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Approximately 8.6% of Swedish agricultural soils are classified as organic soils (Berglund et al. 2010). In the early 19th century, the Swedish government drained peatlands to make land suitable for agricultural production (Berglund 2008). When drained, organic soils are a significant source of CO2 because of the breakdown of organic materials (Ballantyne et al. 2014). In order to reach climate national and international climate goals, the agricultural sector has the important task of reducing its climate impact and thus greenhouse gas (GHG) emissions. For this purpose, the European Union and some Nordic countries see potential in changing land use on organic soils to ley production or perennial green fallow as an alternative to rewetting peatlands. However, there is lacking scientific consensus about the effectiveness of reducing GHG emissions using these interventions. In many studies, different sites or years are compared, which limits the comparability between land uses because of the many variables that influence the outcome (Kasimir-Klemedtsson et al. 1997; Maljanen et al. 2001; Lohila et al. 2004; Beetz et al. 2013), and thus the conclusions that can be taken for future policies. This systematic review aims to answer the question of which land use(s) can be suggested as a valid alternative for decreased GHG emissions on organic soils in temperate and boreal climates. The review will be conducted by establishing a detailed review protocol, following the Collaboration for Environmental Evidence (CEE) guidelines (Pullin et al. 2022), including a methodology for literature search, eligibility screening, data extraction, and critical appraisal. After implementation of the protocol, and if enough valid data can be found, data synthesis, interpretation and a scientific publication about the outcomes will follow. Sources: Beetz, S., Liebersbach, H., Glatzel, S., Jurasinski, G., Buczko, U., & Höper, H. (2013). Effects of land use intensity on the full greenhouse gas balance in an Atlantic peat bog. Biogeosciences, 10(2), 1067–1082. https://doi.org/10.5194/bg-10-1067-2013Berglund, K. (2008). Torvmarken, en resurs i jordbruket igår, idag och även i morgon. In Svensk mosskultur - Odling, torvanvändning och landskapets förändring. (Vol. 41, pp. 483–498). Runefelt, Leif.Berglund, Ö., & Berglund, K. (2010). Distribution and cultivation intensity of agricultural peat and gyttja soils in Sweden and estimation of greenhouse gas emissions from cultivated peat soils. Geoderma, 154(3), 173–180. https://doi.org/https://doi.org/10.1016/j.geoderma.2008.11.035Andrew S Pullin, Geoff K Frampton, Barbara Livoreil, & Gillian Petrokofsky. (2022). Guidelines and Standards for Evidence Synthesis in Environmental Management. Guidelines and Standards for Evidence synthesis in Environmental Management. Version 5.1. https://environmentalevidence.org/information-for-authors/ [5-01-23]Kasimir-Klemedtsson, Å., Klemedtsson, L., Berglund, K., Martikainen, P., Silvola, J., & Oenema, O. (1997). Greenhouse gas emissions from farmed organic soils: a review. Soil Use and Management, 13(s4), 245–250. https://doi.org/https://doi.org/10.1111/j.1475-2743.1997.tb00595.xLohila, A., Aurela, M., Tuovinen, J.-P., & Laurila, T. (2004). Annual CO2 exchange of a peat field growing spring barley or perennial forage grass. Journal of Geophysical Research: Atmospheres, 109(D18). https://doi.org/https://doi.org/10.1029/2004JD004715Maljanen, M., Martikainen, P. J., Walden, J., & Silvola, J. (2001). CO2 exchange in an organic field growing barley or grass in eastern Finland. Global Change Biology, 7(6), 679–692. https://doi.org/https://doi.org/10.1111/j.1365-2486.2001.00437.x

Recent meta-analyses suggest a global potential of cover crops to increase soil organic carbon (SOC), thus contributing to climate change mitigation. However, some studies also found that cover cropping did not affect or even reduced SOC, thus it is uncertain how this effect is controlled. Here we aimed at comprehensively evaluating the potential and mechanisms of carbon (C) sequestration from cover crops in a Danish long-term crop rotation field experiment (LTE) initiated in 1997. We quantified SOC to 1-m depth, and also operationally divided soil organic matter into fractions of particulate organic matter (POM) and mineral associated organic matter (MAOM) to investigate the C saturation status of soils. Moreover, we performed a mescosm study with topsoils where the fate of varying doses (0.1-1.6 mg C g-1 soil) of 14C-labeled cover crop residues (fodder radish, FR; Raphanus sativus L.) and SOC priming were traced in two texturally similar soils having the same long-term management, but different SOC contents (2.0 vs. 2.6% SOC). Our LTE results showed that cover cropping for up two decades had negligible effect on SOC contents in POM and MAOM fractions in the topsoil and in the subsoil. However, the mesocosm study showed considerable net C increases (20-25% of added) when the cover crop C input exceeded 0.3 and 0.6 mg C g-1 in soils with 2.0 and 2.6% SOC, respectively. This was due a combination of new SOC formation and priming effects shifting from positive to negative. Collectively the LTE and mesocosm study suggests that buildup of SOC stock was not essentially constrained by soil C saturation, but rather by the low productivity and C input from cover crops. Our study suggests that agricultural management practices should be adopted (e.g., species choice and sowing time) to achieve a cover crop C input that exceeds a certain threshold to ensure effective C sequestration.

Perennial crops can be as a sustainable alternative to annual crops owing to plant traits and management practices that improve productivity and may contribute to soil carbon (C) accumulation. However, our understanding of the mechanisms behind the potential differences in C stocks between perennials and annuals is incomplete, especially in terms of how the changers and drivers vary at different soil depths. Based on a 10-year cropping experiment in Denmark with perennials (tall fescue, grass-legume mixture) and annuals (triticale monoculture, triticale in a crop rotation), we investigated soil C stock changes and driving mechanisms at depths of 0-20 cm (topsoil) and 20-50 cm (subsoil). We observed that tall fescue and grass-legume mixture systems increased soil C stock by 6-20% in the topsoil as compared to annual crops. In the subsoil, the tall fescue system even enhanced soil C storage by up to 56%, but there was no difference in soil C stock between grass-legume mixture, triticale, and triticale in a rotation. Most importantly, we found that the major determinants of soil C stock depended on soil depth. In the topsoil, enzymes exerted a dominant effect on soil C stock. Perennials with low C/N for aboveground biomass and high root biomass seemed to depress oxidase (phenol oxidase and peroxidase) activities and stimulated the nutrient-acquiring enzymes (leucine amino peptidase, β-1,4-N-acetylglucosaminidase), thus depressing the decomposition of recalcitrant C and maintaining plant growth, which facilitated soil C storage. In the subsoil, microbial biomass, rather than the balance between functional enzymes, seemed to be controlling the soil C storage. In their entirety, our results highlight that it is feasible to enhance soil C storage in systems with perennials with higher aboveground biomass quality and root biomass. Furthermore, there is a link to biological drivers (i.e., extracellular enzyme activity and microbial biomass), which may play a differential role in topsoil and subsoil. With improved mechanistic understanding, such biological drivers of soil C stock for agricultural systems should be considered in Earth system models to improve the accuracy of predicting agricultural soil C dynamics.

