Concepts in modelling N2O emissions from land use

Modelling nitrous oxide (N₂O) emissions from soil is challenging because multiple biological processes are involved that each respond differently to various environmental and soil factors. Soil water content, organic carbon, temperature and pH are often used in models that predict N₂O emissions, yet for each of these factors there are concepts that are not fully understood. Though a ubiquitous measure of soil water for models, the application of functions based on water filled pore space across soils that vary in bulk density is not ideal. Diffusion of gases and solutes in soil are controlled by the volume fractions of air and water present. Across soils with different bulk densities, both of these terms vary at constant water filled pore space. Soil organic carbon influences N₂O emissions in two ways: as a source of energy for denitrifiers and also by driving biological oxygen demand and the creation of anaerobic zones in the soil. Soil temperature influences N₂O emissions through its effect on the activity of microorganisms and enzymes. A variety of temperature response functions have been proposed. The preferred response function should contain a temperature optimum that can be varied in response to climatic conditions to account for microbial adaptation. Soil pH can have direct and indirect influences on rates and product ratios of nitrification and denitrification. The concepts of pH optima and microbial adaptation need to be considered in modelling. Methodological issues such as microsite versus bulk soil measurements and apportioning N₂O fluxes to the various N transformation processes remain an impediment to characterising the influence of pH and other factors on N₂O emissions. Quantifying the response of N₂O emissions to individual factors using regression analysis requires all other factors to be controlled experimentally. Boundary line analysis provides a way of defining the response to a single input variable where other influencing variables are not controlled. Such analyses can aid in the definition of the shape and magnitude of response functions to be incorporated into process simulation models. Process/mechanistic simulation models offer a greater transferability than empirical models but careful consideration of temporal and spatial scale and the availability of data to run these models is critical in developing model structure.     

Spatial variability of nitrous oxide emissions from an Australian irrigated dairy pasture

Understanding spatial variability of emissions of nitrous oxide (N₂O) is essential to understanding of N₂O emissions from soils to the atmosphere and in the design of statistically valid measurement programs to determine plot, farm and regional emission rates. Two afternoon, ‘snap-shot’ experiments were conducted; one in the summer and one in the autumn of 2004, to examine the statistics and soil variables affecting the spatial variability of N₂O emissions at paddock scale. Small, static chambers (mini-chambers) were placed at 100 locations over an 8,100 m² area of irrigated dairy pasture in northern Victoria, Australia. Chamber headspace was sampled for N₂O and soil samples taken below each mini-chamber were analysed for soil nitrate (NO₃ ^-), ammonium (NH₄ ⁺) and other chemical and physical properties known to affect N₂O emissions. The experiments took place immediately after the sequence of grazing, urea application and irrigation. Nitrous oxide emissions and soil variables were analysed using classical statistics to investigate the effect of soil variables on N₂O emissions. Geostatistics were used to investigate spatial patterns of N₂O emissions and soil variables over the measurement area. Nitrous oxide emissions were extremely variable; 45–765 ng N₂O–N m^?2 s^?1 and 20–953 ng N₂O–N m^?2 s^?1 for the two experiments with corresponding averages of 165 and 138 ng N₂O–N m^?2 s^?1. Nitrous oxide emissions showed spatial dependence up to 73 and 51 m for the two experiments. Nitrous oxide emissions showed significant correlation with soil nutrients in decreasing order of NO₃ ^-, NH₄ ⁺ and available-P concentrations. There was no significant correlation of N₂O emissions with measured soil physical properties.     

