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Nitrous oxide (N 2 O) is a potent greenhouse gas (GHG) whose emission from soil can be enhanced by ruminant excretal returns in grasslands. The default (Tier 1) emission factors (EF 3PRP ; i.e. proportion of deposited nitrogen emitted as N 2 O) for ruminant excreta deposition are associated with a wide range of uncertainties and the development of country-specific (Tier 2) EF 3PRP is encouraged. In Ireland, a Tier 2 EF 3PRP has been developed for cattle excreta but no data are available for sheep. The aim of this study was to generate data to contribute to the derivation of a Tier 2 EF 3PRP for sheep excreta, while assessing the effect of excreta type, grassland type and season of deposition on N 2 O emissions. An experiment was carried out on two sites in the west of Ireland: a managed lowland grassland (LOW) and an extensively grazed hill pasture (HILL), characterised by mineral and acid peat soils, respectively. For each season, four treatments were applied to the soil in a fully randomized block design: control (C), sheep urine (U), sheep dung (D), and artificial urine (AU). Nitrous oxide fluxes were assessed over a full year following each application of treatments, using a static chambers methodology. Results showed a brief initial peak following each application of U/AU in LOW but not in HILL. Cumulative N 2 O emissions were significantly higher from the lowland site. Average EF 3PRP for combined excreta was negligible on both sites, thus lower than the IPCC Tier 1 EF 3PRP . Causes of low emissions are likely to depend on site characteristics (e.g. soil acidity in HILL) and season of application (i.e. ammonia volatilisation in summer). This study showed very low N 2 O emissions from sheep excretal returns in Irish grasslands and highlighted the importance of developing Tier 2, animal-specific EF 3PRP . More experimental grasslands should be assessed to confirm these results., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In grazing systems, urine patches deposited by livestock are hotspots of nutrient cycling and the most important source of nitrous oxide (N 2 O) emissions. Studies of the effects of urine deposition, including, for example, the determination of country-specific N 2 O emission factors, require natural urine for use in experiments and face challenges obtaining urine of the same composition, but of differing concentrations. Yet, few studies have explored the importance of storage conditions and processing of ruminant urine for use in subsequent gaseous emission experiments. We conducted three experiments with sheep urine to determine optimal storage conditions and whether partial freeze-drying could be used to concentrate the urine, while maintaining the constituent profile and the subsequent urine-derived gaseous emission response once applied to soil. We concluded that filtering of urine prior to storage, and storage at - 20 °C best maintains the nitrogen-containing constituent profile of sheep urine samples. In addition, based on the 14 urine chemical components determined in this study, partial lyophilisation of sheep urine to a concentrate represents a suitable approach to maintain the constituent profile at a higher overall concentration and does not alter sheep urine-derived soil gaseous emissions.

Non-steady-state (NSS) chamber techniques have been used for decades to measure nitrous oxide (N 2 O) fluxes from agricultural soils. These techniques are widely used because they are relatively inexpensive, easy to adopt, versatile, and adaptable to varying conditions. Much of our current understanding of the drivers of N 2 O emissions is based on studies using NSS chambers. These chamber techniques require decisions regarding multiple methodological aspects (e.g., chamber materials and geometry, deployment, sample analysis, and data and statistical analysis), each of which may significantly affect the results. Variation in methodological details can lead to challenges in comparing results between studies and assessment of reliability and uncertainty. Therefore, the New Zealand Government, in support of the objectives of the Livestock Research Group of the Global Research Alliance on Agricultural Greenhouse Gases (GRA), funded two international projects to, first, develop standardized guidelines on the use of NSS chamber techniques and, second, refine them based on the most up to date knowledge and methods. This introductory paper summarizes a collection of papers that represent the revised guidelines. Each article summarizes existing knowledge and provides guidance and minimum requirements on chamber design, deployment, sample collection, storage and analysis, automated chambers, flux calculations, statistical analysis, emission factor estimation and data reporting, modeling, and "gap-filling" approaches. The minimum requirements are not meant to be highly prescriptive but instead provide researchers with clear direction on best practices and factors that need to be considered. Health and safety considerations of NSS chamber techniques are also provided with this introductory paper., (© 2020 The Authors. Journal of Environmental Quality published by Wiley Periodicals LLC on behalf of American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.)

