Increased N2O Fluxes Research Articles

Increased nitrous oxide emissions from intertidal soil receiving wastewater from dredging shrimp pond sediments

The quantities of greenhouse gas emissions and the activity of functional microbes in coastal soils receiving nutrient-rich wastewaters from mariculture activities have seldom been reported. We investigated the effects of wastewater discharge resulting from dredging shrimp pond sediment on the soil fluxes of methane (CH4) and nitrous oxide (N2O) in intertidal areas and on the functional microorganisms and physio-chemical characteristics of soil. The temporal variations in gas fluxes and soil characteristics following wastewater discharge were also evaluated with the tidal regime on the day of discharge taken into account. The results showed that wastewater discharge immediately resulted in higher levels of ammonia (NH4+-N) deposited and N2O emissions from the soil at the discharge site than at the non-discharge site, while the CH4 flux was not affected. The increase in N2O flux lasted for a longer time when the discharge was performed during a neap tide day than when it was performed during a spring tide day. Wastewater discharge also increased the abundance of ammonia-oxidizing bacterial (AOB) amoA genes and nosZ genes in soil rather than increasing the abundance of narG and nirK genes. The pattern of temporal variations between the N2O flux and soil NH4+–N content was similar to that between the flux and the AOB-amoA gene abundance, suggesting that bacterial nitrification was important for N2O production in soil receiving the dredging wastewater. The results suggest that the wastewater discharge impacts nitrogen metabolism processes and causes a significant N2O emission problem; therefore, pollutant management is essential in shrimp culturing activities to reduce greenhouse gas emissions into the atmosphere.

Graminoid Removal Reduces the Increase in N2O Fluxes Due to Nitrogen Fertilization in a Boreal Peatland

The increase in nitrogen (N) deposition may affect nitrous oxide (N2O) emission in boreal peatlands by increasing N availability and/or altering vegetation composition. However, the effects of changes in vegetation composition (due to the increase in N deposition) on N2O emission are still unknown. Therefore, we used a factorial design, comprising the removal of plant functional groups and N fertilization to investigate their independent effects and combined effects on N2O fluxes at an ombrotrophic blanket bog in Western Newfoundland, Canada. The results reveal that N2O emission significantly increased with N fertilization. Notably, the effect of N addition on N2O fluxes was modulated by the removal of graminoids. In particular, the removal of graminoids reduced 69% of the N2O emission increases due to N fertilization in the third year of the experiment. This result suggests that the enhancement of graminoid abundance due to increases in N deposition/fertilization may dramatically increase N2O emissions in boreal peatlands leading to stronger impacts on the global climate and stratospheric ozone depletion in the future if no countermeasures to reduce reactive N emission are undertaken.

Greenhouse gas fluxes and porewater geochemistry following short-term pulses of saltwater and Fe(III) in a subtropical tidal freshwater estuarine marsh

Estuarine tidal freshwater riparian marshes experience short-term saltwater intrusions (caused by typhoons and hurricanes) and Fe(III) inputs (associated with run-off from upstream basins during rain events). However, the effects of Fe(III) inputs and of simultaneous increases of saltwater and Fe(III) on greenhouse gas (GHG) fluxes and porewater geochemistry of estuarine tidal freshwater wetlands have not yet been studied. This study applied monthly in-situ experimental manipulations of saltwater and Fe(III) additions within a tidal freshwater wetland (the Min River estuary, southeast China) dominated by Sagittaria trifolia, over two years. We examined changes in porewater geochemistry (i.e., SO42–, Cl–, NH4+, and NO3–), ecosystem GHG (i.e., CH4, CO2, and N2O) fluxes, and soil denitrification rates. Short-term saltwater intrusions did not cause a large increase in soil salinity but did decrease CH4 fluxes. Fe(III) additions also greatly decreased CH4 fluxes, however, the combined addition of saltwater and Fe(III) did not have a synergistic effect on the reduction of CH4 emissions. Further, CO2 emissions did not significantly change in response to either the individual or the combined saltwater and Fe(III) additions. Most interestingly, we found that the combined addition of saltwater and Fe(III) led to an increase in N2O fluxes of an order of magnitude compared to the control. Based on the results of sustained-flux global warming potential (SGWP) and sustained-flux global cooling potential (SGCP) models, our findings suggest that short-term pulses of both individual saltwater and the combination of saltwater and Fe(III) caused by storm tides and soil erosion following a typhoon or hurricane event could reduce the CO2-equivalent flux from subtropical estuarine tidal freshwater wetlands, although the effects may be temporary.