Global soil nitrous oxide emissions since the preindustrial era estimated by an ensemble of terrestrial biosphere models: Magnitude,attribution, and uncertainty

Our understanding and quantification of global soil nitrous oxide (N₂O) emissions and the underlying processes remain largely uncertain. Here, we assessed the effects of multiple anthropogenic and natural factors, including nitrogen fertilizer (N) application, atmospheric N deposition, manure N application, land cover change, climate change, and rising atmospheric CO₂ concentration, on global soil N₂O emissions for the period 1861–2016 using a standard simulation protocol with seven process‐based terrestrial biosphere models. Results suggest global soil N₂O emissions have increased from 6.3 ± 1.1 Tg N₂O‐N/year in the preindustrial period (the 1860s) to 10.0 ± 2.0 Tg N₂O‐N/year in the recent decade (2007–2016). Cropland soil emissions increased from 0.3 Tg N₂O‐N/year to 3.3 Tg N₂O‐N/year over the same period, accounting for 82% of the total increase. Regionally, China, South Asia, and Southeast Asia underwent rapid increases in cropland N₂O emissions since the 1970s. However, US cropland N₂O emissions had been relatively flat in magnitude since the 1980s, and EU cropland N₂O emissions appear to have decreased by 14%. Soil N₂O emissions from predominantly natural ecosystems accounted for 67% of the global soil emissions in the recent decade but showed only a relatively small increase of 0.7 ± 0.5 Tg N₂O‐N/year (11%) since the 1860s. In the recent decade, N fertilizer application, N deposition, manure N application, and climate change contributed 54%, 26%, 15%, and 24%, respectively, to the total increase. Rising atmospheric CO₂ concentration reduced soil N₂O emissions by 10% through the enhanced plant N uptake, while land cover change played a minor role. Our estimation here does not account for indirect emissions from soils and the directed emissions from excreta of grazing livestock. To address uncertainties in estimating regional and global soil N₂O emissions, this study recommends several critical strategies for improving the process‐based simulations.     

Using the NZ–DNDC model to estimate agricultural N2O emissions in the Manawatu–Wanganui region

Nitrous oxide (N₂O) emissions are difficult to quantify at regional and national scales. There is considerable spatial and temporal variability in N₂O emissions from soil, partly because of variability in the underlying biogenic processes responsible for soil N₂O production. The process-based NZ–DNDC (New Zealand Denitrification-Decomposition) model was used, with georeferenced input data on soils, climate and land use, to map and predict net N₂O emissions from farming in the Manawatu–Wanganui region. The Manawatu–Wanganui region has a temperate, maritime climate and the major agricultural land use is pastoral grazing. We created databases of regional soil, climate and farm management information from various available data sources including national databases of climate, soil type and land use, and national agricultural production statistics. The error introduced by upscaling the model was assessed by comparing results using measured site data with the corresponding predictions using the regional approximations. We also examined the effect of climate conditions by rerunning the 2003 simulation using the climate data for the years ended June 1990 and 2004. The modelled net N₂O emissions for this region for the year ended June 2003 were 4.6?±?1.5 Gg N₂O–N per year. The total fertiliser and excretal N inputs for the region were approximately 224,140 tonnes, so the percentage emitted as N₂O was 2.0?±?0.7%. The modelled net N₂O emissions for the region for the year ended June 1990 were 3.8?±?2.1 Gg N₂O–N per year, indicating annual net N₂O emissions in the Manawatu–Wanganui region between 1990 and 2003 had increased by 0.8?±?0.6 Gg N₂O–N (an increase of about 20%). This change can be attributed to both changes in weather conditions and land use and farm management between 1990 and 2003.     

Afforestation does not necessarily reduce nitrous oxide emissions from managed boreal peat soils