Agricultural soils are a major source of the potent greenhouse gas and ozone depleting substance, N2O. To implement management practices that minimize microbial N2O production and maximize its consumption (i.e., complete denitrification), we must understand the interplay between simultaneously occurring biological and physical processes, especially how this changes with soil depth. Meaningfully disentangling of these processes is challenging and typical N2O flux measurement techniques provide little insight into subsurface mechanisms. In addition, denitrification studies are often conducted on sieved soil in altered O2 environments which relate poorly to in situ field conditions. Here, we developed a novel incubation system with headspaces both above and below the soil cores and field‐relevant O2 concentrations to better represent in situ conditions. We incubated intact sandy clay loam textured agricultural topsoil (0–10 cm) and subsoil (50–60 cm) cores for 3–4 days at 50% and 70% water‐filled pore space, respectively. 15N‐N2O pool dilution and an SF6 tracer were injected below the cores to determine the relative diffusivity and the net N2O emission and gross N2O emission and consumption fluxes. The relationship between calculated fluxes from the below and above soil core headspaces confirmed that the system performed well. Relative diffusivity did not vary with depth, likely due to the preservation of preferential flow pathways in the intact cores. Gross N2O emission and uptake also did not differ with depth but were higher in the drier cores, contrary to expectation. We speculate this was due to aerobic denitrification being the primary N2O consuming process and simultaneously occurring denitrification and nitrification both producing N2O in the drier cores. We provide further evidence of substantial N2O consumption in drier soil but without net negative N2O emissions. The results from this study are important for the future application of the 15N‐N2O pool dilution method and N budgeting and modelling, as required for improving management to minimize N2O losses. [ABSTRACT FROM AUTHOR]

Background: Brachiaria humidicola (Bh) has the ability to produce biological nitrification inhibitors (NIs) and release NIs from the root to the soil. Aims: To compare the effects of growing Bh with Brachiaria ruziziensis (Br, which is not able to produce NIs) on soil nitrogen (N) dynamics, N gases and carbon dioxide (CO2) emissions and nitrifiers and denitrifiers following sheep urine application, a laboratory incubation was conducted in a He/O2 continuous flow denitrification system (DENIS). This incubation was conducted in the absence of light. Hence the measured effects of Bh and Br on N cycling were the residual effect of biological NIs released into the soil prior to the incubation and released via root death. Methods: The treatments were: (1) Bh with water application (Bh + W); (2) Bh with sheep urine (Bh + U); (3) Br with water application (Br + W); (4) Br with sheep urine (Br + U). Results: Results showed that soil NO3– concentration increased significantly in the soil with sheep urine application after the incubation. Soil nitrous oxide (N2O) and nitric oxide (NO) emissions increased immediately after the sheep urine application and peaked twice during the incubation. Cumulative emissions for the first peak were significantly lower from the Bh + U treatment (0.054 kg N ha–1) compared with the Br + U treatment (0.111 kg N ha–1), but no significant differences were observed in the total cumulative N2O and NO emissions between the Bh + U and Br + U treatment at the end of the incubation. Sheep urine addition did not affect the AOA, nirS and nosZ gene copies, but significantly increased the AOB gene copies after the incubation. Conclusions: We conclude that the residual effect of Bh to mitigate N2O emissions in a highly nitrifying soil is short‐lived. [ABSTRACT FROM AUTHOR]

The potential for soils to produce nitrous oxide (N2O) is impacted by past moisture conditions; however, the extent of this impact is not fully understood. We conducted the first review of this, using two literature searches and a meta‐analysis. We found 36 studies out of a possible 735 that described experiments where soil moisture conditions had been controlled, such that the impact of antecedent moisture levels could be separated from contemporary moisture levels and attributed to N2O emissions. Of those studies, 14 (130 data points) used the appropriate experimental design and presented suitable data that could be standardized for the meta‐analysis. We found that the degree to which the soil was rewetted and the water