Biochar is an aspirational strategy for long-term carbon sequestration in soil, emergent in guidelines by the Intergovernmental Panel on Climate Change (IPCC). Yet, the rate and pathways of biochar mineralization remain uncertain, and information is scarce on the role of soil temperature. Recent studies predicting the 100-yr stability of biochar in soil use a profile of temperature sensitivity (Q10) for biochar mineralization that deviates markedly from common biochemical temperature relationships, especially at mean annual temperatures of 0-10°C, which prevail in many temperate soils. Here, we compared estimates of biochar stability using (i) empirical Q10 data and (ii) Arrhenius activation energies for biochar mineralization similar to those for other recalcitrant biomolecules. The results indicate that empirical Q10 data used so far overestimate the long-term stability of biochar in soils at 0-10°C, but underestimate the stability at >10°C. The size of these effects increases with higher molar ratio of hydrogen to organic carbon (H/Corg) in the biochar, meaning that predictions for labile biochars are more uncertain. We conclude that care should be taken when normalizing biochar stability data to prevailing soil temperatures and call for further studies to document the temperature sensitivity of biochar mineralization

Under the framework of Circular Economy, EU Green Deal and UN Sustainable Development Goals the use of organic amendments is highly promoted as a cost-efficient solution to improve soil quality and agrosystem sustainability. Nonetheless, their agronomic use comes with an uncertainty of their potential to release ample plant-available N, and to emit soil greenhouse gasses.We investigated short-term (90 d) soil N dynamics of a sandy soil mesocosms receiving four organic amendments (50 t/ha) (i) cow manure compost (CMC), (ii) food waste compost (FWC), (iii) used digestate substrate (UDS) and (iv) municipal sewage sludge (MSS), without and with N fertilization (200 kg/ha; urea-N). An unamended soil mesocosm was included as control. During the incubation soil NO3-, NH4+, N2O and CO2 were regularly monitored. At the end of the incubation potential N mineralization (AMN), total Kjeldahl N, organic C, and trace-metals were determined.During the incubation, organic amendments increased ~6x soil NO3- availability (AUC) than C, however soils receiving MSS had similar AUC NO3- to the fertilized soils (~13x). Only MSS increased ~5x available soil NH4+ (AUC) relative to unamended soil. Co-application of urea and organic amendments increased AUC NO3- and AUC NH4+ ~5x, relative to the unamended soil. Organic amendments increased cum. N2O emission by 10, 20, 1% and 13x relative to the control, and additional 20% increase when urea was co-applied for CMC, FWC, UDS and MSS, respectively. Similarly, organic amendments increased cum. CO2 emission by 20, 40, 140, and 1% relative to the control, and additional 10% increase when urea was co-applied for CMC, FWC, UDS and MSS, respectively.At the end, total N increased by 37, 23, 80 and 20% relative to the unamended soil, and an additional 20% when urea was co-applied for CMC, FWC, UDS and MSS, respectively. Org. C increased by 30, 20, 4 and 50%, and an additional 7% when urea was co-applied for CMC, FWC, UDS and MSS, respectively. AMN increased 2, 50 and 220% and an additional 20% when urea was co-applied for CMC, FWC and MSS, respectively, whereas no change was observed for soil receiving UDS relative to the control. Treatments receiving MSS without and with N had relatively higher amounts of total Mg, Zn, Mn, Cu and S than the unamended soil, whereas no significant differences in trace-metals were observed for CMC, FWC and UDS treatments.In conclusion, our preliminary results indicate that co-application of organic amendments with urea-N could potentially fuel CO2 and N2O emissions from soils, thus offsetting any favorable aspects of the aforementioned policies. Organic amendment, urea-N and their interaction were significant factors (p≤0.05) driving CO2 and N2O emissions. The quality and composition of the amendments may stimulate soil microbial N transformations, and further investigation will elucidate the intrinsic role of soil microbes and their dynamics regulating CO2 and N2O emissions from soils.The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the 2nd Call for H.F.R.I. Post-Doctoral Research Projects; #01053 awarded to Dr Georgios Giannopoulos. This project was co-implemented with Corteva Agriscience Hellas SA.

The substantial climate change mitigation potential of restoring peatlands, through rewetting and extensifying agriculture to reduce greenhouse gas (GHG) emissions is largely recognized. The green deal in Denmark aims at restoring 100,000 ha peatlands by 2030. This area corresponds to more than half of the Danish peatland, with an expected reduction of GHG emissions of almost half of the entire land use, land use change and forestry (LULUFC) emissions. Recent advances established the functional relationship between hydrological regimes, i.e. water table depth (WTD), and CO2 and CH4 emissions. This builds the basis for science-based tools to evaluate and prioritize peatland restoration projects. With this article, we lay the foundation of such a development by developing a high-resolution WTD map for Danish peatlands. Further we define WTD repose functions (CO2 and CH4) fitted to Danish flux data to derive a national GHG emission estimate for peat soils. We estimate the annual GHG emissions to be 2.6 Mt CO2-eq, which is around 15 % lower than previous estimates. Lastly, we investigate alternative restoration scenarios and identify substantial differences in the GHG reduction potential depending on the prioritization of fields in the rewetting strategy. If wet fields are prioritized, which is not unlikely in a context of a voluntary bottom-up approach, the GHG reduction potential is just 30 % for the first 10,000 ha with respect to a scenario that prioritizes drained fields. This underpins the importance of the proposed framework linking WTD and GHG emissions to guide a spatially differentiated peatland restoration.