Animal performance, and enteric methane, manure methane and nitrous oxide emissions from Murrah buffalo calves fed diets with different forage-to-concentrate ratios

The present study aimed to evaluate the effects of dietary forage:concentrate ratios on growth performance and enteric and faecal greenhouse-gas emissions from growing buffalo calves. Fifteen Murrah male calves (bodyweight = 233.35 ± 30.92 kg; 8–12 months age) were randomly assigned to three dietary groups that were fed a mixture of berseem fodder, wheat straw and concentrate at the ratios of 20:60:20 (C20), 20:40:40 (C40) and 20:20:60 (C60) respectively, for 120 days. Enteric methane (CH4) production was estimated by the sulfur hexafluoride tracer technique. Faeces were stored for 12 weeks and CH4 and nitrous oxide (N2O) fluxes from stored faeces were estimated every 14 days. Dry-matter intake, feed conversion efficiency and nitrogen retention were not affected (P > 0.05) but average daily gain and urinary nitrogen loss (g/day) were higher for C60 than the C20 diet (P < 0.05). Daily enteric CH4 emission (g/day) was not affected but CH4 yield (g/kg dry-matter intake) and energy loss through CH4 as a proportion of energy intake were lower for C60 than the C20 diet (P < 0.05). Faeces composition was not affected, and large variations of greenhouse-gas emissions were observed for first 10 days of storage. Methane emissions from stored faces were 1.28 ± 0.40, 1.94 ± 0.34 and 3.90 ± 0.27 mg/kg faeces per day for C20, C40 and C60 diets respectively, being higher for C60 than the C40 and C20 diets (P < 0.05). Methane-flux rate from faeces was greater for C60 than the C20 and C40 diets (0.75 vs 0.26 and 0.37 g/animal respectively; P < 0.05). Diet C60 increased N2O fluxes from stored faeces by 63% and 58% respectively, expressed in mg/kg faeces per day and mg/animal per day, compared with C20 diet (P < 0.05). Overall, dietary concentrate proportion of up to 60% in growing buffalo calf diets improved growth performance without increasing enteric CH4 emission, but CH4 and N2O production from faeces were increased. This work has provided information for gas emissions factors from open storage of faeces. More detailed studies on gaseous emissions from open lots on farms are required.

Manure application increased denitrifying gene abundance in a drip-irrigated cotton field.

Application of inorganic nitrogen (N) fertilizer and manure can increase nitrous oxide (N2O) emissions. We tested the hypothesis that increased N2O flux from soils amended with manure reflects a change in bacterial community structure and, specifically, an increase in the number of denitrifiers. To test this hypothesis, a field experiment was conducted in a drip-irrigated cotton field in an arid region of northwestern China. Treatments included plots that were not amended (Control), and plots amended with urea (Urea), animal manure (Manure) and a 50/50 mix of urea and manure (U+M). Manure was broadcast-incorporated into the soil before seeding while urea was split-applied with drip irrigation (fertigation) over the growing season. The addition treatments did not, as assessed by nextgen sequencing of PCR-amplicons generated from rRNA genes in soil, affect the alpha diversity of bacterial communities but did change the beta diversity. Compared to the Control, the addition of manure (U+M and Manure) significantly increased the abundance of genes associated with nitrate reduction (narG) and denitrfication (nirK and nosZ). Manure addition (U+M and Manure) did not affect the nitrifying enzyme activity (NEA) of soil but resulted in 39–59 times greater denitrifying enzyme activity (DEA). In contrast, urea application had no impact on the abundances of nitrifier and denitrifier genes, DEA and NEA; likely due to a limitation of C availability. DEA was highly correlated (r = 0.70–0.84, P < 0.01) with the abundance of genes narG, nirK and nosZ. An increase in the abundance of these functional genes was further correlated with soil NO3−, dissolved organic carbon, total C, and total N concentrations, and soil C:N ratio. These results demonstrated a positive relationship between the abundances of denitrifying functional genes (narG, nirK and nosZ) and denitrification potential, suggesting that manure application increased N2O emission by increasing denitrification and the population of bacteria that mediated that process.

