Labile Carbon from Artificial Roots Alters the Patterns of N2O and N2 Production in Agricultural Soils.

Labile carbon (C) continuously delivered from the rhizosphere profoundly affects terrestrial nitrogen (N) cycling. However, nitrous oxide (N2O) and dinitrogen (N2) production in agricultural soils in the presence of continuous root C exudation with applied N remains poorly understood. We conducted an incubation experiment using artificial roots to continuously deliver small-dose labile C combined with 15N tracers to investigate N2O and N2 emissions in agricultural soils with pH and organic C (SOC) gradients. A significantly negative exponential relationship existed between N2O and N2 emissions under continuous C exudation. Increasing soil pH significantly promoted N2 emissions while reducing N2O emissions. Higher SOC further promoted N2 emissions in alkaline soils. Native soil-N (versus fertilizer-N) was the main source of N2O (average 67%) and N2 (average 80%) emissions across all tested soils. Our study revealed the overlooked high N2 emissions, mainly derived from native soil-N and strengthened by increasing soil pH, under relatively real-world conditions with continuous root C exudation. This highlights the important role of N2O and N2 production from native soil-N in terrestrial N cycling when there is a continuous C supply (e.g., plant-root exudate) and helps mitigate emissions and constrain global budgets of the two concerned nitrogenous gases.

Reducing the environmental impact of rice production in subtropical India by minimising reactive nitrogen loss.

The future of reactive nitrogen (N) for subtropical lowland rice to be characterised under diverse N-management to develop adequate sustainable practices. It is a challenge to increase the efficiency of N use in lowland rice, as N can be lost in various ways, e.g., through nitrous oxide (N2O) or dinitrogen (N2) emissions, ammonia (NH3) volatilization and nitrate (NO3-) leaching. A field study was carried out in the subsequent wet (2021) and dry (2022) seasons to assess the impacts of different N management strategies on yield, N use efficiency and different N losses in a double-cropped rice system. Seven different N-management practices including application of chemical fertilisers, liquid organic fertiliser, nitrification inhibitors, organic nutrient management and integrated nutrient management (INM) were studied. The application of soil test-based neem-coated urea (NCU) during the wet season resulted in the highest economic yield, while integrated nutrient management showed the highest economic yield during the dry season. Total N losses by volatilization of NH3, N2O loss and leaching were 0.06-4.73, 0.32-2.14 and 0.25-1.93 kg ha-1, corresponding to 0.06-5.84%, 0.11-2.20% and 0.09-1.81% of total applied N, respectively. The total N-uptake in grain and straw was highest in INM (87-89% over control) followed by the soil test-based NCU (77-82% over control). In comparison, recovery efficiency of N was maximum from application of NCU + dicyandiamide during both the seasons. The N footprint of paddy rice ranged 0.46-2.01 kg N-eq. t-1 during both seasons under various N management. Ammonia volatilization was the process responsible for the largest N loss, followed by N2O emissions, and NO3- leaching in these subtropical lowland rice fields. After ranking the different N management practices on a scale of 1-7, soil test-based NCU was considered the best N management approach in the wet year 2021, while INM scored the best in the dry year 2022.

Challenges of accounting nitrous oxide emissions from agricultural crop residues.

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2 O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2 O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2 O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N2 O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2 O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2 O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N2 O emissions.

Using nitrification inhibitors and deep placement to tackle the trade-offs between NH3 and N2 O emissions in global croplands.