Denitrifying woodchip bioreactors are used at field-scale to remove nitrate from agricultural drainage water before it contaminates aquatic ecosystems. However, low microbial activity at cold temperatures during the winter half-year may cause the efficiency of nitrate removal to drop below 20%. Here, we isolated 134 strains of denitrifying bacteria from woodchips and wetland soil to search for candidates for bioaugmentation to improve nitrate removal at cold temperatures. The strains were identified by analyses of 16S rRNA genes and were dominated by Pseudomonadaceae (n = 63), notably the genus Pseudomonas (n = 58). Two candidates for bioaugmentation, P. proteolytica WBRS105 and P. chlororaphis PSS4, were selected among six strains that were characterized in detail by assays of growth and denitrification at 5 and 10 °C, combined with genome analysis. Both strains possessed the genes for complete denitrification and degradation of carbon substrates such as cellulose, hemicellulose and lignin. Also, both strains were able to grow with extracts from woodchips as sole sources of energy and carbon. Bioaugmentation under laboratory conditions indicated that the two strains were able to compete with an established microbial community in enrichment cultures at 5 °C and enhance nitrate removal rates by 27–37% when applied to woodchips from an operating full-scale bioreactor. The successful bioaugmentation of woodchip microbiomes at low temperature is encouraging, although the increased rate obtained under laboratory conditions should be further tested, e.g., under pilot-scale conditions, to verify the potential of the technology to increase the efficiency of bioreactors treating agricultural drainage water.

Acidic soils cover about 30% of the world's land. Liming is a management practice applied worldwide to reduce the negative effects of acidification on soil fertility and plant growth. Liming also affects the biotic and abiotic soil properties controlling the production and consumption of the greenhouse gases (GHGs) carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). Although our understanding of how liming regulates net GHG emissions is increasing, the impact of liming on soil biological drivers of GHG emissions has not been quantitatively synthesized. Here we conducted a global meta-analysis using 1474 paired observations from 124 studies to explore the responses of GHG emissions to liming, with a focus on soil biological factors. We show that the N2O mitigation capacity of liming could be linked to (i) increases in bacterial abundance of N2O reductase genes (NosZ) and decreases in fungi:bacteria ratio, both contributing to a lower N2O:N2 product ratio of denitrification; and (ii) reductions in soil mineral nitrogen (N) via stimulation of plant N uptake. The limited evidence available indicates that liming reduced CH4 emissions and the abundance of methanogens, but it had no effect on CH4 uptake and abundance of methanotrophs. Liming-induced increases in soil CO2 emissions can be explained by higher heterotrophic and/or autotrophic respiration. The strong coupling between liming effects on GHG emissions and on soil microbial communities involved in GHG production and consumption can be used to identify strategies to reduce GHGs in response to liming, and to improve process-based models for better predictions of soil GHG emissions.

Water table depth (WTD) modulates greenhouse gas (GHG) emissions from drained peatland soils and rewetting peatlands has been identified as a cost-effective mitigation measure to reduce emissions from the agricultural sector. However, detailed knowledge of the spatial variability of WTD is needed to guide the planning of rewetting measures as well as to upscale GHG emissions from peatlands for national inventories. In this study we developed a high-resolution (10 m) map of long-term mean summertime WTD for Danish peatlands (~9,000 km2) using a gradient boosting decision tree algorithm. The machine learning (ML) model was trained against more than 10,000 WTD observations as well as water levels in over 10,000 groundwater connected lakes and rivers. The WTD observations were transformed to better account for the non-linear relationship between WTD and GHG emissions and the limited WTD range (such as 0 – 50 below ground) in which GHG emissions are most sensitive. Over 20 high-resolution explanatory variables, many of which are satellite based, provided diverse information on topography, groundwater, moisture conditions, land-use and geology to the model. Cross validation was applied to evaluate the accuracy of the trained ML model with special focus on the shallow WTD (mean error= -8cm and mean absolute error = 18 cm). The horizontal and vertical distance to the nearest waterbody as well as organic content of the soil and land surface temperature were among the most important explanatory variables of the trained ML model. The WTD map was subsequently applied as input to two recently developed WTD-dependent GHG emission models to upscale GHG emissions from Danish peatlands. For this purpose, the mean summertime WTD map had to be corrected to represent mean annual conditions. Lastly, simple rewetting scenarios, i.e. decrease in WTD, were applied to elucidate the potentials of rewetting as mitigation measure.

Denitrifying woodchip bioreactors (WBRs) remove nitrate (NO3−) from agricultural drainage water at field-scale, but their efficacy at cold temperatures remains uncertain. This study shows how hydraulic residence time (HRT) controls NO3− removal and environmental side-effects of WBRs at low water temperature under pilot-scale conditions with controlled operation of nine WBRs (94 dm3). Hydraulic properties were assessed by a bromide tracer test, and NO3− removal, emissions of nitrous oxide (N2O) and methane (CH4), and losses of dissolved organic carbon (DOC) were measured at HRTs of 5–30 h. Inlet NO3− concentrations were increasingly reduced at higher HRTs. The relationship between HRT and the efficiency (%) of NO3− removal was linear (Radj2 = 0.94), while the relationship between HRT and NO3− reduction rates (NRR) was logistic (Radj2 = 0.88). Gaseous emissions of N2O were equally low at HRTs of 10–30 h, but higher at 5 h (P < 0.05). Methane fluxes were small, but with consistent emissions at HRTs of 20–30 h and uptake at 5–15 h. HRT had limited effect on effluent DOC concentrations, but strong effect on mass losses that were five-fold higher (320 mg L−1) at the HRT of 5 h than at 30 h. In summary, at cold temperatures HRTs of ≤ 20 h resulted in suboptimal NRR, accelerating DOC losses, and increased risk of N2O losses at least below a threshold HRT of 5–10 h. HRTs of 20–30 h gave maximal NRR, smallest losses of DOC and N2O, but an increased risk of CH4 emissions.

Liming of acidic soils to regulate pH for crop growth may decrease emissions of nitrous oxide (N2O) due to direct effects of pH on the synthesis of N2O reductases by denitrifying bacteria. However, liming also changes general pH-dependent soil properties, including availability of phosphorus (P), with a feedback on N2O fluxes that remains largely unknown. Here we used a mesocosm approach to study the combined role of liming and P in regulating N2O fluxes from denitrification in an arable coarse sandy soil where N2O emissions under field condition coincided with rainfall events and irrigation, which facilitated anoxia. Soils from three long-term liming treatments (0, 4, and 12 Mg ha-1) with resulting pH(CaCl2) of 3.6, 4.7 and 6.3 were incubated at original bulk density first at 60% water filled pore space (WFPS) and successively at 75% WFPS with added nitrate, inorganic P (0 and 10 μg P g-1 soil) and glucose as labile carbon. N2O fluxes were measured during 28 days and were supplemented with measurements of CO2 fluxes, microbial biomass, potential denitrification, and acid phosphatase activity. The results showed a nonlinear response of N2O fluxes to liming rates, with highest fluxes at the intermediate liming level (4 Mg ha-1). Furthermore, inorganic P stimulated N2O fluxes only at the intermediate liming level. Assays of potential denitrification indicated that the N2O/(N2O + N2) product ratio decreased consistently with increasing liming rates, but total N2O fluxes responded nonlinearly likely due to combined effects on N2O/(N2O + N2) product ratios and total denitrification rates. The results suggest that liming and P addition interact on microbial properties and N2O emissions from acidic arable soils and may not follow linear trends. This makes it uncertain to predict and model the resulting net effect, which may depend on the actual pH range and P availability from the unlimed to the limed treatments.