filled pore space (WFPS) the soil was brought to were significant explanatory variables (p = < 0.0001). The larger the difference between the dry and wet states of the soil and the higher the WFPS of the soil after rewetting, the larger the hot moment, with an exponential increase once the soil is anaerobic. Substrate availability and fertiliser quantity and type were also important controls on the amount of N2O emitted during the hot moment (p = < 0.0001). However, controls with a constant WFPS can have the same anaerobicity and substrate concentrations, yet much lower emissions, so we suggest that it is the bioavailability of and how the substrates are utilized by the microbial community, and thus how they are primed by the drought, that is the main causal mechanism. Unfortunately, there is still a large uncertainty regarding how microbial population structure, relative gene abundances and gene expression profiles change according to antecedent dry/wet cycles. We suggest several areas of improvement for future studies and the development of drought‐impact curves. These would show the relationship between N2O emissions and the length of drought (we found no studies that have investigated this) and N2O emissions and the severity of drought (e.g. the difference between 20% and 40% WFPS). Highlights: We investigated the main causal mechanisms of N2O hot moments after soil is rewetted.This is the first meta‐analysis and synthesis of the literature regarding N2O hot moments.The more anaerobic the soil and the more C and N substrates, the larger the hot moment.We provide recommendations for future studies and outline key gaps in the research. [ABSTRACT FROM AUTHOR]

Biological nitrification inhibition is a plant‐mediated rhizosphere process where natural nitrification inhibitors can be produced and released by roots to suppress nitrifier activity in soil. Nitrification is one of the critical soil processes in the nitrogen (N) cycle, but unrestricted and rapid nitrification in agricultural systems can result in major losses of N from the plant–soil system (i.e., by NO3− leaching and gaseous N emissions). In this study, we explored the potential efficacy of biological nitrification inhibitors (linoleic acid [LA] and linolenic acid [LN]) and a proven efficient synthetic (dicyandiamide [DCD]) nitrification inhibitor on N dynamics, nitrous oxide (N2O) and carbon dioxide (CO2) emissions in a highly nitrifying soil. 14C‐labelled LA, LN and DCD mineralization was determined in a parallel experiment to explore the fate of inhibitors after application. We found that LA and LN had no effect on soil NH4+ concentrations, but significantly decreased NO3− concentrations. Soil that received DCD had lower NO3− and higher NH4+ concentrations than the control (soil without nitrification inhibitors). LA and LN increased the cumulative N2O and CO2 emissions when they were applied at high concentrations (635 or 1,270 mg kg−1 dry soil). LA and LN had a much greater mineralization rate than that of DCD: 47–56%, 37–61% and 2.7–5.5%, respectively, after 38 days incubation. We conclude that in contrast to the direct inhibition of nitrification caused by DCD, addition of LA and LN may cause apparent nitrification inhibition by promoting microbial immobilization of soil NH4+ and/or NO3−. Future studies on nitrification inhibitors need to clearly differentiate between the direct and indirect effects that result from addition of these compounds to soil. Highlights: The efficacy and stability of nitrification inhibitors in a highly nitrifying soil were explored.This study supports efforts to mitigate N losses and improve nitrogen use efficiency of inputs.Addition of LA, LN and DCD can decrease NO3− concentration, but their modes of action may be different.The apparent effect of LA and LN on soil NO3− concentration could be indirect. [ABSTRACT FROM AUTHOR]

The response of nitrifier and denitrifier populations and associated N2O emissions to different carbon-to-nitrogen (C/N = 17 or 45) straw amendments was monitored under flooding-drying and non-flooding conditions. A 10-week laboratory mesocosm study was conducted in two soils, (i) a paddy soil with a long history of managed flooding-drying (CN), and (ii) a wheat cropping soil with no previous history of flooding (UK). We measured N2O fluxes and the abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB), nitrite reductase (nirK and nirS) genes, and nitrous oxide reductase (nosZI and nosZII) genes during flooding (4 weeks) and post-flooding (6 weeks). Straw addition enhanced N2O emissions, with higher fluxes apparent after incorporation of narrow C/N residues. Moreover, the