Ammonia (NH3 ) and nitrous oxide (N2 O) are two important air pollutants that have major impacts on climate change and biodiversity losses. Agriculture represents their largest source and effective mitigation measures of individual gases have been well studied. However, the interactions and trade-offs between NH3 and N2 O emissions remain uncertain. Here, we report the results of a two-year field experiment in a wheat-maize rotation in the North China Plain (NCP), a global hotspot of reactive N emissions. Our analysis is supported by a literature synthesis of global croplands, to understand the interactions between NH3 and N2 O emissions and to develop the most effective approaches to jointly mitigate NH3 and N2 O emissions. Field results indicated that deep placement of urea with nitrification inhibitors (NIs) reduced both emissions of NH3 by 67% to 90% and N2 O by 73% to 100%, respectively, in comparison with surface broadcast urea which is the common farmers' practice. But, deep placement of urea, surface broadcast urea with NIs, and application of urea with urease inhibitors probably led to trade-offs between the two gases, with a mitigation potential of -201% to 101% for NH3 and -112% to 89% for N2 O. The literature synthesis showed that deep placement of urea with NIs had an emission factor of 1.53%-4.02% for NH3 and 0.22%-0.36% for N2 O, which were much lower than other fertilization regimes and the default values recommended by IPCC guidelines. This would translate to a reduction of 3.86-5.47 Tg N yr-1 of NH3 and 0.41-0.50 Tg N yr-1 of N2 O emissions, respectively, when adopting deep placement of urea with NIs (relative to current practice) in global croplands. We conclude that the combination of NIs and deep placement of urea can successfully tackle the trade-offs between NH3 and N2 O emissions, therefore avoiding N pollution swapping in global croplands.

Effects of pharmaceuticals on the nitrogen cycle in water and soil: a review.

The effects of pharmaceuticals on the nitrogen cycle in water and soil have recently become an increasingly important issue for environmental research. However, a few studies have investigated the direct effects of pharmaceuticals on the nitrogen cycle in water and soil. Pharmaceuticals can contribute to inhibition and stimulation of nitrogen cycle processes in the environment. Some pharmaceuticals have no observable effect on the nitrogen cycle in water and soil while others appeared to inhibit or stimulate for it. This review reports on the most recent evidence of effects of pharmaceuticals on the nitrogen cycle processes by examination of the potential impact of pharmaceuticals on nitrogen fixation, nitrification, ammonification, denitrification, and anammox. Research studies have identified pharmaceuticals that can either inhibit or stimulate nitrification, ammonification, denitrification, and anammox. Among these, amoxicillin, chlortetracycline, ciprofloxacin, clarithromycin, enrofloxacin, erythromycin, narasin, norfloxacin, and sulfamethazine had the most significant effects on nitrogen cycle processes. This review also clearly demonstrates that some nitrogen transformation processes such as nitrification show much higher sensitivity to the presence of pharmaceuticals than other nitrogen transformations or flows such as mineralization or ammonia volatilization. We conclude by suggesting that future studies take a more comprehensive approach to report on pharmaceuticals' impact on the nitrogen cycle process.

Characterising the biophysical, economic and social impacts of soil carbon sequestration as a greenhouse gas removal technology.

To limit warming to well below 2°C, most scenario projections rely on greenhouse gas removal technologies (GGRTs); one such GGRT uses soil carbon sequestration (SCS) in agricultural land. In addition to their role in mitigating climate change, SCS practices play a role in delivering agroecosystem resilience, climate change adaptability and food security. Environmental heterogeneity and differences in agricultural practices challenge the practical implementation of SCS, and our analysis addresses the associated knowledge gap. Previous assessments have focused on global potentials, but there is a need among policymakers to operationalise SCS. Here, we assess a range of practices already proposed to deliver SCS, and distil these into a subset of specific measures. We provide a multidisciplinary summary of the barriers and potential incentives towards practical implementation of these measures. First, we identify specific practices with potential for both a positive impact on SCS at farm level and an uptake rate compatible with global impact. These focus on: (a) optimising crop primary productivity (e.g. nutrient optimisation, pH management, irrigation); (b) reducing soil disturbance and managing soil physical properties (e.g. improved rotations, minimum till); (c) minimising deliberate removal of C or lateral transport via erosion processes (e.g. support measures, bare fallow reduction); (d) addition of C produced outside the system (e.g. organic manure amendments, biochar addition); (e) provision of additional C inputs within the cropping system (e.g. agroforestry, cover cropping). We then consider economic and non-cost barriers and incentives for land managers implementing these measures, along with the potential externalised impacts of implementation. This offers a framework and reference point for holistic assessment of the impacts of SCS. Finally, we summarise and discuss the ability of extant scientific approaches to quantify the technical potential and externalities of SCS measures, and the barriers and incentives to their implementation in global agricultural systems.