Woodchip bioreactors are increasingly used as tools to mitigate nitrogen (N) pollution from agricultural drainage water. They consist of a basin filled with woodchip material through which N contaminated drainage water can flow. During the water transport through the filter matrix, oxygen is rapidly depleted and denitrification removes a fraction of the nitrate N present in the water. However, the N removal efficiency of the bioreactors varies significantly both across systems and seasonally. Furthermore, denitrification can also produce nitrous oxide, which is a potent greenhouse gas. Here, we investigated how variation in hydraulic residence time influenced N removal efficiency and nitrous oxide emissions at eight woodchip bioreactors of different flow designs, monitored for 2–4 years. We also characterised the relative abundance of genes involved in the N cycle at three of the bioreactors using metagenomics. Our results showed that total N removal was 17–73% of the yearly incoming N and that it was influenced by hydraulic residence time and water temperature. Nitrous oxide emissions were variable among the different bioreactors and were higher when the hydraulic residence time was less than 60 h. However, the yearly nitrous oxide release did not exceed 2.4% of the nitrate removal (on N atom basis) and the mean among the bioreactors was 0.6%. Although there were marked differences in nitrate removal and nitrous oxide emissions, there were no clear differences in the relative abundance of N-cycling genes among and within three tested bioreactors. Yet, denitrification genes greatly outnumbered genes related to dissimilatory nitrate reduction to ammonium. Overall, our study showed that all eight bioreactors were effective in removing N from agricultural drainage water and that nitrous oxide emissions were low, especially at hydraulic residence times of 60 h or more.

Detailed knowledge of the nitrate (NO3−) attenuation capacity of different landscapes is essential to support current aspirations of locally differentiated fertilizer nitrogen (N) regulation for protection of aquatic ecosystems. In this context, we studied the spatial heterogeneity of NO3−, ammonium (NH4+), and total N concentrations in pore water collected in an artificially drained sub-catchment and complemented this with measurements of apparent electrical conductivity (ECa), potential denitrification rates, and basic soil physico-chemical properties. Piezometer nests were installed in transects to represent a range of ECa and slopes in the sub-catchment and were then categorized according to predetermined classifications to delineate the different redox environments. Water samples were collected (February to August 2017) from piezometers that were installed in nests with screen depths at 25–75 cm, 85–115 cm and 125–165 cm. For transects with an average ECa higher than 20 mS m−1, NO3− concentrations were −1 at the depth slightly above the drainage depth, whereas transects with an average ECa of −1 had NO3− concentrations ranging from 0.2 to 15 mg N L−1. Transects with ECa higher than 20 mS m−1 had the highest average clay content and estimated water contents and, possibly, higher macropore connectivity across the soil profile, which could enhance NO3− transport and the potential for development of anaerobic microsites facilitating microbial denitrification. Potential denitrification rates correlated with the presence of amorphous and poorly crystalline iron (Fe) oxides, yet the results were inconclusive regarding the possible role of Fe(II) mediated NO3− reduction. By categorizing the piezometer transects according to their average ECa values, patterns of N concentrations were found that reflected differences in nitrate attenuation capacity, thereby supporting the further pursuit of ECa as a simple and operational tool for mapping the variability in nitrate reduction patterns in the unsaturated zone on a sub-catchment scale.

Organic soils drained for crop production or grazing land are agroecosystems with potentially high but variable emissions of nitrous oxide ( N2O ). The present study investigated the regulation of N2O emissions in a raised bog area drained for agriculture, which is classified as potentially acid sulfate soil. We hypothesised that pyrite ( FeS2 ) oxidation was a potential driver of N2O emissions through microbially mediated reduction of nitrate ( NO 3 - ). Two sites with rotational grass, and two sites with a potato crop, were equipped for monitoring of N2O emissions and soil N2O concentrations at the 5, 10, 20, 50 and 100 cm depth during weekly field campaigns in spring and autumn 2015. Further data acquisition included temperature, precipitation, soil moisture, water table (WT) depth, and soil NO 3 - and ammonium ( NH 4 + ) concentrations. At all sites, the soil was acidic, with pH ranging from 4.7 to 5.4. Spring and autumn monitoring periods together represented between 152 and 174 d, with cumulative emissions of 4–5 kg N2O -N ha −1 at sites with rotational grass and 20–50 kg N2O -N ha −1 at sites with a potato crop. Equivalent soil gas-phase concentrations of N2O at grassland sites varied between 0 and 25 µ L L −1 except for a sampling after slurry application at one of the sites in spring, with a maximum of 560 µ L L −1 at the 1 m depth. At the two potato sites the levels of below-ground N2O concentrations ranged from 0.4 to 2270 µ L L −1 and from 0.1 to 470 µ L L −1 , in accordance with the higher soil mineral N availability at arable sites. Statistical analyses using graphical models showed that soil N2O concentration in the capillary fringe (i.e. the soil volume above the water table influenced by tension saturation) was the strongest predictor of N2O emissions in spring and, for grassland sites, also in the autumn. For potato sites in autumn, there was evidence that NO 3 - availability in the topsoil and temperature were the main controls on N2O emissions. Chemical analyses of intact soil cores from the 0 to 1 m depth, collected at adjacent grassland and potato sites, showed that the total reduction capacity of the peat soil (assessed by cerium(IV) reduction) was much higher than that represented by FeS2 , and the concentrations of total reactive iron (TRFe) were higher than those of FeS2 . Based on the statistical graphical models and the tentative estimates of reduction capacities, FeS2 oxidation was unlikely to be important for N2O emissions. Instead, archaeal ammonia oxidation and either chemodenitrification or nitrifier denitrification were considered to be plausible pathways of N2O production in spring, whereas in the autumn heterotrophic denitrification may have been more important at arable sites.