impact of crop amendment on N2O emission was exacerbated when soil was under flooding-drying conditions. The abundances of nirS and nosZI genes in CN soil and AOA gene in UK soil were increased by straw amendment, with highest in narrow C/N straw amendments. Structural equation modeling showed that the impact of denitrifier gene abundance on the N2O flux was stronger than that of nitrifier gene abundance in the two soils, and significant correlations were observed between N2O fluxes and the consumption of DOC and NO3−, indicating that denitrification was the dominant N2O production pathway during the drying phase. The ratio of (nirS + nirK)/(nosZI + nosZII) in the narrow C/N amendment was greater than in the wide C/N treatment after flooding, suggesting that the straw C/N ratio had an effect on the capacity for N2O production via denitrification. We conclude that crop amendments with an appropriate C/N ratio could minimize N2O fluxes through regulating the denitrification process when soils are subjected to regular flooding and drying and also experiencing greater frequencies of flooding. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is an air pollutant of major environmental concern, with agriculture representing 60% of anthropogenic global N2O emissions. Much of the N2O emissions from livestock production systems result from transformation of N deposited to soil within animal excreta. There exists a substantial body of literature on urine patch N2O dynamics, we aimed to identify key controlling factors influencing N2O emissions and to aid understanding of knowledge gaps to improve GHG reporting and prioritize future research. We conducted an extensive literature review and random effect meta‐analysis (using REML) of results to identify key relationships between multiple potential independent factors and global N2O emissions factors (EFs) from urine patches. Mean air temperature, soil pH and ruminant animal species (sheep or cow) were significant factors influencing the EFs reviewed. However, several factors that are known to influence N2O emissions, such as animal diet and urine composition, could not be considered due to the lack of reported data. The review highlighted a widespread tendency for inadequate metadata and uncertainty reporting in the published studies, as well as the limited geographical extent of investigations, which are more often conducted in temperate regions thus far. Therefore, here we give recommendations for factors that are likely to affect the EFs and should be included in all future studies, these include the following: soil pH and texture; experimental set‐up; direct measurement of soil moisture and temperature during the study period; amount and composition of urine applied; animal type and diet; N2O emissions with a measure of uncertainty; data from a control with zero‐N application and meteorological data. [ABSTRACT FROM AUTHOR]

Abstract The frequency and intensity of extreme weather events (e.g. flood, drought) are predicted to increase for the foreseeable future and it is expected that these will negatively impact upon agroecosystem functioning. Our understanding of how grassland ecosystems respond to extreme weather events occurring at different times of the year, however, is lacking. To better understand the seasonal response of grassland to flooding, we subjected an agricultural grassland to an 8-week extreme flood event at three different temperatures (5 °C-winter, 15 °C-spring/autumn and 25°C-summer) and then followed its subsequent recovery for 9 weeks after floodwater removal. We focused on key indicators of ecosystem functioning including primary production, nutrient cycling, greenhouse gas (GHG) emissions, ammonia (NH 3) volatilization, and soil microbial communities. The experiment used intact soil mesocosms (1 kg) with indigenous vegetation collected from a grassland with no previous history of flooding. Flooding reduced biomass production by 18% at 5 °C, 50% at 15 °C and 95% at 25 °C. Flooding also significantly disrupted elemental cycling (nitrogen, phosphorus and carbon) as evidenced by an increased release of P, Fe and NH 4 + into the soil and overlying floodwater and large amounts of CH 4 and NH 3 released to the atmosphere (mainly during the flooding). These effects were more pronounced at higher temperatures (e.g. 45–700 kg CH 4 C ha−1 and 1–5 kg NH 3 N ha−1 at 15 and 25 °C, respectively). In addition, after floodwater removal this NH 4 + was rapidly nitrified leading to large losses of N 2 O (1.0–14.2 kg N 2 O N ha−1 at 5–25 °C, respectively). Especially at higher temperatures, flooding resulted in a reduction in soil microbial