A critical review of the impacts of cover crops on nitrogen leaching, net greenhouse gas balance and crop productivity.

Cover crops play an increasingly important role in improving soil quality, reducing agricultural inputs and improving environmental sustainability. The main objectives of this critical global review and systematic analysis were to assess cover crop practices in the context of their impacts on nitrogen leaching, net greenhouse gas balances (NGHGB) and crop productivity. Only studies that investigated the impacts of cover crops and measured one or a combination of nitrogen leaching, soil organic carbon (SOC), nitrous oxide (N2 O), grain yield and nitrogen in grain of primary crop, and had a control treatment were included in the analysis. Long-term studies were uncommon, with most data coming from studies lasting 2-3 years. The literature search resulted in 106 studies carried out at 372 sites and covering different countries, climatic zones and management. Our analysis demonstrates that cover crops significantly (p < 0.001) decreased N leaching and significantly (p < 0.001) increased SOC sequestration without having significant (p > 0.05) effects on direct N2 O emissions. Cover crops could mitigate the NGHGB by 2.06 ± 2.10 Mg CO2 -eq ha-1  year-1 . One of the potential disadvantages of cover crops identified was the reduction in grain yield of the primary crop by ≈4%, compared to the control treatment. This drawback could be avoided by selecting mixed cover crops with a range of legumes and non-legumes, which increased the yield by ≈13%. These advantages of cover crops justify their widespread adoption. However, management practices in relation to cover crops will need to be adapted to specific soil, management and regional climatic conditions.

Weakened growth of cropland-N2 O emissions in China associated with nationwide policy interventions.

China has experienced rapid agricultural development over recent decades, accompanied by increased fertilizer consumption in croplands; yet, the trend and drivers of the associated nitrous oxide (N2 O) emissions remain uncertain. The primary sources of this uncertainty are the coarse spatial variation of activity data and the incomplete model representation of N2 O emissions in response to agricultural management. Here, we provide new data-driven estimates of cropland-N2 O emissions across China in 1990-2014, compiled using a global cropland-N2 O flux observation dataset, nationwide survey-based reconstruction of N-fertilization and irrigation, and an updated nonlinear model. In addition, we have evaluated the drivers behind changing cropland-N2 O patterns using an index decomposition analysis approach. We find that China's annual cropland-N2 O emissions increased on average by 11.2 Gg N/year2 (p < .001) from 1990 to 2003, after which emissions plateaued until 2014 (2.8 Gg N/year2 , p = .02), consistent with the output from an ensemble of process-based terrestrial biosphere models. The slowdown of the increase in cropland-N2 O emissions after 2003 was pervasive across two thirds of China's sowing areas. This change was mainly driven by the nationwide reduction in N-fertilizer applied per area, partially due to the prevalence of nationwide technological adoptions. This reduction has almost offset the N2 O emissions induced by policy-driven expansion of sowing areas, particularly in the Northeast Plain and the lower Yangtze River Basin. Our results underline the importance of high-resolution activity data and adoption of nonlinear model of N2 O emission for capturing cropland-N2 O emission changes. Improving the representation of policy interventions is also recommended for future projections.

Oxygen Regulates Nitrous Oxide Production Directly in Agricultural Soils.

