Average N2O Fluxes Research Articles

Tillage and nitrogen fertilization effects on nitrous oxide yield-scaled emissions in a rainfed Mediterranean area

There is a strong need to identify the combination of tillage and N fertilization practices that reduce the amount of nitrous oxide (N2O) emissions while maintaining crop productivity in dryland Mediterranean areas. We measured the fluxes of N2O in two field experiments with 3 and 15 years since their establishment. In the long-term experiment, two types of tillage (NT, no-tillage, and CT, conventional intensive tillage) and three mineral N fertilization rates (0, 60 and 120kgNha−1) were compared. In the short-term experiment, the same tillage systems (CT and NT) and three N fertilization doses (0, 75 and 150kgNha−1) and two types of fertilizers (mineral N and organic N with pig slurry) were compared. N2O emissions, water-filled pore space, soil mineral N content, grain yields, N-biomass inputs and soil total nitrogen (STN) stocks were quantified and the N2O yield-scaled ratio as kg of CO2 equivalents per kg of grain produced was calculated. In both experiments tillage treatments significantly affected the dynamics of N2O fluxes. Cumulative losses of N as N2O were similar between tillage treatments in the long-term field experiment. Contrarily, although not significant, cumulative N losses were about 35% greater under NT than CT in the short-term experiment. NT significantly increased the production of grain and the inputs of N to the soil as above-ground biomass in both experiments. Averaged across fertilizer treatments, CT emitted 0.362 and 0.104kg CO2 equiv.kggrain−1 in the long-term and the short-term experiment, respectively, significantly more than NT that emitted 0.033 and 0.056kg CO2 equiv.kggrain−1, respectively. Nitrogen fertilization rates did not affect the average N2O fluxes or the total N losses during the period of gas measurement in the long-term experiment. Contrarily, in the short-term experiment, N2O emissions increased with application rate for both mineral and organic fertilizers. The use of pig slurry increased grain production when compared with the mineral N treatment, thus reducing the yield-scaled emissions of N2O by 44%. Our results showed that in rainfed Mediterranean agroecosystems, the use of NT and pig slurry are effective means of yield-scaled N2O emissions reduction.

Mitigating nitrous oxide emissions from tea field soil using bioaugmentation with a Trichoderma viride biofertilizer.

Land-use conversion from woodlands to tea fields in subtropical areas of central China leads to increased nitrous oxide (N2O) emissions, partly due to increased nitrogen fertilizer use. A field investigation of N2O using a static closed chamber-gas chromatography revealed that the average N2O fluxes in tea fields with 225 kg N ha−1 yr−1 fertilizer application were 9.4 ± 6.2 times higher than those of woodlands. Accordingly, it is urgent to develop practices for mitigating N2O emissions from tea fields. By liquid-state fermentation of sweet potato starch wastewater and solid-state fermentation of paddy straw with application of Trichoderma viride, we provided the tea plantation with biofertilizer containing 2.4 t C ha−1 and 58.7 kg N ha−1. Compared to use of synthetic N fertilizer, use of biofertilizer at 225 kg N ha−1 yr−1 significantly reduced N2O emissions by 33.3%–71.8% and increased the tea yield by 16.2%–62.2%. Therefore, the process of bioconversion/bioaugmentation tested in this study was found to be a cost-effective and feasible approach to reducing N2O emissions and can be considered the best management practice for tea fields.

Impact of sampling time on chamber-based measurements of riverine nitrous oxide emissions using relative difference analysis

The daily mean riverine N2O flux at a field site is often assumed to be equivalent to the magnitude of occasional once-per-day measurements made using the chamber method. The resulting time series data set therefore has a low temporal resolution. However, significant temporal variations in the N2O fluxes are typically found in streams and rivers, and high-frequency sampling is needed. To minimize the number of observations required without introducing significant bias from the daily averaging of values, we extend the temporal stability method, which has been widely used in soil physics and hydrology. We present the first analysis of the effects of sampling time on the determination of riverine N2O emissions based on high-frequency sampling of N2O fluxes. The results showed that: (1) daytime sampling can overestimate the daily average riverine N2O emissions, and emissions were more spatially stable at nighttime; (2) the spatial stability of N2O was controlled by the patterns of sewage inputs and amounts of dissolved oxygen (DO), NH4+–N, Cl−, and SO42−; and (3) sampling in the early morning (around 05:00h) and evening (around 19:00h) is typically spatially stable and is able to consistently represent the daily average N2O flux rate at a study site. Our methodology potentially provides a simple and rapid sampling protocol that could be used to identify the most suitable sampling times for floating chambers in estimating daily mean N2O flux rate from rivers.