Pristine peatlands have generally low nitrous oxide (N₂O) emissions but drainage and management practices enhance the microbial processes and associated N₂O emissions. It is assumed that leaving peat soils from intensive management, such as agriculture, will decrease their N₂O emissions. In this paper we report how the annual N₂O emission rates will change when agricultural peat soil is either left abandoned or afforested and also N₂O emissions from afforested peat extraction sites. In addition, we evaluated a biogeochemical model (DNDC) with a view to explaining GHG emissions from peat soils under different land uses. The abandoned agricultural peat soils had lower mean annual N₂O emissions (5.5?±?5.4?kg?N?ha^?1) than the peat soils in active agricultural use in Finland. Surprisingly, N₂O emissions from afforested organic agricultural soils (12.8?±?9.4?kg?N?ha^?1) were similar to those from organic agricultural soils in active use. These emissions were much higher than those from the forests on nutrient rich peat soils. Abandoned and afforested peat extraction sites emitted more N₂O, (2.4?±?2.1?kg?N?ha^?1), than the areas under active peat extraction (0.7?±?0.5?kg?N?ha^?1). Emissions outside the growing season contributed significantly, 40% on an average, to the annual emissions. The DNDC model overestimated N₂O emission rates during the growing season and indicated no emissions during winter. The differences in the N₂O emission rates were not associated with the age of the land use change, vegetation characteristics, peat depth or peat bulk density. The highest N₂O emissions occurred when the soil C:N ratio was below 20 with a significant variability within the measured C:N range (13–27). Low soil pH, high nitrate availability and water table depth (50–70?cm) were also associated with high N₂O emissions. Mineral soil has been added to most of the soils studied here to improve the fertility and this may have an impact on the N₂O emissions. We infer from the multi-site dataset presented in this paper that afforestation is not necessarily an efficient way to reduce N₂O emissions from drained boreal organic fields.     

Projecting future N2O emissions from agricultural soils in Belgium

This study analyses the spatial and temporal variability of N₂O emissions from the agricultural soils of Belgium. Annual N₂O emission rates are estimated with two statistical models, MCROPS and MGRASS, which take account of the impact of changes in land use, climate, and nitrogen‐fertilization rate. The models are used to simulate the temporal trend of N₂O emissions between 1990 and 2050 for a 10′ latitude and longitude grid. The results are also aggregated to the regional and national scale to facilitate comparison with other studies and national inventories. Changes in climate and land use are derived from the quantitative scenarios developed by the ATEAM project based on the Intergovernmental Panel on Climate Change‐Special Report on Emissions Scenarios (IPCC‐SRES) storylines. The average N₂O flux for Belgium was estimated to be 8.6 × 10⁶ kg N₂O‐N yr⁻¹ (STD = 2.1 × 10⁶ kg N₂O‐N yr⁻¹) for the period 1990–2000. Fluxes estimated for a single year (1996) give a reasonable agreement with published results at the national and regional scales for the same year. The scenario‐based simulations of future N₂O emissions show the strong influence of land‐use change. The scenarios A1FI, B1 and B2 produce similar results between 2001 and 2050 with a national emission rate in 2050 of 11.9 × 10⁶ kg N₂O‐N yr⁻¹. The A2 scenario, however, is very sensitive to the reduction in agricultural land areas (−14% compared with the 1990 baseline), which results in a reduced emission rate in 2050 of 8.3 × 10⁶ kg N₂O‐N yr⁻¹. Neither the climatic change scenarios nor the reduction in nitrogen fertilization rate could explain these results leading to the conclusion that N₂O emissions from Belgian agricultural soils will be more markedly affected by changes in agricultural land areas.     

Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation

In this paper we discuss three topics concerning N₂O emissions from agricultural systems. First, we present an appraisal of N₂O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N₂O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N₂O emissions from N-fertilized fields (Mitigation).To assess the global emission of N₂O from agricultural soils, the total flux should represent N₂O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N₂O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N₂O production. The assessment of direct and indirect N₂O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N₂O-N ha^-1. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N₂O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N₂O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N₂O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N₂O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N₂O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N₂O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.     

Regionally differentiated estimates of cropland N2O emissions reduce uncertainty in global calculations

Nitrogen (N) additions to cropland soils are the largest source of anthropogenic nitrous oxide (N₂O) emissions and are an important contributor to global greenhouse gas radiative forcing. Progress in understanding controls on N₂O fluxes from soils is demonstrated in increasingly sophisticated emissions estimates with improved spatial and source resolution. These methods build upon ongoing field, laboratory, and modeling advances that are restricted to just a handful of countries. Thus, burgeoning new knowledge is of limited utility for improving N₂O emissions estimates for the rest of the world where prospects for near‐term advances are constrained by the limited breadth of observations and availability of model driver data. Here, we use Bayesian inversion to leverage information from recent national‐level N₂O emission inventories and reduce uncertainty by up to 65% for estimates of regional and global direct cropland N₂O emissions. Our estimates for the proportion of N inputs lost as N₂O vary by a factor of two between regions and depart from existing default emission factors, yet regional emissions estimates based on these factors are consistent with global, regional, and local observations. Improved regional emission factors will enhance national greenhouse gas inventories in information‐poor countries and guide efforts to reduce agricultural N₂O emissions.     