Drained riparian wetlands have been rewetted and restored in recent decades to remove nutrients, increase biodiversity, and mitigate soil carbon dioxide (CO2) emissions. Yet, few studies have documented the long-term effects of rewetting on complete greenhouse gas (GHG) balances including emissions of CO2, methane (CH4), and nitrous oxide (N2O). Here, we report the complete annual GHG balance of an extensively managed riparian wetland, dominated by creeping bentgrass (Agrostis stolonifera), 12 years after rewetting. Net ecosystem exchange (NEE) of CO2 was measured by transparent closed chambers, and fluxes were partitioned into gross primary production (GPP) and ecosystem respiration (ER) for modelling and extrapolation to annual emissions based on photosynthetically active radiation, ratio vegetation index and temperature. Fluxes of CH4 and N2O were monitored with opaque chambers. Groundwater table (GWT) was close to soil surface for most of the growing period, whereas the site was inundated during winter. Biomass was cut in late summer (8.5 Mg dry weight ha−1), but left on-site according to current management in the area. Annual ER (1360 g CO2-C m−2) exceeded GPP (–1140 g CO2-C m−2), and the ecosystem was a net source of CO2 with NEE of 220 g CO2-C m−2 yr−1. However, fluxes of CH4 (53 g CH4 m−2 yr−1) dominated the annual GHG balance with 405 g CO2-Ceq m−2 yr−1 which contributed 60% to the total GHG balance. Fluxes of N2O were primarily found at times of changing GWT with annual emission of 0.7 g N2O m−2 (50 g CO2-Ceq m−2) equal to 7% of the complete GHG balance. With proper management, rewetting and restoration of wetlands is expected to eventually resume the carbon sink function of natural wetlands, but this was not found in the present study as net fluxes of both CO2 and CH4 were positive. This was mainly attributed to on-site deposition of biomass which apparently stimulated both CO2 and CH4 emissions and partly reduced GPP by acting as a mulch layer. Future studies should focus on managements that increase CO2 uptake and biomass yield, and at the same time reduce CH4 emissions; such managements should avoid on-site deposition of aboveground biomass at rewetted sites. Drained riparian wetlands have been rewetted and restored in recent decades to remove nutrients, increasebiodiversity, and mitigate soil carbon dioxide (CO2) emissions. Yet, few studies have documented the long-termeffects of rewetting on complete greenhouse gas (GHG) balances including emissions of CO2, methane (CH4), andnitrous oxide (N2O). Here, we report the complete annual GHG balance of an extensively managed riparianwetland, dominated by creeping bentgrass (Agrostis stolonifera), 12 years after rewetting. Net ecosystem exchange(NEE) of CO2 was measured by transparent closed chambers, and fluxes were partitioned into grossprimary production (GPP) and ecosystem respiration (ER) for modelling and extrapolation to annual emissionsbased on photosynthetically active radiation, ratio vegetation index and temperature. Fluxes of CH4 and N2Owere monitored with opaque chambers. Groundwater table (GWT) was close to soil surface for most of thegrowing period, whereas the site was inundated during winter. Biomass was cut in late summer (8.5 Mg dryweight ha−1), but left on-site according to current management in the area. Annual ER (1360 g CO2-Cm−2)exceeded GPP (–1140 g CO2-Cm−2), and the ecosystem was a net source of CO2 with NEE of 220 g CO2-Cm−2 yr−1. However, fluxes of CH4 (53 g CH4m−2 yr−1) dominated the annual GHG balance with 405 g CO2-Ceqm−2 yr−1 which contributed 60% to the total GHG balance. Fluxes of N2O were primarily found at times ofchanging GWT with annual emission of 0.7 g N2Om−2 (50 g CO2-Ceqm−2) equal to 7% of the complete GHGbalance. With proper management, rewetting and restoration of wetlands is expected to eventually resume thecarbon sink function of natural wetlands, but this was not found in the present study as net fluxes of both CO2and CH4 were positive. This was mainly attributed to on-site deposition of biomass which apparently stimulatedboth CO2 and CH4 emissions and partly reduced GPP by acting as a mulch layer. Future studies should focus onmanagements that increase CO2 uptake and biomass yield, and at the same time reduce CH4 emissions; suchmanagements should avoid on-site deposition of aboveground biomass at rewetted sites.

Increasing subsoil organic carbon inputs could potentially mitigate climate change by sequestering atmospheric CO2. Yet, microbial turnover and stabilization of labile carbon in subsoils are regulated by complex mechanisms including the availability of nitrogen (N), phosphorous (P), and sulfur (S). The present study mimicked labile organic carbon input using a versatile substrate (i.e. glucose) to address the interaction between carbon-induced mineralization, N-P-S availability, and microbial physiology in topsoil and subsoils from a temperate agricultural sandy loam soil. A factorial incubation study (42 days) showed that net losses of added carbon in topsoil were constant, whereas carbon losses in subsoils varied according to nutrient treatments. Glucose added to subsoil in combination with N was fully depleted, whereas glucose added alone or in combination with P and S was only partly depleted, and remarkably 59–92% of the added glucose was recovered after the incubation. This showed that N limitation largely controlled carbon turnover in the subsoil, which was also reflected by microbial processes where addition of glucose and N increased β-glucosidase activity, which was positively correlated to the maximum CO2 production rate during incubation. The importance of N limitation was substantiated by subsoil profiles of carbon source utilization, where microbial metabolic diversity was mainly related to the absence or presence of added N. Overall, the results documented that labile carbon turnover and microbial functions in a temperate agricultural subsoil was controlled to a large extent by N availability. Effects of glucoseinduced microbial activity on subsoil physical properties remained ambiguous due to apparent chemical effects of N (nitrate) on clay dispersibility.

Drainage of peatlands for agriculture causes substantial degradation and finally loss, including associated ecosystem functions, but also creates emission hotspots of carbon dioxide (CO2). Mean CO2 emission from drained temperate grassland on peat was reported by IPCC as 22.4 (18.3-26.7) MgCO2-eq ha-1 y-1 (95% CI) while methane (CH4) emissions were close to zero. Rewetting of peatlands reduces CO2 emissions while at the same time favouring CH4 emissions. From wet or rewetted nutrient-rich grassland, emissions of CO2 and CH4 were reported by IPCC as 1.8 (-2.8-2.8) and9.8 (0-39) Mg CO2-eq ha-1 y-1, respectively (GWP CH4 = 34). The uncertainties of the estimates reflect the large variation among the reported studies, which could be caused by different climate conditions, vegetation, groundwater table (GWT), peat composition and biogeochemistry. A mesocosm experiment was established to assess biogeochemical causes of variation in CO2 and CH4 flux dynamics under controlled GWT for peatsoils derived from five different Danish bogs and fens. A total number of 75 mesocosms were grouped into three treatments: GWT -40 cm, bare; GWT -5 cm, bare; and GWT -5 cm, cultivated with reed canary grass (RCG). GHG fluxes were measured using opaque chambers at biweekly intervals from July 2019 to 2020 and extrapolated to annual values. Preliminary results indicate significant differences regarding CO2 and CH4 fluxes across all sites and depending on soil biogeochemical and physical properties. Rewetting raised the contribution of CH4 most on soils from Store Vildmose and Vejrumbro with 1.9 to 12.9 t CO2eq ha-1 yr-1 and 0.1 to 5.7 t CO2eq ha-1 yr-1, respectively. On an annual average, these high emissions were with 69 % and 48 % mitigated by the cultivation of RCG in a paludiculture scenario. Further, the results show that CH4 spikes of up to 37.5 mg m-2 h-1 at elevated GWT during warmer summer months may be mitigated by cultivation with RCG, with maximum peaks of 2.1 mg m-2 h-1. Soil analyses highlighted distinct differences in the soil mineralogical composition across sites and soil depths.