Importance of annual monitoring for evaluating the direct nitrous oxide emission factor in temperate mono-rice paddy fields

In temperate mono-rice paddy fields, rice is cultivated for 100–140 days under submerged conditions during the summer period, and thereafter, the field remains under dry conditions during the winter and spring seasons. However, the early developed nitrous oxide (N2O) emission factor (EF) was only based on seasonal (rice cropping period) N2O fluxes, which resulted in lower N2O EF than the default value (0.3%) used by the Intergovernmental Panel on Climate Change (IPCC). Furthermore, the long fallow season may be favorable for nitrification and substantially result in increased N2O emissions. A two-year field experiment was conducted to evaluate the effect of N2O emissions during the dry fallow season on the annual N2O EF. The N2O emission rates were sequentially characterized during the rice cropping and the fallow season under four different levels of nitrogen (N) fertilizer for rice cultivation. The urea was applied at four different (0, 45, 90 and 180 kg N ha−1) levels, and rice was cultivated under submerged conditions during late May to early October. The seasonal N2O fluxes during the rice cropping and fallow seasons clearly increased with increasing N application rates. In the N fertilized plots, the mean N2O emission rates were higher during the fertilized cropping season than the fallow season, but the seasonal fluxes were much higher during the unfertilized fallow season, due to the long dry period. The seasonal N2O EF, which was estimated by the increased N2O flux with N fertilizer, was only 0.0015–0.0017 kg N2O-N kg−1 N during rice cropping. However, the annual N2O EF combining the two seasonal N2O fluxes markedly increased to 0.0028–0.0031 kg N2O-N kg−1 N, which was very close to the N2O EF of the IPCC. Conclusively, the N2O EF in mono-rice paddy fields should be developed using the annual N2O fluxes and not only the cropping seasonal N2O fluxes.

Differences in N2O Fluxes and Denitrification Gene Abundance in the Wet and Dry Seasons Through Soil and Plant Residue Characteristics of Tropical Tree Crops

The conversion of forest to agricultural soils is a widespread activity in tropical systems, and its link to nitrous oxide (N2O) fluxes and nitrogen cycling gene abundance is relevant to understand environmental drivers that may interact with climate change. A current challenge to estimating N2O emissions from land use conversion is an incomplete understanding of crop-specific impacts on denitrifier communities and the N2O fluxes driven by differences in the above- and below-ground inputs with crop type. To address this knowledge gap in tree crops, we evaluated N2O fluxes and denitrification gene abundance and their relationships with soil and plant residue characteristics in citrus and eucalyptus plantations in the field and in soil incubations. We found that the accumulated N2O fluxes from soil were lower for the two agricultural field sites than those for their adjacent forest sites in dry and wet seasons. The N2O fluxes were higher in the wet season, and this seasonal difference persisted even when the soils collected from both seasons were incubated under the same moisture and temperature conditions in the lab for 30 days. Increased N2O fluxes in the wet season were accompanied by an increase in soil nirK and nosZ gene abundance, the dissolved organic carbon (DOC) concentration, and the total soil carbon (C) and nitrogen (N) content. In turn, the abundance of denitrifiers, as indicated by nirK, nirS and nosZ gene copy numbers, showed a low but significant positive correlation with soil bulk density. Our results suggest that soil moisture, leaf litter and crop residues influence the seasonal differences in both N2O fluxes and abundance of denitrifiers in citrus- and eucalyptus-cultivated soils, likely through effects on soil physicochemical characteristics. These findings highlight the overwhelming role of environmental drivers that can make investigating microbial drivers difficult in the field and open the possibility for a better understanding of N cycling processes in tropical soils based on paired field- and incubation-based experimentation.