biomass (more than 58% of the equivalent unflooded treatment at 25 °C) and changes in microbial community structure (assessed by PLFAs). Further, some of these changes persisted after flood removal including a loss of actinomycetes, arbuscular mycorrhizal fungi and fungi. Overall, we conclude that ecosystem responses to extreme weather events are critically dependent on temperature with those occurring at higher temperatures having a greater negative impact than those at the lowest temperature (5 °C). The large potential release of CH 4 and N 2 O also suggests that flood events should be considered as a potential source of GHGs when comparing top-down and bottom-up calculations of national inventories, and that further work is needed to better refine GHG emission estimates for these events. Highlights • Flooding induced a rapid release of nutrients, especially at higher temperatures. • 700 kg CH 4 C ha−1 and 5 kg NH 3 N ha−1 were released in the flood phase at 25 °C. • During soil recovery, nitrification led to 1.0–14.2 kg N 2 O N ha−1 losses at 5–25 °C. • Flooding reduced soil microbial biomass, actinomycetes and arbuscular mycorrhiza. • Flooding reduced biomass production by 18% at 5 °C, 50% at 15 °C and 95% at 25 °C. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) makes the single largest contribution to greenhouse gas (GHG) emissions from UK and European Union agriculture. Ambitious government targets for GHG mitigation are leading to the implementation of changes in agricultural management in order to reduce these emissions (mitigation measures). We review the evidence for the contribution of those measures with the greatest mitigation potential which provide an estimated 4.3 t CO2e ha−1 y−1GHG reduction in the UK. The mitigation options considered were: using biological fixation to provide nitrogen (N) inputs (clover,Trifolium), reducing N fertilizer, improving land drainage, avoiding N excess, fully accounting for manure/slurry N, species introduction (including legumes), improved timing of mineral fertilizer N application, nitrification inhibitors, improved timing of slurry and manure application, and adopting systems less reliant on inputs. These measures depend mostly on increasing the efficiency of N fertilizer use and improving soil conditions; however, they provide the added benefit of increasing the economic efficiency of farming systems, and can often be viewed as “win-win” solutions. [ABSTRACT FROM PUBLISHER]

Nitrification inhibitors are a potential technology to mitigate N 2 O emissions from the urine patches of grazing animals. At present, there is limited information regarding the efficacy of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) in reducing N 2 O emissions from ruminant urine patches, as opposed to the well-studied nitrification inhibitor, dicyandiamide. In practical terms, urine patches would be deposited to soil at various times following the application of a nitrification inhibitor to soil. We hypothesised that the effectiveness of DMPP in reducing cumulative N 2 O emissions would decrease the longer the time since DMPP application. This study utilised an automated closed chamber technique, to monitor fluxes of N 2 O from sheep urine patches (725 kg N ha −1 ; 150 ml; 300 cm 2 ) deposited to a Eutric Cambisol, where DMPP was applied (1 kg ha −1 ) on the same day, 2 weeks before and 4 weeks before urine application. Fluxes were monitored continuously from 4 weeks before, to 9 weeks after, urine application. DMPP was found to be ineffective at reducing cumulative N 2 O emissions and 9-week urine-N 2 O emission factors when applied at the same time as the sheep urine, although a low number of replicates were used in this study. Some effect of DMPP in delaying the accumulation of soil NO 3 − was observed, with effects being greater the shorter the time since DMPP application. The temporal dynamics of N 2 O fluxes were also altered where DMPP was applied on the same day as the urine. Heterogeneity in soil conditions were deemed responsible for the large spatial variability of N 2 O emissions observed in this study. The use of Rhizon samplers were useful for detecting spatial variability within the soil solution directly beneath the urine patches (within the flux chambers), which may not have been detected within duplicate urine patches (outside the chambers), where soil sampling was conducted. Further work is required to determine the loading rate and duration of efficacy of DMPP to reduce emissions from urine patches under temperate summer conditions. [ABSTRACT FROM AUTHOR]