Measuring and modeling the effects of drainage water management on soil greenhouse gas fluxes from corn and soybean fields

Controlled tile drainage can boost crop yields and improve water quality, but it also has the potential to increase GHG emissions. This study compared in-situ chamber-based measures of soil CH4, N2O, and CO2 fluxes for silt loam soil under corn and soybean cropping with conventional tile drainage (UTD) and controlled tile drainage (CTD). A semi-empirical model (NEMIS-NOE) was also used to predict soil N2O fluxes from soils using observed soil data. Observed N2O and CH4 fluxes between UTD and CTD fields during the farming season were not significantly different at 0.05 level. Soils were primarily a sink for CH4 but in some cases a source (sources were associated exclusively with CTD). The average N2O fluxes measured ranged between 0.003 and 0.028 kg N ha−1 day−1. There were some significantly higher (p ≤ 0.05) CO2 fluxes associated with CTD relative to UTD during some years of study. Correlation analyses indicated that the shallower the water table, the greater the CO2 fluxes. Higher corn plant C for CTD tended to offset estimated higher CTD CO2 C losses via soil respiration by ∼100–300 kg C ha−1. There were good fits between observed and predicted (NEMIS–NOE) N2O fluxes for corn (R2 = 0.70) and soybean (R2 = 0.53). Predicted N2O fluxes were higher for CTD for approximately 70% of the paired-field study periods suggesting that soil physical factors, such as water-filled pore space, imposed by CTD have potentially strong impacts on net N fluxes. Model predictions of daily cumulative N2O fluxes for the agronomically-active study period for corn-CTD and corn-UTD, as a percentage of total N fertilizer applied, were 3.1% and 2.6%, respectively. For predicted N2O fluxes on basis of yield units, indices were 0.0005 and 0.0004 (kg N kg−1 crop grain yield) for CTD and UTD corn fields, respectively, and 0.0011 and 0.0005 for CTD and UTD soybean fields, respectively.

Exchange of the Greenhouse Gases Methane and Nitrous Oxide Between the Atmosphere and a Temperate Peatland in Central Europe

Fluxes of methane (CH4), nitrous oxide (N2O), and respired CO2 were measured with an automated chamber system (SIGMA) over the autumn of 2009 and growing season of 2010 in an undisturbed peatland with a floating carpet of mosses in western Poland. The main goal of our investigation was to deliver estimations of CH4 and N2O emission rates for this unique peatland type in Central Europe. CO2 and CH4 fluxes were also measured using a non-steady-state-flow-through manual chamber systems equipped with infrared gas analyzers and used to validate and adjust results obtained with the SIGMA system. The average methane fluxes for the 2009–2010 (excluding winter) period was 95.4 (±32.4) mg CH4-C m−2 d−1 resulting in a cumulative CH4 emission of 23.4 (±2.4) g CH4-C m−2 day growing season and 29.2 (±3.2) g CH4-C m−2 year−1 for the whole of 2010. The average seasonal N2O fluxes were 13.6 (± 28.2) μg N2O-N m−2 d−1. N2O fluxes were highly variable in time and sign (both uptake and emissions of N2O were observed during the accumulation periods). Maximum estimates of the annual emission rates of N2O were 0.004 g N2O-N m−2 year−1. Methane fluxes correlated significantly with the air and peat temperatures, but they were not dependent on water table depth. The N2O fluxes did not demonstrate a clear response to any environmental variables. Both CH4 and N2O emission rates were in the range of fluxes recorded in pristine peatlands in other climatic zones.