Oxygen (O2) plays a critical and yet poorly understood role in regulating nitrous oxide (N2O) production in well-structured agricultural soils. We investigated the effects of in situ O2 dynamics on N2O production in a typical intensively managed Chinese cropping system under a range of environmental conditions (temperature, moisture, ammonium, nitrate, dissolved organic carbon, and so forth). Climate and management (fertilization, irrigation, precipitation, and temperature), and their interactions significantly affected soil O2 and N2O concentrations (P < 0.05). Soil O2 concentration was the most significant factor correlating with soil N2O concentration (r = -0.71) when compared with temperature, water-filled pore space, and ammonium concentration (r = 0.30, 0.25, and 0.26, respectively). Soil N2O concentration increased exponentially with decreasing soil O2 concentrations. The exponential model of N treatments and fertilization with irrigation/precipitation events predicted 74-90% and 58% of the variance in soil N2O concentrations, respectively. Our results highlight that the soil O2 status is the proximal, direct, and the most decisive environmental trigger for N2O production, outweighing the effects of other factors and could be a key variable integrating the aggregated effects of various complex interacting variables. This study offers new opportunities for developing more sensitive approaches to predicting and through appropriate management interventions mitigating N2O emissions from agricultural soils.

Chinese cropping systems are a net source of greenhouse gases despite soil carbon sequestration.

Soil carbon sequestration is being considered as a potential pathway to mitigate climate change. Cropland soils could provide a sink for carbon that can be modified by farming practices; however, they can also act as a source of greenhouse gases (GHG), including not only nitrous oxide (N2 O) and methane (CH4 ), but also the upstream carbon dioxide (CO2 ) emissions associated with agronomic management. These latter emissions are also sometimes termed "hidden" or "embedded" CO2 . In this paper, we estimated the net GHG balance for Chinese cropping systems by considering the balance of soil carbon sequestration, N2 O and CH4 emissions, and the upstream CO2 emissions of agronomic management from a life cycle perspective during 2000-2017. Results showed that although soil organic carbon (SOC) increased by 23.2 ± 8.6 Tg C per year, the soil N2 O and CH4 emissions plus upstream CO2 emissions arising from agronomic management added 269.5 ± 21.1 Tg C-eq per year to the atmosphere. These findings demonstrate that Chinese cropping systems are a net source of GHG emissions and that total GHG emissions are about 12 times larger than carbon uptake by soil sequestration. There were large variations between different cropping systems in the net GHG balance ranging from 328 to 7,567 kg C-eq ha-1  year-1 , but all systems act as a net GHG source to the atmosphere. The main sources of total GHG emissions are nitrogen fertilization (emissions during production and application), power use for irrigation, and soil N2 O and CH4 emissions. Optimizing agronomic management practices, especially fertilization, irrigation, plastic mulching, and crop residues to reduce total GHG emissions from the whole chain is urgently required in order to develop a low-carbon future for Chinese crop production.

Nitrous Oxide Emissions Increase Exponentially When Optimum Nitrogen Fertilizer Rates Are Exceeded in the North China Plain.

The IPCC assume a linear relationship between nitrogen (N) application rate and nitrous oxide (N2O) emissions in inventory reporting, however, a growing number of studies show a nonlinear relationship under specific soil-climatic conditions. In the North China plain, a global hotspot of N2O emissions, covering a land as large as Germany, the correlation between N rate and N2O emissions remains unclear. We have therefore specifically investigated the N2O response to N applications by conducting field experiments with five N rates, and high-frequency measurements of N2O emissions across contrasting climatic years. Our results showed that cumulative and yield-scaled N2O emissions both increased exponentially as N applications were raised above the optimum rate in maize ( Zea mays L.). In wheat ( Triticum aestivum L.) there was a corresponding quadratic increase in N2O emissions with the magnitude of the response in 2012-2013 distinctly larger than that in 2013-2014 owing to the effects of extreme snowfall. Existing empirical models (including the IPCC approach) of the N2O response to N rate have overestimated N2O emissions in the North China plain, even at high N rates. Our study therefore provides a new and robust analysis of the effects of fertilizer rate and climatic conditions on N2O emissions.