Greenhouse gas fluxes from coastal wetlands at the intersection of urban pollution and saltwater intrusion: A soil core experiment

Wetlands serve an important role in regulating greenhouse gases globally. Anthropogenic stressors including elevated nutrients and contaminants from urban pollution and elevated salinity from saltwater intrusion have the potential to alter greenhouse gas emissions, particularly in tidal wetlands that experience these stressors simultaneously. We designed a laboratory soil core experiment to disentangle the separate and combined effects of saltwater intrusion (elevated salinity versus elevated sulfate) and urban pollution (elevated nitrogen versus elevated copper) on carbon and nitrogen cycling in wetland soils. We collected 45 intact soil cores from a brackish tidal wetland and applied chemical treatments for seven weeks in the laboratory. We measured greenhouse gas soil-to-atmosphere fluxes (CO2, CH4, N2O) and porewater chemistry weekly. Soils were harvested at the end of the experiment to analyze soil chemistry and microbial carbon and nitrogen processing rates. Compared to the freshwater control, we found that elevated copper and sulfate (without elevated salinity) decreased CH4 fluxes. Treatments with elevated salinity, sulfate, or both also reduced CO2 fluxes. Likewise, carbon mineralization at the end of the experiment was lower for all treatments with elevated copper, sulfate, or salinity. In contrast, elevated salinity enhanced N2O fluxes but did not affect potential denitrification rates at the end of the experiment. Combined urban pollution and sea salt enrichment suppressed carbon gas fluxes but enhanced N2O emissions. Decreased carbon gas fluxes offset increased N2O fluxes such that the saltwater, urban, and combined treatments reduced the total global warming potential by 6.9×, 2.6×, and 3.6×, respectively, relative to the freshwater control. Thus our laboratory experiment suggests in the short-term, saltwater intrusion could reduce global warming potential of coastal wetland soils in Southern New England with typical levels of urban pollution. Future research that addresses the response of coastal wetland ecosystems to combined effects of urban pollution and saltwater intrusion will be critical for predicting long-term coastal wetland function under future land use and climatic conditions.

Salinity effects on greenhouse gas emissions from wetland soils are contingent upon hydrologic setting: a microcosm experiment

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO42−). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO42−, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO42−, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO2 production, in both drought and flooded treatments. Contrary to our expectations, CH4 fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO42− treatments respectively. In the drought treatments, we saw virtually no CH4 release from any core, while artificial seawater with sulfate increased N2O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N2O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology.

Impact of elevated tropospheric ozone on soil C, N and microbial dynamics of winter wheat

Tropospheric O3 can impact soil systems mediated by changing root biomass and exudates. This study investigated how elevated O3 alters soil CO2 and N2O emissions from a winter wheat field. Winter wheat (Triticum aestivum L.) were fumigated using elevated O3 concentration (EO3) and using ambient air within open-top chambers during 2015 and 2016. Soil CO2 and N2O emissions were measured using a static closed-chamber technique. Rhizosphere soils were collected to determine dissolved organic carbon (DOC) content, ammonium-nitrogen (NH4+-N) and nitrate-nitrogen (NO3−-N) concentrations, microbial biomass carbon (MBC) and nitrogen (MBN), and activities of nitrate reductase (NR), nitrite reductase (NiR) and hydroxylamine reductase (HR). Moreover, volumetric soil water content (θv) was monitored and plant dry biomass was quantified. The EO3 treatment exhibited decreased CO2 and increased N2O emissions from winter wheat soil. Soil NH4+-N, NO3−-N, NR and NiR increased under EO3, whereas DOC, MBC, MBN and HR decreased. Soil θv was higher under EO3 before irrigation, due to reduced transpiration. Elevated O3 decreased soil CO2 flux potentially due to reduced root biomass and associated carbon input. It increased N2O fluxes apparently through enhanced denitrification due to greater substrate availability and soil θv, despite the inhibition of nitrification-related routes.

Effects of warming on N2O fluxes in a boreal peatland of Permafrost region, Northeast China

Climate warming is expected to increasingly influence boreal peatlands and alter their greenhouse gases emissions. However, the effects of warming on N2O fluxes and the N2O budgets were ignored in boreal peatlands. Therefore, in a boreal peatland of permafrost zone in Northeast China, a simulated warming experiment was conducted to investigate the effects of warming on N2O fluxes in Betula. Fruticosa community (B. Fruticosa) and Ledum. palustre community (L. palustre) during the growing seasons from 2013 to 2015. Results showed that warming treatment increased air temperature at 1.5m aboveground and soil temperature at 5cm depth by 0.6°C and 2°C, respectively. The average seasonal N2O fluxes ranged from 6.62 to 9.34μgm−2h−1 in the warming plot and ranged from 0.41 to 4.55μgm−2h−1 in the control plots. Warming treatment increased N2O fluxes by 147% and transformed the boreal peatlands from a N2O sink to a source. The primary driving factors for N2O fluxes were soil temperature and active layer depth, whereas soil moisture showed a weak correlation with N2O fluxes. The results indicated that warming promoted N2O fluxes by increasing soil temperature and active layer depth in a boreal peatland of permafrost zone in Northeast China. Moreover, elevated N2O fluxes persisted in this region will potentially drive a noncarbon feedback to ongoing climate change.