The growth of deep-rooted crops has the potential to increase carbon deposition within deep subsoil layers, thereby contributing to climate change mitigation. Yet, the mechanisms of labile carbon stabilization in deep subsoils remain uncertain. Here we studied the effect of nitrogen, phosphorus and sulphur limitations on the microbial mineralization of added glucose and an artificial root exudate mixture (both spiked with 14C-glucose) in intact soil samples from a deep subsoil (5–6 m) over a 10-week incubation period, where the temporal respiration of CO2 and 14CO2 were quantified. We found that mineralization was co-limited by nitrogen and phosphorus, but the cumulative amount of CO2 emitted was significantly larger from artificial root exudates than glucose, suggesting that the nitrogen carried in amino acids in root exudates (here, L-arginine) was sufficient to overcome the nitrogen limitation in this subsoil. Highlights: Labile C substrates and nutrients from deep-rooted crops may affect the potential for C sequestration in subsoil. Deep subsoil was incubated with glucose and artificial root exudates amended with N, P, and S. C mineralization was co-limited by N and P, but less so in soil amended with glucose. Amino acid derived-N in exudates from deep-rooted crops may alleviate the N limitation in deep subsoils.

The German Agricultural Soil Inventory (BZE-LW) is running a general project evaluation as this network is currently in the phase between the first and second sampling campaign, the latter starting in 2023. A one-day meeting was held in Braunschweig to discuss the general results of the first campaign and the organisation planned for the next campaign. The following scientists were present:●Bas van Wesemael, Earth and Life Institute, UCLouvain, Belgium●Nicolas Saby, unité Infosol, INRAE, Orléans, France●Claudy Jolivet, unité Infosol, INRAE, Orléans, France●Heikkinen Jaakko, Natural Resources Institute, Luke, Finland●Lars Elsgaard, Department of Agroecology, Aarhus University, Denmark●Axel Don and Christopher Poeplau, Thünen Institute This report gathers some general comments and ideas organised according to the list of questions raised before and during the workshop.

The implementation of a targeted, site-specific approach on reducing the nitrogen load from agriculture requires fine-scale assessment of soil heterogeneity. In this study, the soil heterogeneity was described through the spatial heterogeneity of the apparent electrical conductivity values (ECa) and its implications to the patterns of redox potential values (Eh) in an intensely tile drained agricultural area. Piezometers and platinum-tipped redox probes for water level and Eh measurements were installed in transects along the assumed subsurface lateral water flow direction at different depths overlying the tile drains. Water level and Eh were determined from February to August 2017. A statistical analysis of the average ECa and slope measurements of each transect showed that they could be classified into four operational soil combinations (SC): high ECa, low slope (SC-1), low ECa, low slope (SC-2), high ECa, high slope (SC-3), low ECa, high slope (SC-4). A significant correlation between the inverted electrical conductivity (EC) values, clay and soil organic carbon (SOC) content suggested that the EC correlates with clay content. The high clay percentage associated with high ECa is assumed to facilitate development of anoxic microsites which led to more reducing conditions. Due to the generally reducing conditions and higher occurrence of saturated piezometers, transects under SC-1 were considered to have the most potential for intense nitrate reduction. In a tile drained setting, classifications based on average ECa and slope values may be utilized as parameters for a simple mapping tool for improved fine scale prediction of potential hotspots for nitrate reduction.

Liming is a common agricultural practice to improve crop yields by raising soil pH. Liming also modulates the production and consumption of greenhouse gases (GHGs) in soils, but the direction and strength of such effects are largely unknown. Since lime application is not a dichotomous variable (application or not), but rather can be dosed according to crop requirements, a critical research gap is whether liming effects on GHG emissions are linear or follow alternate relationships. Based on two-years of data from a long-term field-liming experiment (initiated in 1942), we show that the relationship between an established soil pH gradient (pH of 3.77, 4.92, 6.39, and 6.84 in 1 M KCl extracts) and the emission of GHGs deviates from linearity. Liming increased carbon dioxide (CO2) fluxes from ecosystem respiration, but there were no differences between limed treatments. The increased respiration was due to higher plant carbon (C) inputs and root respiration, enhanced decomposition activity (β-glucosidase) and increased abundance of specific microbial groups (e.g., cellulose-decomposing bacteria). Liming decreased nitrous oxide (N2O) emissions due to increased N2O reduction via denitrification and by promoting plant growth. Compared with native acidic soil, lime application stimulated methane (CH4) oxidation, but the highest liming rate was least stimulatory. This aligned with the soil CH4 oxidation potential, which was modified by liming-induced changes in soil microbial structure as indicated by phospholipid fatty acid analyses. There were no differences in soil organic C (SOC) content between liming levels, apparently because increased ecosystem respiration rates compensated for higher plant C inputs from limed plots. Overall, our results reveal a fundamental ecological trade-off: applying lime at a rate that maximizes crop yield with low N2O emissions may increase CO2 emissions (not compensated by higher SOC in a sandy soil) and decrease CH4 oxidation compared to liming applications below this level.

Despite the importance of subsoil carbon (C) deposition by deep-rooted crops in mitigating climate change and maintaining soil health, the quantification of root C input and its microbial utilization and stabilization below 1 m depth remains unexplored. We studied C input by three perennial deep-rooted plants (lucerne, kernza, and rosinweed) grown in a unique 4-m deep RootTower facility. 13C multiple pulse labeling was applied to trace C flows in roots, rhizodeposition, and soil as well as 13C incorporation into microbial groups by phospholipid fatty acids and the long-term stabilization of microbial residues by amino sugars. The ratio of rhizodeposited 13C in the PLFA and amino sugar pools was used to compare the relative microbial stability of rhizodeposited C across depths and plant species. Belowground C allocation between roots, rhizodeposits, and living and dead microorganisms indicated depth dependent plant investment. Rhizodeposition as a fraction of the total belowground C input declined from the topsoil (0–25 cm) to the deepest layer (360 cm), i.e., from 35%, 45%, and 36%–8.0%, 2.5%, and 2.7% for lucerne, kernza, and rosinweed, respectively, where lucerne had greater C input than the other species between 340 and 360 cm. The relative microbial stabilization of rhizodeposits in the subsoil across all species showed a dominance of recently assimilated C in microbial necromass, thus indicating a higher microbial stabilization of rhizodeposited C with depth. In conclusion, we traced photosynthates down to 3.6 m soil depth and showed that even relatively small C amounts allocated to deep soil layers will become microbially stabilized. Thus, deep-rooted crops, in particular lucerne are important for stabilization and storage of C over long time scales in deep soil.