The legacy effect of long-term nitrogen fertilization on nitrous oxide emissions.

The primary driver of increasing atmospheric concentrations of nitrous oxide (N2O) is the use of organic and synthetic fertilizer to increase agricultural crop production. Current global estimates are based on IPCC N2O emission factor (EF) calculations, although there are shortcomings as many of the N2O EFs are derived from measurements during the cropping season. These neglect the fallow season, and do not adequately account for double or even triple cropping systems or legacy effects on soil N2O emissions in the following year. In this study, we assessed the legacy effect of fertilization on soil N2O fluxes using data from a long-term double-cropping field experiment with summer maize and winter wheat in rotation, in which no nitrogen (N; NN) and balanced manure with synthetic N (MN) fertilized treatments were switched to allow an assessment of legacy effects. Based on high-frequency measurements of N2O and previous data, we calculated that the historical N fertilization, or legacy effect, explained 23 % of the annual flux of 0.81 kg N ha-1 yr-1 in the first season of observation. In the following three seasons, the legacy effect of the previous N fertilization regime decreased to a negligible level, with N2O emissions mainly driven by in-season fertilization. Our data show that, on average, the seasonal EF for N2O was about 0.11 % higher in response to the previous N fertilization. Our study indicates that the current N2O EF may severely underestimate emissions because studies ignore legacy effects on N2O emissions from zero N plots and only compare zero N with N fertilization treatments for a given season or year to derive seasonal or annual N2O EF.

Assessing the Occurrence and Distribution of Microplastics in Surface Freshwater and Wastewaters of Latvia and Lithuania.

Microplastic concentrations in surface water and wastewater collected from Daugavpils and Liepaja cities in Latvia, as well as Klaipeda and Siauliai cities in Lithuania, were measured in July and December 2021. Using optical microscopy, polymer composition was characterized using micro-Raman spectroscopy. The average abundance of microplastics in surface water and wastewater samples was 16.63 ± 20.29 particles/L. The dominant shape group of microplastics in water was fiber, with dominant colors found to be blue (61%), black (36%), and red (3%) in Latvia. Similar distribution in Lithuania was found, i.e., fiber (95%) and fragments (5%) with dominant colors, such as blue (53%), black (30%), red (9%), yellow (5%), and transparent (3%). The micro-Raman spectroscopy spectra of visible microplastics were identified to be polyethylene terephthalate (33%) and polyvinyl chloride (33%), nylon (12%), polyester (PS) (11%), and high-density polyethylene (11%). In the study area, municipal and hospital wastewater from catchment areas were the main reasons for the contamination of microplastics in the surface water and wastewater of Latvia and Lithuania. It is possible to reduce pollution loads by implementing measures such as raising awareness, installing more high-tech wastewater treatment plants, and reducing plastic use.

Are single global warming potential impact assessments adequate for carbon footprints of agri-food systems?

The vast majority of agri-food climate-based sustainability analyses use global warming potential (GWP100) as an impact assessment, usually in isolation; however, in recent years, discussions have criticised the 'across-the-board' application of GWP100 in Life Cycle Assessments (LCAs), particularly of food systems which generate large amounts of methane (CH4) and considered whether reporting additional and/or alternative metrics may be more applicable to certain circumstances or research questions (e.g. Global Temperature Change Potential (GTP)). This paper reports a largescale sensitivity analysis using a pasture-based beef production system (a high producer of CH4 emissions) as an exemplar to compare various climatatic impact assessments: CO2-equivalents using GWP100 and GTP100, and 'CO2-warming-equivalents' using 'GWP Star', or GWP*. The inventory for this system was compiled using data from the UK Research and Innovation National Capability, the North Wyke Farm Platform, in Devon, SW England. LCAs can have an important bearing on: (i) policymakers' decisions; (ii) farmer management decisions; (iii) consumers' purchasing habits; and (iv) wider perceptions of whether certain activities can be considered 'sustainable' or not; it is, therefore, the responsibility of LCA practitioners and scientists to ensure that subjective decisions are tested as robustly as possible through appropriate sensitivity and uncertainty analyses. We demonstrate herein that the choice of climate impact assessment has dramatic effects on interpretation, with GWP100 and GTP100 producing substantially different results due to their different treatments of CH4 in the context of carbon dioxide (CO2) equivalents. Given its dynamic nature and previously proven strong correspondence with climate models, out of the three assessments covered, GWP* provides the most complete coverage of the temporal evolution of temperature change for different greenhouse gas emissions. We extend previous discussions on the limitations of static emission metrics and encourage LCA practitioners to consider due care and attention where additional information or dynamic approaches may prove superior, scientifically speaking, particularly in cases of decision support.