Simulating soil N2O emissions and heterotrophic CO2 respiration in arable systems using FASSET and MoBiLE-DNDC

Modelling of soil emissions of nitrous oxide (N₂O) and carbon dioxide (CO₂) is complicated by complex interactions between processes and factors influencing their production, consumption and transport. In this study N₂O emissions and heterotrophic CO₂ respiration were simulated from soils under winter wheat grown in three different organic and one inorganic fertilizer-based cropping system using two different models, i.e., MoBiLE-DNDC and FASSET. The two models were generally capable of simulating most seasonal trends of measured soil heterotrophic CO₂ respiration and N₂O emissions. Annual soil heterotrophic CO₂ respiration was underestimated by both models in all systems (about 10?C30% by FASSET and 10?C40% by MoBiLE-DNDC). Both models overestimated annual N₂O emissions in all systems (about 10?C580% by FASSET and 20?C50% by MoBiLE-DNDC). In addition, both models had some problems in simulating soil mineral nitrogen, which seemed to originate from deficiencies in simulating degradation of soil organic matter, incorporated residues of catch crops and organic fertilizers. To improve the performance of the models, organic matter decomposition parameters need to be revised.     

Stimulation of N2O emission by manure application to agricultural soils may largely offset carbon benefits: a global meta‐analysis

Animal manure application as organic fertilizer does not only sustain agricultural productivity and increase soil organic carbon (SOC) stocks, but also affects soil nitrogen cycling and nitrous oxide (N₂O) emissions. However, given that the sign and magnitude of manure effects on soil N₂O emissions is uncertain, the net climatic impact of manure application in arable land is unknown. Here, we performed a global meta‐analysis using field experimental data published in peer‐reviewed journals prior to December 2015. In this meta‐analysis, we quantified the responses of N₂O emissions to manure application relative to synthetic N fertilizer application from individual studies and analyzed manure characteristics, experimental duration, climate, and soil properties as explanatory factors. Manure application significantly increased N₂O emissions by an average 32.7% (95% confidence interval: 5.1–58.2%) compared to application of synthetic N fertilizer alone. The significant stimulation of N₂O emissions occurred following cattle and poultry manure applications, subsurface manure application, and raw manure application. Furthermore, the significant stimulatory effects on N₂O emissions were also observed for warm temperate climate, acid soils (pH < 6.5), and soil texture classes of sandy loam and clay loam. Average direct N₂O emission factors (EFs) of 1.87% and 0.24% were estimated for upland soils and rice paddy soils receiving manure application, respectively. Although manure application increased SOC stocks, our study suggested that the benefit of increasing SOC stocks as GHG sinks could be largely offset by stimulation of soil N₂O emissions and aggravated by CH₄ emissions if, particularly for rice paddy soils, the stimulation of CH₄ emissions by manure application was taken into account.     

Simulation of Effects of Soils,Climate and Management on N<Subscript>2</Subscript>O Emission from Grasslands

Nitrous oxide (N₂O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate, crops and management practices. The N₂O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems. An algorithm for N₂O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources for N₂O production, and the N₂O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N₂O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management. The model simulated the general time course of N₂O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N fertilisation on N₂O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha⁻¹. The simulated N₂O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated emissions increased with increasing soil clay contents. N₂O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing N₂O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils.     

Urbanization can accelerate climate change by increasing soil N2O emission while reducing CH4 uptake

Urban land-use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to (1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N₂O) and methane (CH₄) fluxes, (2) quantify direct N₂O emission factors (EF_d) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N₂O emissions by 153%, to 3.0 kg N ha⁻¹ year⁻¹, while rates of soil CH₄ uptake are reduced by 50%, to 2.0 kg C ha⁻¹ year⁻¹. The global mean annual N₂O EF_d of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N₂O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N₂O emission by 0.46 Tg N₂O-N year⁻¹ and decreased soil CH₄ uptake by 0.58 Tg CH₄-C year⁻¹. Urbanization driven changes in soil N₂O emission and CH₄ uptake are associated with changes in soil properties (bulk density, pH, total N content, and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N₂O emissions and reduces the role of soils as a sink for atmospheric CH₄. These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.     