Urine patches contribute greatly to greenhouse gas emissions within livestock grazed ecosystems. The effective area of a ruminant urine patch comprises the wetted area, the diffusional area and the pasture response area. This study specifically assesses the importance of considering the diffusional area for monitoring urine patch N2O emissions. Spatial and temporal changes in N2O emissions and potential drivers of emissions (soil pH, EC, redox potential, dissolved organic carbon and nitrogen, NO- 3 and NH+ 4) were measured in sheep urine amended Eutric Cambisol mesocosms, maintained at 50% or 70% waterfilled pore space (WFPS). At 70% WFPS, over 10 weeks, the emission factor (EF) was greater when considering the wetted area plus a 9 cm diffusional area (EF = 2.75 ± 0.72% of applied N) than when considering the wetted area alone (EF = 1.44 ± 0.30% of applied N); differences were not statistically significant at 50% WFPS. Redox potential, total extractable N and WFPS contributed significantly to the observed variation in daily N2O fluxes from the urine patch. We conclude that the urine patch diffusional area is an extremely important source of emissions from urine patches. This has implications when measuring EFs, as the lateral diffusion of solutes may be restricted by chamber walls resulting in an underestimate of N2O emissions, particularly at higher soil moisture contents. Site-specific assessments of the urine patch diffusional area should be made, and accounted for, prior to monitoring emissions and calculating emission factors from urine patches applied within chambers. [ABSTRACT FROM AUTHOR]

A large number of sheep graze extensively managed grasslands, including upland and hill areas. Excretal deposition of nitrogen (N) to upland soil is a potentially large source of the powerful greenhouse gas (GHG), nitrous oxide (N 2 O), however, few studies have assessed urine-patch N 2 O emissions from upland areas. Current default excretal N 2 O emission factors (EFs) are based on intensively managed lowland systems, with cattle excreta as the N source. We hypothesised that N 2 O emissions could differ substantially from those of lowland systems, due to differences in soil type, climate, topography, pasture composition and management factors along altitudinal and productivity gradients. We investigated N 2 O emission factors across two seasons (spring and autumn), for an extensive semi-improved, temperate grassland using IPCC-compliant and representative sheep urine patches (in terms of urine chemical composition, urine patch size and N loading rates). An automated GHG monitoring system provided high-frequency GHG data from sheep urine patches (756 and 1112 kg N ha −1 applied in spring and autumn, respectively), reference artificial sheep urine patches (1066 and 1004 kg N ha −1 applied in spring and autumn, respectively) and control treatments. In spring, urine patch N 2 O emission factors were −0.02 ± 0.04 (artificial sheep urine) and 0.03 ± 0.09% (real sheep urine) of the applied N; in autumn emission factors were 0.02 ± 0.03 (artificial sheep urine) and 0.08 ± 0.04% (real sheep urine) of the applied N. These values are much lower than default inventory values (1% of applied N) for excreta deposited by grazing livestock. There was a greater pasture foliar N content following urine application in spring as opposed to autumn, and a significantly longer residence time of extractable mineral N in autumn. Our findings demonstrate the importance of generating country-specific N 2 O EFs based on altitude/productivity gradients of livestock production, with implications for national inventories and the accuracy of sustainability metrics of lamb produced in the UK uplands. [ABSTRACT FROM AUTHOR]

Microplastic contamination in agroecosystems is becoming more prevalent due to the direct use of plastics in agriculture (e.g., mulch films) and via contamination of amendments (e.g., compost, biosolids application). Long-term use of agricultural plastics and microplastic pollution could lead to soil degradation and reduced crop health due to the slow degradation of conventional plastics creating legacy plastic. Biodegradable plastics are more commonly being used, both domestically and in agriculture, to minimise plastic pollution due to their biodegradable nature. However, the influence of a biodegradable plastics on soil function at the field scale is largely unknown. We investigated the effect of conventional (polyethylene) and biodegradable (PHBV) microplastics on N 2 O emissions and soil biochemical processes in a field trial of winter barley. Microplastic was added to the soil at realistic levels (100 kg ha-1) for both conventional and biodegradable treatments. N 2 