Evaluating the effect of liming on N2O fluxes from denitrification in an Andosol using the acetylene inhibition and 15N isotope tracer methods

We assessed the effect of liming on (1) N2O production by denitrification under aerobic conditions using the 15N tracer method (experiment 1); and (2) the reduction of N2O to N2 under anaerobic conditions using the acetylene inhibition method (experiment 2). A Mollic Andosol with three lime treatments (unlimed soil, 4 and 20 mg CaCO3 kg−1) was incubated at 15 and 25 °C for 22 days at 50% and then 80% WFPS with or without 200 mg N kg−1 added as 15N enriched KNO3 in experiment 1. In experiment 2, the limed and unlimed soils were incubated under completely anaerobic conditions for 44 h (with or without 100 mg N kg−1 as KNO3). In experiment 1, limed treatments increased N2O fluxes at 50% WFPS but decreased these fluxes at 80% WFPS. At 25 °C, cumulative N2O and 15N2O emissions in the high lime treatment were the lowest (with at least 30% less 15N2O and total N2O than the unlimed soil). Under anaerobic conditions, the high lime treatment showed at least 50% less N2O than the unlimed treatment at both temperatures with or without KNO3 addition but showed enhanced N2 production. Our results suggest that the positive effect of liming on the mitigation of N2O evolution from soil was influenced by soil temperature and moisture conditions.

Soil mineral N dynamics and N 2 O emissions following grassland renewal

Abstract Managed grasslands are periodically renewed in north-western Europe, primarily in response to a decline in yield and nutritive value or sward damage. Grassland conversion to arable land is also a common agricultural practice on intensively used grassland sites. However, depending on the soil and its management, grassland break-up (i.e. the destruction of the grass sward and soil disturbance) is associated with the mineralisation of soil organic nitrogen (N) and the decomposition of stubbles and roots from the old grass sward, with both leading to enhanced nitrous oxide (N2O) emissions and nitrate (NO3−) leaching. Two sites were set up to investigate the effects of different grassland renewal techniques (keeping and improving the old sward, chemical sward killing and chemical killing of the sward followed by ploughing) with grassland conversion to maize cropping and permanent grassland as the reference treatments. The sites (Histic Gleysol and a Plaggic Anthrosol) differed in their organic matter content and groundwater level. N2O fluxes were measured weekly using static closed chambers for a period of two years. The relationship between N2O fluxes and explanatory/controlling variables was investigated using generalised additive models (GAM). The potential NO3− losses via NO3− leaching were quantified by taking weekly measurements of the soil mineral N (Nmin) from the topsoil layer (0–30 cm) and from depth profiles (0–90 cm) in the autumn (pre-winter) and spring (post-winter). The aboveground biomass in the different treatments was also measured. Grassland renewal was not a significant source of direct N2O emissions at either experimental site during the two years of the study. There was only a short two-month period during which there were significantly increased N2O fluxes (up to a maximum of 1.6 kg N2O ha−1 day−1 in the Histic Gleysol) and treatment differences. N fertilisation (as reflected in the Nmin content in soil), soil moisture, and microbial and plant respiratory activity were identified as important drivers of N2O emission. The destruction of the old grass sward (i.e. chemical killing by herbicide application and ploughing and conversion to maize cropping) resulted in an increased net N release of Nmin during the first year, which indicated losses via indirect N2O emission and a higher risk of NO3− leaching, especially on the sandy Plaggic Anthrosol. No yield effects were found after grassland renewal at either site. With respect to N2O mitigation and the prevention of NO3− leaching, it is recommended that the new grass sward should be rapidly established as a sink for Nmin and that the amount of available mineralised N following grassland renewal is taken into account when applying N fertiliser, as mineralisation following sward destruction provides high amounts of plant-available N.

Evidence for fungal and chemodenitrification based N2O flux from nitrogen impacted coastal sediments

Although increasing atmospheric nitrous oxide (N2O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N2O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N2O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N2O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N2O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N2O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N2O fluxes, especially in redox-dynamic sediments of coastal ecosystems.