The effort to increase the sustainable supply of food and fibre is challenged by the potential for increased greenhouse gas (GHG) emissions from farming systems with intensified production systems. This study aimed at quantifying soil N

Increased human‐derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N‐induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N‐induced P limitation. Here we show, using a meta‐analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short‐term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long‐term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short‐ or long‐term studies. Together, these results suggest that N‐induced P limitation in ecosystems is alleviated in the long‐term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought. This study was funded by Aarhus University Centre for Circular Bioeconomy, Aarhus University Research Foundation AUFF Starting Grants (AUFF-E-2019-7-1), and Marie Skłodowska-Curie Individual Fellowship H2020-MSCA-IF-2018 (no. 839806). Ji Chen acknowledges funding support from the National Natural Science Foundation of China (41701292) and China Postdoctoral Science Foundation (2017M610647, 2018T111091) when constructing the databases. César Terrer was supported by a Lawrence Fellow award through Lawrence Livermore National Laboratory (LLNL). This work was performed under the auspices of the U.S. Department of Energy by LLNL under contract DEAC52-07NA27344 and was supported by the LLNL-LDRD Program under Project No. 20-ERD-055. Fernando T. Maestre was supported by the European Research Council (ERC Grant agreement 647038 [BIODESERT]) and Generalitat Valenciana (CIDEGENT/2018/041).

Nitrogen (N) fertilization is an essential agronomic practice, which increases crop yields and improves soil fertility. Globally, more than 110 x 109 kg of chemical N fertilizers are applied each year with urea-N being one of the most affordable options. Upon urea hydrolysis, any portion not assimilated by crops is either volatilized as NH3 or microbially nitrified (i.e., NH4+ oxidized) to leachable NO3- and NO2-. Nitrification inhibitors (NI) are increasingly co-applied as a sustainable agricultural practice and block the process of nitrification, resulting in a temporal increase of NH4+ in the soils. Several studies have documented the effectiveness of NIs in retaining soil NH4+ and increasing crop yields, but less is known about the effects of NIs on the fate of urea–N and the overall impact of NIs on the soil microbial community.In a 60 day soil mesocosm experiment, we investigated the effects of Nitrapyrin (NI; 2-chloro-6-(trichloromethyl)pyridine) co-applied with a selection of urea-based fertilizers: urea (U); U with urease inhibitors (U+UI); methylene-urea (MU); and zeolite-coated urea (ZU), on a typical Mediterranean soil under ambient summer conditions. We showed that NI applied with urea fertilizers resulted in a slower decay of extractable NH4+ with a concurrent increase in NH3 volatilization. Integrated measures of soil NH4+ were 1.5 to 3.3-fold greater when NI was applied. At the same time, there was a 10 to 60% reduction in integrated measures of NO3- and NO2- when NI was applied with the tested fertilizer types, except MU fertilizer where the integrated measures of NO3- and NO2- doubled. Upon urea hydrolysis, the released NH4+ was transformed to NO3- and NO2-, which subsequently decreased in concentration following a typical nitrification - denitrification pathway in the absence of plants. Soil N2O emissions from urea fertilizers were reduced by 40% with UI, 50% with NI, and 66% with NI + UI.Interestingly, 15 days after the application of NI, there was a decrease in bacterial abundance (eub genes; qPCR) in all fertilized treatments. NI dramatically reduced the abundance of ammonia-oxidizing microbes (amoA genes) and there were fewer bacteria associated with denitrification genes (nirK, nirS, nosZ) when NI was applied. At the end of the experiment, there was no significant difference in total N among all fertilized soils. Total N was in excess when compared to the control, and it was a considerable N pool potentially immobilized in microbial biomass in the absence of crops.In conclusion, the use of NI doubled NH4+ retention in the soil and decreased soil N2O emission by 50%, through negatively affecting ammonia oxidizing and denitrifying microbes and subsequently reducing soil available NO3- and NO2-. The application of NIs should be carefully planned and synchronized (timing) with crop growth to reduce subsequent N transformations and N loss to the environment.Keywords: urea, zeolite, methylene-urea, nitrification inhibitor, nitrapyrin, calcareous soil, soil nitrogen

Biochar is related to multiple greenhouse gas (GHG) mitigation potentials, namely nitrous oxide (N2O) mitigation, carbon (C) sequestration and the possible green electricity produced in the pyrolysis process. Whereas the mechanisms behind potential N2O mitigation effects of biochar are still unclear, the mechanisms behind C sequestration is agreed to be caused by C compound conversion towards aromatic structures in the biochar C as a consequence of the heating treatment in the pyrolysis process. The specific recalcitrance of the biochar is, however, difficult to estimate. The possible electricity production from the syn-gas and bio-oil pyrolysis products depends on the feedstock and process temperature, and can contribute considerably to total system GHG mitigation. However, the multiple effects on GHG balances of biochar and pyrolysis represent a complexity, which may best be analysed by a life cycle assessment (LCA) approach. In this study, the average Danish oilseed rape cultivation was set as the reference scenario in an LCA of cultivation related GHG emissions. The reference was compared with two scenarios with theoretical inclusion of pyrolysis and biochar, meaning that the oilseed straw residue was transported to a pyrolysis plant and the biochar was returned to the field in a corresponding amount (ca. 1 Mg ha-1). Transportation, additional field operations, N2O mitigation, electricity production and C sequestration was included, and the latter was calculated by using the concept of avoided atmospheric CO2 load. The latter approach resulted in larger mitigation effects than derived from calculations of just the remaining C in soil. In total, GHG emissions were reduced by 73 to 83% in the two biochar scenarios as compared with the reference scenario, mainly due to increased C sequestration. The study suggests that even low application rates and rather conservative biochar C recalcitrance estimates lead to considerably reductions in GHG emissions from oilseed rape and expectedly other crops.