Estimating annual N2O emissions from agricultural soils in temperate climates

The Kyoto protocol requires countries to provide national inventories for a list of greenhouse gases including N₂O. A standard methodology proposed by the Intergovernmental Panel on Climate Change (IPCC) estimates direct N₂O emissions from soils as a constant fraction (1.25%) of the nitrogen input. This approach is insensitive to environmental variability. A more dynamic approach is needed to establish reliable N₂O emission inventories and to propose efficient mitigation strategies. The objective of this paper is to develop a model that allows the spatial and temporal variation in environmental conditions to be taken into account in national inventories of direct N₂O emissions. Observed annual N₂O emission rates are used to establish statistical relationships between N₂O emissions, seasonal climate and nitrogen‐fertilization rate. Two empirical models, MCROPS and MGRASS, were developed for croplands and grasslands. Validated with an independent data set, MCROPS shows that spring temperature and summer precipitation explain 35% of the variance in annual N₂O emissions from croplands. In MGRASS, nitrogen‐fertilization rate and winter temperature explain 48% of the variance in annual N₂O emissions from grasslands. Using long‐term climate observations (1900–2000), the sensitivity of the models with climate variability is estimated by comparing the year‐to‐year prediction of the model to the precision obtained during the validation process. MCROPS is able to capture interannual variability of N₂O emissions from croplands. However, grassland emissions show very small interannual variations, which are too small to be detectable by MGRASS. MCROPS and MGRASS improve the statistical reliability of direct N₂O emissions compared with the IPCC default methodology. Furthermore, the models can be used to estimate the effects of interannual variation in climate, climate change on direct N₂O emissions from soils at the regional scale.     

Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation

Nitrous oxide (N₂O) is a greenhouse gas that also plays the primary role in stratospheric ozone depletion. The use of nitrogen fertilizers is known as the major reason for atmospheric N₂O increase. Empirical bottom‐up models therefore estimate agricultural N₂O inventories using N loading as the sole predictor, disregarding the regional heterogeneities in soil inherent response to external N loading. Several environmental factors have been found to influence the response in soil N₂O emission to N fertilization, but their interdependence and relative importance have not been addressed properly. Here, we show that soil pH is the chief factor explaining regional disparities in N₂O emission, using a global meta‐analysis of 1,104 field measurements. The emission factor (EF) of N₂O increases significantly (p < .001) with soil pH decrease. The default EF value of 1.0%, according to IPCC (Intergovernmental Panel on Climate Change) for agricultural soils, occurs at soil pH 6.76. Moreover, changes in EF with N fertilization (i.e. ΔEF) is also negatively correlated (p < .001) with soil pH. This indicates that N₂O emission in acidic soils is more sensitive to changing N fertilization than that in alkaline soils. Incorporating our findings into bottom‐up models has significant consequences for regional and global N₂O emission inventories and reconciling them with those from top‐down models. Moreover, our results allow region‐specific development of tailor‐made N₂O mitigation measures in agriculture.     

Effects of Biochar Addition on CO2 and N2O Emissions following Fertilizer Application to a Cultivated Grassland Soil

