Area and yield-scaled GWPs were reduced by 67–83 %. The e–AWD regime altered soil redox potentials, resulting in a reduction in grain arsenic and lead concentrations of up to 66 % and 73 % respectively. Grain cadmium levels were also reduced up to 33 % in organic treatments. Structural equation modelling showed that DOC, redox, ammonium and root biomass were the key traits that regulated emissions and maintained yield. Despite the fact that the experiment was conducted in the dry-season when soil moisture conditions can be relatively well-controlled, our findings should be confirmed in multi-year studies in farmers' fields. These results suggest that in flooded rice systems receiving organic amendments or organic management, the e-AWD water regime can achieve multiple environmental and food safety objectives without compromising yield. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an important nutrient for plant growth. However, P is often poorly available for uptake by roots because it strongly adsorbs to the soil mineral phase. Recent research shows that earthworms can temporally and locally increase P availability to plants. However, the pathways through which they do so are not fully understood, and it remains unclear to what extent this capacity varies among earthworm species. Here we study the variation among earthworm species with respect to readily-available P in casts as well as other physico-chemical cast properties, in a greenhouse pot experiment using a soil with a low P status. The earthworms belong to eight commonly occurring earthworm species in the Netherlands: two epigeic species (a mixture of the compost earthworms Dendrobaena veneta/Eisenia fetida; Lumbricus rubellus); four endogeic species (Allolobophora chlorotica , Aporrectodea caliginosa , Aporrectodea rosea , Octolasion lacteum); and two anecic species (Aporrectodea longa; Lumbricus terrestris). For all species, the pH in water extracts of earthworm cast (pH = 7.4–8.2) was significantly higher (p < 0.001) than for the control bulk soil (pH = 6.6) and differed significantly (p = 0.003) among earthworm species. Similarly, the dissolved organic carbon (DOC) concentration in the same water extracts was an order of magnitude higher in earthworm cast compared to the control bulk soil and varied among species (p < 0.001). The size of the total pool of reversibly adsorbed P in earthworm cast was greater than in the control bulk soil, but no significant differences were found among earthworm species. Differences among species were present for the readily-available P pools extracted from casts, including P-Olsen and water-extractable ortho-P. Water-extractable ortho-P concentrations were much higher in the casts of all species as compared to the control bulk soil (0.9–6.8 vs 0.06 mg l−1 or to 9.0–68 vs 0.6 mg kg−1). Highest ortho-P levels were measured in L. rubellus casts and the lowest in casts of A. chlorotica. A positive correlation between the concentrations of DOC and ortho-P was observed (R2 = 0.72, p < 0.001). The observed variation in all measured physico-chemical cast properties could not be explained by conventional ecological earthworm classifications. Our results show that the nature and magnitude of earthworm-induced increased P availability differs dramatically among earthworm species. This strongly suggests that, apart from its size, species composition of the earthworm community is key to optimizing P availability to plants. • Readily-available P is increased in earthworm cast as compared to control bulk soil. • Variability of readily-available P in casts is high among earthworm species. • Physico-chemical cast properties vary strongly among earthworm species. • Conventional ecological earthworm classifications cannot explain this variation. • Earthworm community composition is key to optimizing P availability to plants. [ABSTRACT FROM AUTHOR]

Enhanced weathering (EW) is an emerging carbon dioxide (CO2) removal technology that can contribute to climate change mitigation. This technology relies on accelerating the natural process of mineral weathering in soils by manipulating the abiotic variables that govern this process, in particular mineral grain size and exposure to acids dissolved in water. EW mainly aims at reducing atmospheric CO2 concentrations by enhancing inorganic carbon sequestration. Until now, knowledge of EW has been mainly gained through experiments that focused on the abiotic variables known for stimulating mineral weathering, thereby neglecting the potential influence of biotic components. While bacteria, fungi, and earthworms are known to increase mineral weathering rates, the use of soil organisms in the context of EW remains underexplored. This protocol describes the design and construction of an experimental setup developed to enhance mineral weathering rates through soil organisms while concurrently controlling abiotic conditions. The setup is designed to maximize weathering rates while maintaining soil organisms' activity. It consists of a large number of columns filled with rock powder and organic material, located in a climate chamber and with water applied via a downflow irrigation system. Columns are placed above a fridge containing jerrycans to collect the leachate. Representative results demonstrate that this setup is suitable to ensure the activity of soil organisms and quantify their effect on inorganic carbon sequestration. Challenges remain in minimizing leachate losses, ensuring homogeneous ventilation through the climate chamber, and avoiding flooding of the columns. With this setup, an innovative and promising approach is proposed to enhance mineral weathering rates through the activity of soil biota and disentangle the effect of biotic and abiotic factors as drivers of EW.