Nitrous oxide production and mRNA expression analysis of nitrifying and denitrifying bacterial genes under floodwater disappearance and fertilizer application.

A pulse of nitrous oxide (N2O) emission has been observed following the disappearance of floodwater by drainage. However, its mechanism is not well understood. We conducted a column study to clarify the mechanism for N2O production during floodwater disappearance by using a microsensor and determining the bacterial gene expression. An increase in N2O flux was observed following floodwater disappearance after the addition of NH4+, with a corresponding increase in the concentrations of NO3- and dissolved N2O in the oxic and anoxic soil layers, respectively. The transcription level of the bacterial amoA mRNA did not change, while that of nirK mRNA increased sharply after an hour of floodwater disappearance. An additional anoxic soil slurry experiment demonstrated that the addition of NO3- induced the expression of nirK gene and caused a concomitant increase in N2O production. These findings suggest that NO3- production in the oxic layers is important as it provides a substrate and induces the synthesis of denitrification enzymes in the anoxic layer during N2O production.

Non‐Legume Cover Crops Can Increase Non‐Growing Season Nitrous Oxide Emissions

Core Ideas Nitrous oxide emissions were greater in winter than spring or fall. Tillage radish increased over‐winter N2O fluxes. Non‐legume cover crops increased N2O fluxes under apparent NO3 limiting conditions. Cover crops retain post‐harvest nutrients but how they impact non‐growing season nitrous oxide (N2O) emissions is unclear. Therefore, we quantified how cover crop type (fall rye [Secale cereale L.] or oilseed radish [Raphanus sativus L.]) and fertilizer source (compost or inorganic fertilizer) affected N2O emissions, soil water‐extractable organic C (WEOC) and nitrate (NO3) dynamics over two non‐growing seasons. A treatment with no fertilizer or cover crop was also included. Weekly, N2O fluxes were determined using vented static chambers; soil WEOC and NO3–N concentrations were measured monthly. Each non‐growing season, mean N2O fluxes were 74 to 450% greater in the winter (21 December–20 March) than spring (21 March–20 June) or fall (22 September–20 December). In winter 2014–2015, oilseed radish increased the mean N2O flux by 39 and 323% compared with fall rye and no cover crop, respectively, while the mean N2O fluxes were strongly correlated to the pre‐winter (16 Dec. 2014) NO3 concentrations (r = 0.96; P &lt; 0.001), indicating NO3 levels &lt; 6 mg NO3–N kg–1 limited N2O fluxes. In 2014–2015, fall rye and oilseed radish had 76 and 154% greater cumulative N2O emissions than amended soils with no cover crop, respectively. Across both winters, an exponential model explained 67% of variability between the pre‐winter WEOC to NO3 ratio and N2O fluxes, indicating that organic C and NO3 controlled over‐winter N2O fluxes. Non‐legume cover crops increased non‐growing season N2O emissions, suggesting that cover crops concentrate denitrification substrates in root‐associated soil to enhance N2O fluxes.

Contrasting response of two grassland soils to N addition and moisture levels: N2O emission and functional gene abundance

Nitrification and denitrification processes dominate nitrous oxide (N2O) emission in grassland ecosystems, but their relative contribution as well as the abiotic factors are still not well understood. Two grassland soils from Duolun in Inner Mongolia, China, and Canterbury in New Zealand were used to quantitatively compare N2O production and the abundance of bacterial and archaeal amoA, denitrifying nirK and nirS genes in response to N additions (0 and 100 μg NH4 +–N g−1 dry soil) and two soil moisture levels (40 and 80 % water holding capacity) using microcosms. Soil moisture rather than N availability significantly increased the nitrification rate in the Duolun soil but not in the Canterbury soil. Moreover, N addition promoted denitrification enzyme activities in the Canterbury soil but not in the Duolun soil. The abundance of bacterial and archaeal amoA genes significantly increased as soil moisture increased in the Duolun soil, whereas in the Canterbury soil, only the abundance of bacterial amoA gene increased. The increase in N2O flux induced by N addition was significantly greater in the Duolun soil than in the Canterbury soil, suggesting that nitrification may have a dominant role in N2O emission for the Duolun soil, while denitrification for the Canterbury soil. Microbial processes controlling N2O emission differed in grassland soils, thus providing important baseline data in terms of global change.