Carbon (C) sequestration potential of biochar should be considered together with emission of greenhouse gases when applied to soils. In this study, we investigated CO₂ and N₂O emissions following the application of rice husk biochars to cultivated grassland soils and related gas emissions tos oil C and nitrogen (N) dynamics. Treatments included biochar addition (CHAR, NO CHAR) and amendment (COMPOST, UREA, NO FERT). The biochar application rate was 0.3% by weight. The temporal pattern of CO₂ emissions differed according to biochar addition and amendments. CO₂ emissions from the COMPOST soils were significantly higher than those from the UREA and NO FERT soils and less CO₂ emission was observed when biochar and compost were applied together during the summer. Overall N₂O emission was significantly influenced by the interaction between biochar and amendments. In UREA soil, biochar addition increased N₂O emission by 49% compared to the control, while in the COMPOST and NO FERT soils, biochar did not have an effect on N₂O emission. Two possible mechanisms were proposed to explain the higher N₂O emissions upon biochar addition to UREA soil than other soils. Labile C in the biochar may have stimulated microbial N mineralization in the C-limited soil used in our study, resulting in an increase in N₂O emission. Biochar may also have provided the soil with the ability to retain mineral N, leading to increased N₂O emission. The overall results imply that biochar addition can increase C sequestration when applied together with compost, and might stimulate N₂O emission when applied to soil amended with urea.     

Does a warmer climate with frequent mild water shortages protect grassland communities against a prolonged drought?

While irrigation of farm dairy effluent (FDE) to land is becoming popular in New Zealand, it can lead to increased emissions of the greenhouse gas nitrous oxide (N₂O). This paper reports the results from trials on N₂O emissions from irrigation of FDE to two dairy-grazed pastures on two poorly drained silt-loam soils located at Waikato and Manawatu, New Zealand. These pasture soils were periodically irrigated with FDE under contrasting soil moisture conditions with water-filled pore-space (WFPS) ranging between 26% and 94%. Nitrous oxide emissions were measured from the FDE irrigated and unirrigated sites using large numbers of static chambers (12–20). Irrigation of FDE generally increased N₂O emissions compared to the control. N₂O emissions varied with changes in climatic conditions and soil WFPS. Overall N₂O emissions from effluent-derived N ranged between 0.01% and 4.93% depending on irrigation time and soil WFPS. Lower N₂O emissions from FDE were attributable to very low soil WFPS conditions during the dry seasons. Higher N₂O emissions were measured from application of FDE to a recently grazed pasture on wet soil. Our results suggest strategic application of FDE during dry summer and autumn seasons can reduce N₂O emissions from application of FDE. Delaying effluent-irrigation after grazing events could further reduce N₂O emissions by reducing the levels of surplus mineral-N.     

Simulation of N2O emissions from rain-fed wheat and the impact of climate variation in southeastern Australia

The Water and Nitrogen Management model (WNMM) was applied to simulate N₂O emissions from a rain-fed wheat cropping system on a loam-textured soil for two treatments, conventional cultivation with residue burn (CC?+?BURN?+?N) and direct drill with residue retention (DD?+?RET?+?N), at Rutherglen in southeastern Australia from January 2004 to March 2005. Both treatments received the same amount of nitrogen (N) fertiliser. The WNMM satisfactorily simulated the soil water content, mineral N contents and N₂O emissions from the soil, as compared with the field observations for both treatments. The simulated nitrification-induced N₂O emissions accounted for 45% and 34% of total N₂O emissions for the treatments CC?+?BURN?+?N and DD?+?RET?+?N, respectively. The calibrated WNMM was used to simulate N₂O emissions from this soil using historic daily weather data from 1968 to 2004 and applying seven scenarios of fertiliser N application. Correlation analysis found that the annual N₂O emissions for this rain-fed wheat cropping system were significantly correlated to the annual average of daily maximum air temperature (r?=?0.51 for CC?+?BURN?+?N and 0.56 for DD?+?RET?+?N), annual rainfall (r?=??0.56 for CC?+?BURN?+?N and ?0.59 for DD?+?RET?+?N) and fertiliser N application rate (r?=?0.43 for CC?+?BURN?+?N and 0.31 for DD?+?RET?+?N). Based on the 37-year historic simulations, multivariate regression models for estimating annual N₂O emissions were developed to account for climatic variation, and explained about 50% of variations of annual N₂O emissions estimated by WNMM.     