Microbial necromass is a central component of soil organic matter (SOM), whose management may be essential in mitigating atmospheric CO 2 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., (© 2022 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Intensively managed grasslands are large sources of the potent greenhouse gas nitrous oxide (N 2 O) and important regulators of methane (CH 4 ) consumption and production. The predicted increase in flooding frequency and severity due to climate change could increase N 2 O emissions and shift grasslands from a net CH 4 sink to a source. Therefore, effective management strategies are critical for mitigating greenhouse gas emissions from flood-prone grasslands. We tested how repeated flooding affected the N 2 O and CH 4 emissions from 11 different plant communities (Festuca arundinacea, Lolium perenne, Poa trivialis, and Trifolium repens in monoculture, 2- and 4-species mixtures), using intact soil cores from an 18-month old grassland field experiment in a 4-month greenhouse experiment. To elucidate potential underlying mechanisms, we related plant functional traits to cumulative N 2 O and CH 4 emissions. We hypothesized that traits related with fast nitrogen uptake and growth would lower N 2 O and CH 4 emissions in ambient (non-flooded) conditions, and that traits related to tissue toughness would lower N 2 O and CH 4 emissions in flooded conditions. We found that flooding increased cumulative N 2 O emissions by 97 fold and cumulative CH 4 emissions by 1.6 fold on average. Plant community composition mediated the flood-induced increase in N 2 O emissions. In flooded conditions, increasing abundance of the grass F. arundinacea was related with lower N 2 O emissions; whereas increases in abundance of the legume T. repens resulted in higher N 2 O emissions. In non-flooded conditions, N 2 O emissions were not clearly mediated by plant traits related with nitrogen uptake or biomass production. In flooded conditions, plant communities with high root carbon to nitrogen ratio were related with lower cumulative N 2 O emissions, and a lower global warming potential (CO 2 equivalent of N 2 O and CH 4 ). We conclude that plant functional traits related to slower decomposition and nitrogen mineralization could play a significant role in mitigating N 2 O emissions in flooded grasslands., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)

Global rice production systems face two opposing challenges: the need to increase production to accommodate the world's growing population while simultaneously reducing greenhouse gas (GHG) emissions. Adaptations to drainage regimes are one of the most promising options for methane mitigation in rice production. Whereas several studies have focused on mid-season drainage (MD) to mitigate GHG emissions, early-season drainage (ED) varying in timing and duration has not been extensively studied. However, such ED periods could potentially be very effective since initial available C levels (and thereby the potential for methanogenesis) can be very high in paddy systems with rice straw incorporation. This study tested the effectiveness of seven drainage regimes varying in their timing and duration (combinations of ED and MD) to mitigate CH 4 and N 2 O emissions in a 101-day growth chamber experiment. Emissions were considerably reduced by early-season drainage compared to both conventional continuous flooding (CF) and the MD drainage regime. The results suggest that ED+MD drainage may have the potential to reduce CH 4 emissions and yield-scaled GWP by 85-90% compared to CF and by 75-77% compared to MD only. A combination of (short or long) ED drainage and one MD drainage episode was found to be the most effective in mitigating CH 4 emissions without negatively affecting yield. In particular, compared with CF, the long early-season drainage treatments LE+SM and LE+LM significantly (p<0.01) decreased yield-scaled GWP by 85% and 87% respectively. This was associated with carbon being stabilised early in the season, thereby reducing available C for methanogenesis. Overall N 2 O emissions were small and not significantly affected by ED. It is concluded that ED+MD drainage might be an effective low-tech option for small-scale farmers to reduce GHG emissions and save water while maintaining yield., (Copyright © 2017 The Authors. Published by Elsevier B.V. All rights reserved.)

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 degrees C or at 600 degrees 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.

The greenhouse gas nitrous oxide (N(2)O) can be both formed and consumed by microbial processes in the soil. As these processes fractionate strongly in favour of (14)N and (16)O, delta(15)N and delta(18)O gradients of N(2)O in the soil profile may elucidate patterns of N(2)O formation, consumption or emission to the atmosphere. We present the first in situ data of such gradients over time for a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.). On two adjacent fields in 2002 and 2003, topsoil N(2)O fluxes were measured and the soil atmosphere was regularly sampled for N(2)O concentrations, delta(15)N and delta(18)O signatures of N(2)O at depths of 18, 48 and 90 cm during approximately 400 days. During the entire sampling period, the N(2)O concentrations were the highest and the delta(15)N signatures the lowest in the subsoil (48 or 90 cm depth) as compared with the topsoil, indicating production of N(2)O in the subsoil. For delta(15)N, differences greater than 30 per thousand between topsoil and subsoil on the same date were regularly observed. The highest N(2)O concentration of 100385 microL m(-3) at 90 cm depth on 1 July 2003, was preceded by the lowest delta(15)N value of -43.5 per thousand one week earlier. This was followed by a 150-day general decrease of N(2)O concentrations at 90 cm depth to 1723 microL m(-3) and a simultaneous enrichment of delta(15)N to +7.1 per thousand, mostly without a significant topsoil flux. There was a negative logarithmic relationship between N(2)O concentration at 90 cm depth and its delta(15)N signature. This relationship indicated a delta(15)N signature of -40 to -45 per thousand during the production of N(2)O in the subsoil, and a subsequent enrichment during the consumption of N(2)O. We conclude that the isotopic signature of the N(2)O topsoil flux is the result of various processes of consumption and production at different depths in the soil profile. It is therefore not a reliable estimator for the overall delta(15)N signature of N(2)O in the soil atmosphere, nor for indirect losses of N(2)O to the environment. Therefore, these findings will pose a further challenge to ongoing efforts to draw up a global isotopic budget for N(2)O., (Copyright 2005 John Wiley & Sons, Ltd.)