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain. After 2 years of fertilization with NH4NO3 at six rates (0, 10, 20, 40, 80 and 160 kg N ha−1 yr−1), the responses of plant and soil parameters as well as N2O fluxes were measured. At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5 ± 0.1 g m−2 in the control plots to 1.9 ± 0.2 g m−2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ15N, soil NO3 −-N and N2O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha−1 yr−1 to 160 kg N ha−1 yr−1, the N-use efficiency decreased from 12.3 ± 4.6 kg C kg N−1 to 1.6 ± 0.2 kg C kg N−1, and the N-uptake efficiency decreased from 43.2 ± 9.7 % to 9.1 ± 1.1 %. Biomass N:P ratios increased from 14.4 ± 2.6 in the control plots to 20.5 ± 0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha−1 yr−1. Above this level, soil NO3 −-N began to accumulate. The seasonal average N2O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ15N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant 15N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species. Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha−1 yr−1. For the entire Tibetan Plateau (ca. 2.57 million km2), a low N deposition rate (10 kg N ha−1 yr−1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1 ± 0.3 Tg N yr−1 and 31.5 ± 11.8 Tg C yr−1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.

Spatial and temporal variations of nitrous oxide flux between coastal marsh and the atmosphere in the Yellow River estuary of China.

To investigate the spatial and seasonal variations of nitrous oxide (N2O) fluxes and understand the key controlling factors, we explored N2O fluxes and environmental variables in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in the Yellow River estuary throughout a year. Fluxes of N2O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N2O fluxes ranged from −0.0051 to 0.0805 mg N2O m−2 h−1, and high N2O emissions occurred during spring (0.0278 mg N2O m−2 h−1) and winter (0.0139 mg N2O m−2 h−1) while low fluxes were observed during summer (0.0065 mg N2O m−2 h−1) and autumn (0.0060 mg N2O m−2 h−1). The annual average N2O flux from the intertidal zone was 0.0117 mg N2O m−2 h−1, and the cumulative N2O emission throughout a year was 113.03 mg N2O m−2, indicating that coastal marsh acted as N2O source. Over all seasons, N2O fluxes from the four marshes were significantly different (p < 0.05), in the order of HM (0.0256 ± 0.0040 mg N2O m−2 h−1) > MF (0.0107 ± 0.0027 mg N2O m−2 h−1) > LM (0.0073 ± 0.0020 mg N2O m−2 h−1) > MM (0.0026 ± 0.0011 mg N2O m−2 h−1). Temporal variations of N2O emissions were related to the vegetations (Suaeda salsa, Phragmites australis, and Tamarix chinensis) and the limited C and mineral N in soils during summer and autumn and the frequent freeze/thaw cycles in soils during spring and winter, while spatial variations were mainly affected by tidal fluctuation and plant composition at spatial scale. This study indicated the importance of seasonal N2O contributions (particularly during non-growing season) to the estimation of local N2O inventory, and highlighted both the large spatial variation of N2O fluxes across the coastal marsh (CV = 158.31 %) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N2O emission should be considered before the annual or local N2O inventory was evaluated accurately.

A field trial of nutrient stimulation of methanotrophs to reduce greenhouse gas emissions from landfill cover soils