O emissions were measured throughout the growing season alongside key soil quality indicators (microbial community composition, ammonium, nitrate, moisture content, pH and EC). Overall, microplastic addition had no observable effect on crop yield, microbial communities or soil biochemical properties. Yet, we found cumulative N 2 O emissions were reduced by two-thirds following conventional microplastic addition compared to the no-plastic and biodegradable microplastic treatments. We believe this response is due to the lower soil moisture levels over the winter in the conventional microplastic treatment. Overall, the response of key soil parameters to microplastic addition show fewer negative effects to those seen in high dose laboratory mesocosm experiments. Thus, it is imperative that long-term field experiments at realistic dose rates be undertaken to quantify the real risk that microplastics pose to agroecosystem health. [Display omitted] • We compared conventional and biodegradable microplastics in a field trial with winter barely. • N 2 O emissions with conventional microplastic addition were a third of the control. • Conventional microplastic reduced soil moisture in wetter periods. • Microplastic addition had no effect on crop yields or microbial community composition. [ABSTRACT FROM AUTHOR]

Excess chemical fertilizer application results in substantial N 2 O emission. Manure application into agricultural soil can reduce chemical fertilizer input while maintaining crop yield. However, little is known about the potential effects of reduction in chemical fertilizer with green manure or organic manure on yield-scaled N 2 O emissions in orchards. A two-year experiment was carried out in a citrus orchard, including (i) chemical fertilizer (urea), 0%, 70%, 85% and 100% of N supplied, respectively (CK, C70, C85, CF), (ii) organic manure (rapeseed cake) incorporation with 0%, 70%, 85% and 100% of N supplied from chemical fertilizer (OM, OMC70, OMC85, OMCF), (iii) green manure (smooth vetch) incorporation with 0%, 70%, 85% and 100% of N supplied from chemical fertilizer (GM, GMC70, GMC85, GMCF) treatments. The soil physical-chemical properties, N 2 O emissions and yields were measured during 2018–2019. N 2 O emissions were 1.57 and 1.80 kg N ha−1 yr−1 under CF treatment in 2018 and 2019, respectively, and decreased by 25.4%−31.6% under C70, 32.5%−55.4% under OMC70 and OMC85 treatments and by 16.1–32.2% under GMC70 and GMC85 treatments. The 70% N rate of chemical fertilizer was the optimal N rate for maximizing yield in no manure, organic manure and green manure treatments, respectively. The C70, OMC70 and GMC70 treatments both significantly decreased yield-scaled N 2 O emission with the lowest emission in OMC70 treatment relative to CF treatment. N 2 O fluxes in OMC70 and OMC85 treatments were mainly controlled by the DOC/NO 3 - ratio, while soil NH 4 + was the dominant factor controlling N 2 O fluxes in GMC70 and GMC85 treatments, suggesting that more NH 4 + provided from green manure than from organic manure contributed to higher N 2 O emission. We conclude that applying organic manure incorporation with 30% reductions of chemical fertilizer is a fertilizer management strategy worth pursuing for sustainable productivity and environmental protection in citrus orchards. • Organic manure amendment decreased yield-scaled N 2 O emission relative to urea alone. •.N 2 O flux under organic manure amendment was controlled by soil DOC/NO 3 -. • Soil NH 4 + was main factor regulating N 2 O flux under green manure amendment. • NH 4 + from green manure contributed to higher N 2 O emission relative to organic manure. [ABSTRACT FROM AUTHOR]

• Statistical relationships of pasture N 2 O, CO 2 & CH 4 fluxes & environmental data. • Extensive site N 2 O flux predictors consistent with controls & literature. • Use of a spatial regression changed predictors for Intensive site N 2 O fluxes. • Significant predictors of GHG fluxes are site- & GHG-specific. • Missing predictors or more complex (e.g. non-stationary) relationships. Greenhouse gas (GHG) fluxes from livestock grazed pasture soils are highly variable in both space and time but the quantitative importance of the factors regulating this variation remain poorly understood. Our aim was to explore this variability on contrasting extensively (low input) and intensively managed sheep-grazed 'case-study' pastures. We quantified (through standard and spatially-informed regressions) the statistical relationships between GHG fluxes (nitrous oxide (N 2 O), carbon dioxide (CO 2) and methane (CH 4)) and a range of soil, field and management characteristics. Fluxes