The influence of tillage on N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O fluxes from an intensively managed grazed grassland in Scotland

Abstract. Intensively managed grass production in high-rainfall temperate climate zones is a globally important source of N2O. Many of these grasslands are occasionally tilled to rejuvenate the sward, and this can lead to increased N2O emissions. This was investigated by comparing N2O fluxes from two adjacent intensively managed grazed grasslands in Scotland, one of which was tilled. A combination of eddy covariance, high-resolution dynamic chamber and static chamber methods was used. N2O emissions from the tilled field increased significantly for several days immediately after ploughing and remained elevated for approximately 2 months after the tillage event contributing to an estimated increase in N2O fluxes of 0.85 ± 0.11 kg N2O-N ha−1. However, any influence on N2O emissions after this period appears to be minimal. The cumulative N2O emissions associated with the tillage event and a fertiliser application of 70 kg N ammonia nitrate from one field were not significantly different from the adjacent untilled field, in which two fertiliser applications of 70 kg N ammonia nitrate occurred during the same period. Total cumulative fluxes calculated for the tilled and untilled fields over the entire 175-day measurement period were 2.14 ± 0.18 and 1.65 ± 1.02 kg N2O-N ha−1, respectively.

Water management reduces greenhouse gas emissions in a Mediterranean rice paddy field

Rice paddy fields are one of the largest anthropogenic sources of methane (CH4), the second most important anthropogenic greenhouse gas (GHG) after carbon dioxide (CO2). For this reason most studies on the GHG budget in these agricultural systems focus on the evaluation of CH4 production. However, these systems also exchange other GHGs with the atmosphere, such as CO2 and nitrous oxide (N2O). To estimate the total global warming potential (GWP) of rice cultivation, a field experiment was carried out in a Mediterranean rice paddy field in the Po Valley (Italy), the largest rice producing region in Europe. For two consecutive years, ecosystem CO2 and CH4 fluxes were assessed using the eddy covariance technique and CH4 and N2O fluxes were measured with closed chambers. The net biome productivity indicated a nearly carbon (C) neutral system in 2009 while it accumulated C in 2010, due to the application of organic fertilizers and the midseason drainage of the otherwise flooded field, the latter having the additional benefit of leading to lower water consumption. The rice paddy field acted as a strong GHG source with a GWP of 1148g CO2-eqm−2yr−1 in 2009 and decreased four-fold in 2010 (289g CO2-eqm−2yr−1). In both years, the site was a large CH4 source. Differences in the GHG budget between the two years of measurements were mainly caused by the lower CH4 emissions in 2010 (21.0g CH4 m−2 compared to 37.4gCH4m−2 in 2009), probably driven by drainage of the otherwise flooded field in the middle of the growing season during 2010 and moderately larger CO2 uptake. The increased N2O fluxes (29%), had a marginal contribution to the GWP. However, midseason drainage, which needs to be evaluated in combination with the concurrent application of organic fertilizers, resulted in small decreases of yield. Our results therefore suggest that an adequate management of the water table level reduces CH4 fluxes and has the potential to decrease the GWP and water losses through evapotranspiration of rice paddy fields, confirming that full GHG budgets should be assessed in combination with yields in order to develop and evaluate effective mitigation strategies.

How gut-stimulated denitrifiers influence soil N2O emission without earthworm activity

Gut-stimulated denitrifiers are responsible for the in vivo emission of N2O by earthworms, and contribute to the N2O that is emitted from certain terrestrial ecosystems. To evaluate the separate effect of gut-stimulated denitrifiers on N2O emissions, earthworms or their casts were inoculated into sterilized and unsterilized soil, respectively, and compared with control (soil without inoculation) in this study, to find out if there was a difference in N2O emissions during the 15 days incubation period. Compared with the control, earthworm inoculation significantly increased N2O flux and relative densities of nirS and narG gene copy numbers both in sterilized and unsterilized soil, while no significant difference was found when we transited and recolonized those gut-stimulated denitrifiers into soil artificially. In contrast, cast inoculation did not change CO2 emission in the unsterilized soil, but significant increased cumulative CO2 emission in the sterilized soil. Our data suggest that the effect of gut-stimulated denitrifiers on soil N2O produce was strongly diminished when separated from earthworm activity.