Landfills are among the major sources of anthropogenic methane (CH4) estimated to reach 40 × 109kg per year worldwide by 2015 (IPCC, 2007). A 2½-year field experiment was conducted at a closed landfill in western Michigan where methanotrophs, methane-consuming bacteria, were stimulated by nutrient addition to the soil without significantly increasing biogenic nitrous oxide (N2O) production. The effects of the nitrogen amendments (KNO3 and NH4Cl), phenylacetylene (a selective inhibitor of nitrifying bacteria that contribute to N2O production), and a canopy (to reduce direct water infiltration) on the vertical soil gas profiles of CH4, CO2, and O2 were measured in the top meter of the soil. Methane and nitrous oxide fluxes were calculated from the corresponding soil gas concentration gradients with respect to depth and a Millington–Quirk diffusivity coefficient in soil derived empirically from soil porosity, water content, and diffusivity coefficients in air from the literature. Methane flux estimates were as high as 218.4 g m−2 day−1 in the fall and 12.8 g/m−2 day−1 in the summer. During the spring and summer, CH4 fluxes were reduced by more than half by adding KNO3 and NH4Cl into the soil as compared to control plots, while N2O fluxes increased substantially. The concurrent addition of phenylacetylene to the amendment decreased peak N2O production by half and the rate of peak methane oxidation by about one-third. The seasonal average methane and N2O flux data were extrapolated to estimate the reduction of CH4 and N2O fluxes into the atmosphere by nitrogen and inhibitor addition to the cover soils. The results suggest that such additions coupled with soil moisture management may provide a potential strategy to significantly reduce greenhouse gas emissions from landfills. Implications The results of a 2½-year study of effects of nutrient stimulation on methane oxidation in landfill cover soils demonstrates that nutrient addition does decrease methane emissions. The work further underscores the control which soil moisture exerts on methane oxidation. Water management is critical to the success of methane oxidation strategies. Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association.

Occasional large emissions of nitrous oxide and methane observed in stormwater biofiltration systems

Designed, green infrastructures are becoming a customary feature of the urban landscape. Sustainable technologies for stormwater management, and biofilters in particular, are increasingly used to reduce stormwater runoff volumes and peaks as well as improve the water quality of runoff discharged into urban water bodies. Although a lot of research has been devoted to these technologies, their effect in terms of greenhouse gas fluxes in urban areas has not been yet investigated. We present the first study aimed at quantifying greenhouse gas fluxes between the soil of stormwater biofilters and the atmosphere. N2O, CH4, and CO2 were measured periodically over a year in two operational vegetated biofiltration cells at Monash University in Melbourne, Australia. One cell had a saturated zone at the bottom, and compost and hardwood mulch added to the sandy loam filter media. The other cell had no saturated zone and was composed of sandy loam. Similar sedges were planted in both cells. The biofilter soil was a small N2O source and a sink for CH4 for most measurement events, with occasional large emissions of both N2O and CH4 under very wet conditions. Average N2O fluxes from the cell with the saturated zone were almost five-fold greater (65.6μg N2O–Nm−2h−1) than from the other cell (13.7μg N2O–Nm−2h−1), with peaks up to 1100μg N2O–Nm−2h−1. These N2O fluxes are of similar magnitude to those measured in other urban soils, but with larger peak emissions. The CH4 sink strength of the cell with the saturated zone (−3.8μg CH4–Cm−2h−1) was lower than the other cell (−18.3μg CH4–Cm−2h−1). Both cells of the biofilter appeared to take up CH4 at similar rates to other urban lawn systems; however, the biofilter cells displayed occasional large CH4 emissions following inflow events, which were not seen in other urban systems. CO2 fluxes increased with soil temperature in both cells, and in the cell without the saturated zone CO2 fluxes decreased as soil moisture increased. Other studies of CO2 fluxes from urban soils have found both similar and larger CO2 emissions than those measured in the biofilter. The results of this study suggest that the greenhouse gas footprint of stormwater treatment warrant consideration in the planning and implementation of engineered green infrastructures.

Diurnal pattern in nitrous oxide emissions from a sewage-enriched river

Estimates of N2O emission based on limit measurements could be highly inaccurate because of considerable diurnal variations in N2O flux due to rapid transformation of nutrients and diel change of dissolved oxygen (DO). In the present study, the N2O fluxes, dissolved N2O concentrations, and the controlling variables were measured hourly for 3d and night cycles at five sites on a typically sewage-enriched river in the Taihu region. There were no significant diurnal patterns in N2O emissions and dissolved N2O saturation, with respective mean value of 56.1μg N2O–Nm−2h−1 (range=41.1μg N2O–Nm−2h−1 to 87.7μg N2O–Nm−2h−1) and 813% (range=597–1372%), though distinct diurnal patterns were observed in DO concentration and river chemistry. However, the mean N2O emissions and the mean dissolved N2O saturation during the day (61.7μgNm−2h−1 for N2O fluxes and 0.52μgNL−1 for dissolved N2O saturation) were significantly higher than those during the night (50.1μgNm−2h−1for N2O fluxes and 0.44μgNL−1 for dissolved N2O saturation). Factors controlling the N2O flux were pH, DO, NH4+,SO42-, air temperature, and water temperature. Sampling at 19:00h could well represent the daily average N2O flux at the studied river.