Soil oxygen depletion and corresponding nitrous oxide production at hot moments in an agricultural soil.

Hot moments of nitrous oxide (N2O) emissions induced by interactions between weather and management make a major contribution to annual N2O budgets in agricultural soils. The causes of N2O production during hot moments are not well understood under field conditions, but emerging evidence suggests that short-term fluctuations in soil oxygen (O2) concentration can be critically important. We conducted high time-resolution field observations of O2 and N2O concentrations during hot moments in a dryland agricultural soil in Northern China. Three typical management and weather events, including irrigation (Irr.), fertilization coupled with irrigation (Fer.+Irr.) or with extreme precipitation (Fer.+Pre.), were observed. Soil O2 and N2O concentrations were measured hourly for 24 h immediately following events and measured daily for at least one week before and after the events. Soil moisture, temperature, and mineral N were simultaneously measured. Soil O2 concentrations decreased rapidly within 4 h following irrigation in both the Irr. and Fer.+Irr. events. In the Fer.+Pre. event, soil O2 depletion did not occur immediately following fertilization but began following subsequent continuous rainfall. The soil O2 concentration dropped to as low as 0.2% (with the highest soil N2O concentration of up to 180 ppmv) following the Fer.+Pre. event, but only fell to 11.7% and 13.6% after the Fer.+Irr. and Irr. events, which were associated with soil N2O concentrations of 27 ppmv and 3 ppmv, respectively. During the hot moments of all three events, soil N2O concentrations were negatively correlated with soil O2 concentrations (r = -0.5, P < 0.01), showing a quadratic increase as soil O2 concentrations declined. Our results provide new understanding of the rapid short response of N2O production to O2 dynamics driven by changes in soil environmental factors during hot moments. Such understanding helps improve soil management to avoid transitory O2 depletion and reduce the risk of N2O production.

A review and meta-analysis of mitigation measures for nitrous oxide emissions from crop residues.

Crop residues are of crucial importance to maintain or even increase soil carbon stocks and fertility, and thereby to address the global challenge of climate change mitigation. However, crop residues can also potentially stimulate emissions of the greenhouse gas nitrous oxide (N2O) from soils. A better understanding of how to mitigate N2O emissions due to crop residue management while promoting positive effects on soil carbon is needed to reconcile the opposing effects of crop residues on the greenhouse gas balance of agroecosystems. Here, we combine a literature review and a meta-analysis to identify and assess measures for mitigating N2O emissions due to crop residue application to agricultural fields. Our study shows that crop residue removal, shallow incorporation, incorporation of residues with C:N ratio > 30 and avoiding incorporation of residues from crops terminated at an immature physiological stage, are measures leading to significantly lower N2O emissions. Other practices such as incorporation timing and interactions with fertilisers are less conclusive. Several of the evaluated N2O mitigation measures implied negative side-effects on yield, soil organic carbon storage, nitrate leaching and/or ammonia volatilization. We identified additional strategies with potential to reduce crop residue N2O emissions without strong negative side-effects, which require further research. These are: a) treatment of crop residues before field application, e.g., conversion of residues into biochar or anaerobic digestate, b) co-application with nitrification inhibitors or N-immobilizing materials such as compost with a high C:N ratio, paper waste or sawdust, and c) use of residues obtained from crop mixtures. Our study provides a scientific basis to be developed over the coming years on how to increase the sustainability of agroecosystems though adequate crop residue management.