of these three GHGs at two study sites were highly variable, but spatial structure (i.e. autocorrelation) was only observed in the variability of N 2 O fluxes across the intensive site and CO 2 fluxes across the extensive site. The regression analyses identified significant GHG predictor variables for the extensive site as: NO 3 − (p < 0.001) and vegetation-type (p < 0.01) for N 2 O (R 2 = 0.57; p = 0.000); NH 4 + (p < 0.05), slope (p < 0.05) and elevation (p < 0.01) for CO 2 (R 2 = 0.34; p = 0.000); and NO 3 − (p < 0.01), NH 4 + (p < 0.05) and soil moisture (p < 0.05) for CH 4 (R 2 = 0.25; p = 0.005). Significant GHG predictor variables for the intensive site were soil moisture (p < 0.01) and bulk density (p < 0.01) for N 2 O (R 2 = 0.27; p = 0.005); soil moisture (p < 0.001) for CO 2 (R 2 = 0.31; p = 0.001); while none were found for CH 4 (R 2 = 0.10; p = 0.655). Key factors driving GHG variation were both site- and GHG-specific, with fluxes controlled by local conditions leading to differences in limiting factors (possibly even at the within-site scale). Our statistical analyses suggest a larger range of driving variables (e.g. air and soil temperature or other soil chemical properties such as total extractable N) may be required to more fully capture the observed variability in the GHG processes considered here, and that it may also be fruitful for future analyses to consider non-linear, non-stationary and interacting relationships across space- and time-scales. Adequacies of each site's sample design also played a key interpretive role in the GHG processes, requiring further evaluation through additional sampling campaigns. [ABSTRACT FROM AUTHOR]

Bacteria and fungi have been shown to produce nitrous oxide (N 2 O) during denitrification, but their contribution after nitrate (NO 3 −) application to soil is not clearly established. In a microcosm experiment, the relative contribution of bacteria and fungi to N 2 O and carbon dioxide (CO 2) production by four contrasting soils representing different land uses after KNO 3 addition was studied. The soils were daily wetted to 80% water-filled pore space (WFPS) and kept under greenhouse conditions for 10 days. The fungicide cycloheximide and the bactericide streptomycin were used to determine the possible microbial origin of the N 2 O and CO 2 emissions. Non-target effects of the antibiotics on the emission of N 2 O and CO 2 were evaluated using the inhibitor additivity ratio (IAR). The abundance of the bacterial and fungal communities was estimated by quantitative PCR (qPCR) of the bacterial 16S rRNA gene and the fungal internal transcribed spacer (ITS) region, respectively. The gene copy number of bacterial denitrifiers was calculated after quantification of the nirK , nirS , norB , nosZ I and nosZ II genes. After 10 d, regardless of the soil type, the cumulative N 2 O emission from the soils treated with cycloheximide or streptomycin were similar. In all the four soils, N 2 O fluxes were greater (on average 1.8 ± 0.3 times) in soils amended with the fungicide than with the bactericide during incubation for the first 48–96 h. Greater N 2 O emissions (on average 1.7 ± 0.2 times) were detected in soils where bacteria were inhibited in comparison to those treated with the fungicide from 96 to 240 h. On average, 68.5% of the total CO 2 emitted during the 10-d incubation period was produced in soils treated with the fungicide and 31.5% in those treated with the bactericide. The greater contribution of bacteria to the production of N 2 O than fungi during the first 48–96 h was possibly due to a faster used of nitrate. Variations in the abundance of bacterial 16S rRNA genes, the ITS region, and the nirK , nirS , norB and nosZ I bacterial denitrification genes indicated that the antibiotics used to prevent the growth of bacteria and fungi were effective during incubation. These results suggest that both bacteria and fungi should be considered when designing and applying greenhouse gas mitigation strategies in soils and that their relative contribution to produce N 2 O and CO 2 can vary with time and nitrate availability. • In a 10-d microcosm experiment, bacteria dominated fungi for N 2 O emissions during the first 3 d after N-fertilisation. • Fungi dominated bacteria for N 2 O emissions during the remaining 7-d of treatment. • After 10-d, bacteria and fungi similarly contributed to N 2 O production in the four soils analysed. • Bacteria were greater CO 2 producers than fungi after the 10-d treatment in the four soils analysed. • Bacteria and fungi should be considered when designing greenhouse gas mitigation strategies. [ABSTRACT FROM AUTHOR]