Estimates of N2O Emissions and Mitigation Potential from a Spring Maize Field Based on DNDC Model

Agricultural production plays an important role in affecting atmospheric nitrous oxide (N2O) concentrations. Field measurements were conducted in Dalian City, Liaoning Province in Northeast China from two consecutive years (2009 and 2010) to estimate N2O emissions from a spring maize field, a main cropping system across the Chinese agricultural regions. The observed flux data in conjunction with the local climate, soil and management information were utilized to test a process-based model, DeNitrification-DeComposition (DNDC), for its applicability for the cropping system. The validated DNDC was then used for exploring strategies to reduce N2O emissions from the target field. The results showed that the major N2O pulse emissions occurred with duration of about 3-5 d after fertilizer application in both years 2009 and 2010, which on average accounted for about 60% of the total N2O emissions each year. Rainfall and fertilizer application were the major factors influencing the N2O emissions from spring maize field. The average N2O fluxes from the CK (control plot, without fertilization) and FP (traditional chemical N fertilizer) treatments were 23.1 and 60.6 μg m−2 h−1 in 2009, respectively, and 21.5 and 64.3 μg m−2 h−1 in 2010, respectively. The emission factors (EFs) of the applied N fertilizer (270 kg N ha−1) as N2O-N were 0.62% in 2009 and 0.77% in 2010, respectively. The comparison of modeled daily N2O emission fluxes against observations indicated that the DNDC model had a good performance even if without adjusting the internal parameters. The modeled results showed that management practices such as no-till, changing timing or rate of fertilizer application, increasing residue incorporation, and other technically applicable measures could effectively reduce N2O emissions from the tested fields. Our study indicated that avoiding application of N fertilizers at heavy rainfall events or splitting the fertilizer into more applications would be the most feasible approaches to reduce N2O emissions from spring maize production in Northeast China.

Emissions of CH&lt;sub&gt;4&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O from a Wetland in Sanjiang Plain

The seasonal variation and influence factors of emission of greenhouse gasses (CH4 and N2O) from the different water depths were studied during the growing seasons in a marsh wetland of Sanjiang Plain of China. The results showed that the wetland was a source of both CH4 and N2O during the growing seasons and emissions showed conspicuous temporal and spatial variations. Emission rates of CH4 were higher in July and August while N2O were higher in June and July. The highest average CH4 flux among the four sites was in the water depth at 40cm with a rate of 37.00 mg•m-2•h-1 and the lowest was in the water depth at 0cm with a rate of 8.13 mg•m-2•h-1. While the highest average N2O flux was in the water depth at 0cm with a rate of 0.063 mg•m-2•h-1 and the lowest was in the water depth at 60cm with a rate of 0.034 mg•m-2•h-1. Further analysis indicated that water depth at 40cm and the soil temperature at 5cm were the main factors affecting CH4 emission. Water depths at 0 and 40cm were the main factors affecting N2O emission. Air temperature and surface soil temperature were not the main factors affecting CH4 and N2O emissions.

Nitrous oxide fluxes from tropical peat with different disturbance history and management