Predicting field N2O emissions from crop residues based on their biochemical composition: A meta-analytical approach.

Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N2O). Previous meta-analyses have linked various biochemical properties of crop residues to N2O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N2O emissions from specific residues. Here we combine comprehensive databases for N2O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N2O emissions. On average, crop residue incorporation increased soil N2O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N2O emissions. Crop residue effects on N2O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N2O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N2O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N2O emissions, whereas mature residues with opposite characteristics had marginal effects on N2O. The most important crop types belonging to the immature residue group - cover crops, grasslands and vegetables - are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N2O emissions.

Cropping system design can improve nitrogen use efficiency in intensively managed agriculture.

New agronomic and management approaches are urgently required to meet the challenges of improving resource use efficiency and crop yields in intensive agricultural systems. Here we report the fertilizer N use efficiency (FNUE), fate of fertilizer N and N budgets in newly designed cropping systems as compared with conventional winter wheat-summer maize double cropping (Con. W/M) in the North China Plain. A15N labelling approach was used to quantify FNUE by these new cropping systems which included optimized winter wheat-summer maize (Opt. W/M) with two harvests in one year; winter wheat/summer maize-spring maize (W/M-M) and winter wheat/summer soybean-spring maize (W/S-M) with three harvests in two years, and spring maize (M) with one harvest in one year. The results showed that only 18-20% of fertilizer N was recovered by crops in Con. W/M. Although Opt. W/M significantly increased FNUE to 33%-35% with increased crop yields, it consumed as much groundwater as Con. W/M. The W/M-M, W/S-M and M significantly increased FNUE to 27%-44% and reduced groundwater use and fertilizer N losses when compared to Con. W/M. The W/M-M achieved a comparable grain yield, but W/S-M and M had significantly lower grain yields when compared to Con. W/M. However, grain N harvest in W/S-M was comparable with Con. W/M due to higher grain N content in soybean. Post-anthesis fertilizer N uptake provided little contribution to total N uptake, and accounted for 5%, 12%, 7% and 2% of the average N uptake for winter wheat, spring maize, summer maize and summer soybean, respectively. When taking the second crop into account, Con. W/M recovered 27% of fertilizer N, while it increased to 36%-50% under the new cropping systems. We conclude that W/M-M and W/S-M will deliver significant improvements in the environmental footprints and sustainability of intensively managed cropping systems in the North China Plain.

Using milk vetch (Astragalus sinicus L.) to promote rice straw decomposition by regulating enzyme activity and bacterial community.

The present study determined the dynamic changes of enzyme activity and bacterial community in rice straw (RS) and milk vetch (MV) co-decomposing process. Results showed that mixing RS and MV promoted decomposition. The mixture enhanced ß-glucosidase and ß-cellobiohydrolase activities relative to its monospecific residue during the mid-late stage of decomposition. The mixture enhanced Enterobacteriaceae (monosaccharide decomposing bacteria) abundance during the initial stage of decomposition, and the abundance of Hydrogenispora, Bacteroides, Ruminiclostridium, and Acidobacteriaceae that could hydrolyze fiber during the mid-late stage of decomposition relative to single RS and MV, respectively, which would benefit mixture decomposition. Furthermore, more interconnected and competitive relations existed between the bacteria in the mixture. These results indicated that mixing RS and MV promoted residue decomposition by increasing hydrolytic enzyme activities and changing bacterial community. This study concluded that co-incorporating RS and MV may be recommended as a promising practice for the efficient utilization of RS resources.