Controls and models for estimating direct nitrous oxide emissions from temperate and sub-boreal agricultural mineral soils in Europe

Based on a review of N₂O field studies in Europe, major soil, climate and management controls of N₂O release from agricultural mineral soils in the European Union have been identified. Data for these N₂O emission drivers can easily be gathered from statistical services. Using stepwise multivariate linear regression analysis, empirical first order models of N₂O emissions have been established which allow – in contrast to existing large-scale approaches – a regionally disaggregated estimation of N₂O emissions at sub-national, national and continental level in the temperate and boreal climate regions of Europe. Arable soils showed lower mean and maximum emissions in oceanic temperate climate (Temperate West) than in pre-alpine temperate and sub-boreal climate (Sub-boreal Europe). Therefore, two separate regression models were developed. Nitrous oxide emissions from arable soils the Temperate West amount to an average flux rate below 2 kg N₂O-N ha^–1 yr^–1 and rarely exceed 5 kg N₂O-N ha^–1 yr^–1. They are modelled by the parameters fertiliser, topsoil organic carbon and sand content. In Sub-boreal European arable soils, N₂O emissions vary in a much wider range between 0 and 27 kg N₂O-N ha^–1 yr^–1 in dependence of available nitrogen, represented in the model by fertiliser and topsoil nitrogen content. Compared to existing methods for large scale inventories, the regression models allow a better regional fit to measured values since they integrate additional driving forces for N₂O emissions. For grasslands, a fertiliser-based model was established which yields higher emission estimates than existing ones. Due to an extreme variability, no climate, soil nor management parameters could be included in the empirical grasslands model.     

Nitrous oxide emissions from drained organic forest soils––an up-scaling based on C:N ratios

Total emissions of N₂O from drained organic forest soils in Sweden were estimated using an equation linking the C:N ratio of the soil to N₂O emissions. Information on soil C:N ratios was derived from a national database. It was estimated that the emissions from Histosols amount to 2,820 tonnes N₂O a⁻¹. This is almost five times the value calculated for the same soils using the method suggested by the Intergovernmental Panel on Climate Change: 580 tonnes N₂O a⁻¹. The higher value in the present study can mainly be explained by improved accuracy of estimates of N₂O emissions from nutrient-rich soils, including former agricultural soils. In Sweden, in addition to 0.94 Mha of drained Histosols, there are 0.55 Mha of other types of drained organic soils. The annual emissions from these soils were estimated to amount to 1,890 tonnes of N₂O. The total emission value calculated for drained organic forest soils was thus 4,700 tonnes N₂O a⁻¹, which, if added, would increase the current estimate of the Swedish anthropogenic N₂O source strength by 18%. Of these emissions, 88% occur from sites with C:N ratios lower than 25. The exponential relationship between C:N ratio and N₂O emissions, in combination with a scarcity of data, resulted in large confidence intervals around the estimates. However, by using the C:N ratio-based method, N₂O emission estimates can be calculated from a variable that is readily available in databases. Also, the recent findings that there are exceptionally large emissions of N₂O from the most nitrogen-rich drained organic forest soils are taken into account.     

Mitigating N2O emissions from agricultural soils with fungivorous mites

Nitrous oxide (N₂O) is an important greenhouse gas and an ozone-depleting substance. Due to the long persistence of N₂O in the atmosphere, the mitigation of anthropogenic N₂O emissions, which are mainly derived from microbial N₂O-producing processes, including nitrification and denitrification by bacteria, archaea, and fungi, in agricultural soils, is urgently necessary. Members of mesofauna affect microbial processes by consuming microbial biomass in soil. However, how microbial consumption affects N₂O emissions is largely unknown. Here, we report the significant role of fungivorous mites, the major mesofaunal group in agricultural soils, in regulating N₂O production by fungi, and the results can be applied to the mitigation of N₂O emissions. We found that the application of coconut husks, which is the low-value part of coconut and is commonly employed as a soil conditioner in agriculture, to soil can supply a favorable habitat for fungivorous mites due to its porous structure and thereby increase the mite abundance in agricultural fields. Because mites rapidly consume fungal N₂O producers in soil, the increase in mite abundance substantially decreases the N₂O emissions from soil. Our findings might provide new insight into the mechanisms of soil N₂O emissions and broaden the options for the mitigation of N₂O emissions.Subject terms: Climate-change ecology, Climate-change ecology