Liming to increase pH of acidic soils is a common agricultural practice to optimize crop yields, which also modulates greenhouse gas emissions from soils. In particular, soil pH has been identified as a primary regulator of denitrification pathways with enhanced ratio of nitrous oxide (N2O) to dinitrogen (N2) emissions (i.e., enhanced N2O/N2 ratio) at lower soil pH. Therefore liming could represent a potential management option to mitigate soil N2O emissions. However, changes in soil pH have pervasive effects on general microbial activity and on soil properties, including transformations of carbon (C) and bioavailability of phosphorus (P), with a feedback on microbial processes. Thus, the eventual net effects of liming on microbially derived N2O emissions may be complex. The aim of this study was to discern the interaction between liming (soil pH), and availability of C and P in regulating N2O emissions from acidic fertilized agroecosystems. Using coarse sandy soils from a long-term liming field experiment, N2O/N2 ratios from denitrifying enzyme activity was shown to be strongly affected by liming, i.e., with gradually decreasing ratios at increasing soil pH. Although liming acidic soil (pH, 3.6) to almost neutral (pH, 6.4) favored the reduction of N2O to N2, it also enhanced the overall denitrification rate, which eventually resulted in the highest N2O emission from moderately limed treatments (pH, 4.7). Interactions between P availability and denitrification (and N2O emission) occurred, where P addition generally increased cumulative N2O emissions with strongest effect at the moderately limed soil. Mechanistic hypotheses for this effect are discussed. Overall, our results suggest that a critical liming rate should be pursued which may lead to substantial mitigation of N2O emissions from acidic arable soil.

Cover crops represents an agricultural management option, which has the potential to increase soil carbon (C) stocks and contribute to atmospheric CO 2 reduction. However, there is a scarcity of studies quantifying the C inputs from cover crops, particularly into subsoil layers. Furthermore, estimates of plant C inputs to soils often lack accounting of root fragments (here defined as roots in the size class of 0.25–4 mm) and net phyllo- and rhizodeposition. Based on a field experiment with multiple 13C-CO 2 pulse labeling, we investigated the short-term C inputs from cover crop mixtures to 1-m depth in two long-term cropping systems with organic and conventional management. We used a novel 13C-based approach to quantify the amount of C in root fragments, as young fragile cover crop roots are challenging to sample in soils. The results showed that the total belowground C input to 1-m depth (570–1000 kg C ha −1) was similar across long-term management (organic and conventional, with or without a history of cover crops), but with a higher belowground C input for mixtures established without winter vetch (Vicia villosa Roth). The C input to the subsoil (> 25 cm) by cover crops accounted for 11–42% of the total plant belowground C input, which was significantly and negatively correlated to the soil fertility measured as the initial mineral nitrogen (N) content in the topsoil. The results suggest that root exploitation, and thus plant C inputs into the subsoil, was enhanced in relatively N poor cropping systems. Root fragments and net phyllo- and rhizodeposition accounted for 39–59% and 5–37%, respectively, of the total cover crop belowground C input, i.e., both representing significant pools of plant C input. We conclude that future studies on the effects of cover crops on soil C storage need to account for subsoil C pools including root fragments and rhizodeposition to achieve a comprehensive estimation of plant C inputs.

The effort to increase the sustainable supply of food and fibre is challenged by the potential for increased greenhouse gas (GHG) emissions from farming systems with intensified production systems. This study aimed at quantifying soil N2O emissions from smallholder organic and conventional cotton production practices in a semi-arid area, Meatu, Northern Tanzania. Field experiments were conducted to quantify N2O emissions under (i) current practices with organic (3 Mg ha−1 farmyard manure (FYM)) and conventional (30 kg mineral N ha−1) cultivation; (ii) a high input practice with organic (5 Mg ha−1 FYM) and conventional (60 kg mineral N ha−1) cultivation; and (iii) an integrated practice with organic (3 Mg FYM + legume intercropping) and conventional (30 kg N + 3 Mg ha−1 FYM) cultivation. In both organic and conventional farming, control treatments with no fertilizer application were included. The study was performed over two growing seasons, where season 1 was rather wet and season 2 was rather dry. Static chambers were used for in-situ measurement of N2O emission from soil. The current organic and conventional cotton farming practices did not differ (P > 0.05) in cumulative area-scaled and yield-scaled N2O emissions. High input conventional cotton showed higher area scaled N2O emissions than organic cotton during the wetter season, but not during the drier season. The inorganic fertilizer + FYM combination did not differ (P > 0.05) in area- and yield-scaled N2O emissions from conventional practice. Intercropping cotton and legumes did not affect (P > 0.05) N2O emission compared to 3 Mg FYM ha−1. The emission factors for both conventional and organic systems were generally above 1% in the dry season 2, but below 1% in the wetter season 1. The use of organic and inorganic fertilizers at rates up to 60 kg N ha−1, FYM-inorganic fertilizer combination, and cotton-legume intercropping increased yields, while N2O emissions stayed low, in particular with use of mineral fertilizers.

This study was designed to show the effects of harvest time and frequency on biomass nutrient concentrations (total ash, N, P, K, Ca, Mg, Fe, Mn, Cu and Zn) as well as total nutrient removal potential by festulolium and tall fescue cultivated on a nutrient-rich fen peatland. The harvest managements included a three-cut (3C) and three two-cut (2C) systems which differed by two-week delays of first cut as: 2C-early, 2C-mid and 2C-late, representing phenological stages of pre-heading, inflorescence emergence, and flowering, respectively. In the study year, the grasses received 80-16–60 kg N-P-K ha−1 in spring and 80-0–100 kg N-P-K ha−1 after each harvest (except final). Nutrient concentrations and total ash content in the biomass dropped sharply during spring-summer growth. Concentrations of Ca, Mg and Mn were significantly higher in tall fescue biomass during spring-summer growth, but for other nutrients, there were no consistent differences between the two grasses. Total ash in the biomass from 3C managements (1.1 Mg ha−1) was ∼28% higher than in the biomass removed by 2C managements (0.8 Mg ha−1). Similarly, mean annual N removal by 3C (315 kg N ha−1) was 31% higher than by 2C (240 kg N ha−1) managements, but net removals (removed minus applied N) from both managements were similar (75–80 kg ha−1). Net P-removal by 3C (37 kg P ha−1) was higher than by 2C (26 kg P ha−1) managements, whereas total K removals in all managements were close to the applied K amount (i.e., net removals were close to zero). Whereas all crop/management combinations extracted more N and P than applied, the 2C-late approach is recommended since the biomass yield was higher than for the other 2C approaches, and it required less fertilizer and management inputs than the 3C approach.

Nitrous oxide (N2O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N2O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N2O include the application of biochar, but efficacy and mechanisms of N2O suppression are elusive. We measured N2O emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017–2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha−1) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on N2O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N2O emissions (1.71–2.78 kg N ha−1 yr−1). Likewise, no significant effects were found on crop yield, yield-scaled N2O emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed N2O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N2O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha−1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N2O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N2O mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased N2O emissions.