Abstract. Tropical peatlands are one of the most important terrestrial ecosystems in terms of impact on the atmospheric greenhouse gas composition. Currently, greenhouse gas emissions from tropical peatlands following disturbances due to deforestation, drainage or wildfire are substantial. We quantified in situ nitrous oxide (N2O) fluxes during both dry and wet seasons using a closed chamber method at sites that represented differing land uses and land use change intensities in Central Kalimantan, Indonesia. Cumulative N2O fluxes were compared with carbon dioxide (CO2) and methane (CH4) fluxes. The mean N2O flux rates (N2O-N &amp;amp;plusmn: SD, mg m−2 h−1) varied as follows: drained forest (0.112 ± 0.293) &gt; agricultural peat at the Kalampangan site (0.012 ± 0.026) &gt; drained burned peat (0.011 ± 0.018) &gt; agricultural peat at the Marang site (0.0072 ± 0.028) &gt; undrained forest (0.0025 ± 0.053) &gt; clear-felled, drained, recovering forest (0.0022 ± 0.021). The widest N2O flux range was detected in the drained forest (max. 2.312 and min. −0.043 mg N2O-N m−2 h−1). At the other flux monitoring sites the flux ranges remained at about one tenth that of the drained forest site. The highest N2O emission rates were observed at water tables close to the peat surface where also the flux range was widest. Annual cumulative peat surface N2O emissions (expressed in CO2 equivalents as a percentage of the total greenhouse gas (N2O, CO2 and CH4) emissions) were 9.2 % at highest, but typically ~1 %. Average N2O fluxes and also the total of monitored GHG emissions were highest in drainage-affected forest which is characterized by continuous labile nitrogen availability from vegetation, and water tables typically below the surface.

Effect of Assessment Scale on Spatial and Temporal Variations in CH4, CO2, and N2O Fluxes in a Forested Wetland

Emissions of methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) from a forested watershed (160 ha) in South Carolina, USA, were estimated with a spatially explicit watershed-scale modeling framework that utilizes the spatial variations in physical and biogeochemical characteristics across watersheds. The target watershed (WS80) consisting of wetland (23%) and upland (77%) was divided into 675 grid cells, and each of the cells had unique combina- tion of vegetation, hydrology, soil properties, and topography. Driven by local climate, topography, soil, and vegetation conditions, MIKE SHE was used to generate daily flows as well as water table depth for each grid cell across the watershed. Forest-DNDC was then run for each cell to calculate its biogeochemistry including daily fluxes of the three greenhouse gases (GHGs). The simulated daily average CH4 ,C O2 and N2O flux from the watershed were 17.9 mg C, 1.3 g C and 0.7 mg N m −2 , respectively, during the period from 2003-2007. The average contributions of the wetlands to the CH4 ,C O2 and N2O emissions were about 95%, 20% and 18%, respectively. The spatial and temporal variation in the modeled CH4 ,C O 2 and N2 Of luxes were large, and closely related to hydrological con- ditions. To understand the impact of spatial heteroge- neity in physical and biogeochemical characteristics of the target watershed on GHG emissions, we used Forest-DNDC in a coarse mode (field scale), in which the entire watershed was set as a single simulated unit, where all hydrological, biogeochemical, and biophys- ical conditions were considered uniform. The results from the field-scale model differed from those modeled with the watershed-scale model which considered the spatial differences in physical and biogeochemical characteristics of the catchment. This contrast demon- strates that the spatially averaged topographic or biophysical conditions which are inherent with field- scale simulations could mask "hot spots" or small source areas with inherently high GHGs flux rates. The spatial resolution in conjunction with coupled hydro- logical and biogeochemical models could play a crucial role in reducing uncertainty of modeled GHG emissions from wetland-involved watersheds.

Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in on‐farm corn crops of the US Midwest

AbstractRow‐crop agriculture is a major source of nitrous oxide (N2O) globally, and results from recent field experiments suggest that significant decreases in N2O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0–225 kg N ha−1) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N2O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N2O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha−1), average N2O fluxes were 43% (18 g N2O–N ha−1 day−1) and 115% (26 g N2O–N ha−1 day−1) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha−1yielded an average 8.3 Mg grain ha−1. Our study shows the potential to lower agricultural N2O fluxes within a range of N fertilization that does not affect economic return from grain yield.

CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O fluxes from soil of a burned grassland in Central Africa

Abstract. The impact of fire on soil fluxes of CO2, CH4 and N2O was investigated in a tropical grassland in Congo Brazzaville during two field campaigns in 2007–2008. The first campaign was conducted in the middle of the dry season and the second at the end of the growing season, respectively one and eight months after burning. Gas fluxes and several soil parameters were measured in each campaign from burned plots and from a close-by control area preserved from fire. Rain events were simulated at each campaign to evaluate the magnitude and duration of the generated gas flux pulses. In laboratory experiments, soil samples from field plots were analysed for microbial biomass, net N mineralization, net nitrification, N2O, NO and CO2 emissions under different water and temperature soil regimes. One month after burning, field CO2 emissions were significantly lower in burned plots than in the control plots, the average daily CH4 flux shifted from net emission in the unburned area to net consumption in burned plots, no significant effect of fire was observed on soil N2O fluxes. Eight months after burning, the average daily fluxes of CO2, CH4 and N2O measured in control and burned plots were not significantly different. In laboratory, N2O fluxes from soil of burned plots were significantly higher than fluxes from soil of unburned plots only above 70% of maximum soil water holding capacity; this was never attained in the field even after rain simulation. Higher NO emissions were measured in the lab in soil from burned plots at both 10% and 50% of maximum soil water holding capacity. Increasing the incubation temperature from 25 °C to 37 °C negatively affected microbial growth, mineralization and nitrification activities but enhanced N2O and CO2 production. Results indicate that fire did not increase post-burning soil GHG emissions in this tropical grasslands characterized by acidic, well drained and nutrient-poor soil.

Development of an accumulation‐based system for cost‐effective chamber measurements of inert trace gas fluxes

As soil–atmosphere fluxes of greenhouse gases are characterized by high temporal fluctuations, frequent measurements in the range of hours to days need to be deployed, resulting in high analytical costs. We have therefore developed a new low‐cost system that combines high‐frequency automated sampling with low‐frequency chemical analysis. The System for Inert Gas Monitoring by Accumulation (SIGMA) is suited particularly for stand‐alone observations in remote locations. The SIGMA is connected to an automated chamber with headspace sampling several times per day. Air samples are aggregated in sampling bags, which reduces the number of subsequent laboratory analyses and allows calculation of average flux rates over extended sampling periods. The SIGMA was tested under field conditions and compared with a conventional autochamber system, where flux rates were measured several times per day. Sample air fillings of the SIGMA sampling bags varied less than 5% between bags and diverged &lt;1 to 6% of the pre‐set values, justifying the assumption about proportional mixing of chamber headspace samples. When the SIGMA and conventional autochambers were compared simultaneously over the same patch of grassland in Denmark, the estimates agreed within ±12% in the assessment of average N2O fluxes. When deployed to discrete autochambers situated a few metres apart and under various environmental conditions in Denmark and the UK, we found no consistent difference between the two measuring systems. For a N‐fertilized cropland in Denmark the SIGMA N2O and CO2fluxes were 28 and 23% less, respectively than those recorded with the conventional autochamber, while for a N‐fertilized grassland in the UK the SIGMA N2O flux was 47% more than that recorded with the conventional autochamber and 6% less than that recorded with a manual chamber system. Such differences are within the range of natural spatial variability in trace gas fluxes for these ecosystems and indicate the potential for further application of the SIGMA approach as a cost‐effective technique to estimate long‐term trace gas fluxes.

N2O emissions from municipal solid waste landfills with selected infertile cover soils and leachate subsurface irrigation

This study presents the field investigations into the effects of cover soils and leachate subsurface irrigation on N2O emissions from municipal solid waste landfills. Landfill Site A and Site B, covered with carefully chosen infertile soils, were selected to monitor their diurnal and seasonal variations of N2O emissions. The annual average N2O flux was 469 +/- 796 microg N2O-N m(-2) h(-1) in Site B with leachate subsurface irrigation, three times that of Site A without leachate irrigation. When an additional soil containing lower contents of carbon and nitrogen was introduced to cover part of Site B, its N2O fluxes decreased by 1-2 orders of magnitude compared with the left area of Site B. This suggested that carefully selected cover soils could substantially reduce N2O emissions even under leachate subsurface irrigation. Statistical analysis proved that the availabilities of soil moisture and mineralized nitrogen were the key parameters controlling landfill N2O emissions.

Potential N2O emissions from leguminous tree plantation soils in the humid tropics

We compared nitrous oxide (N2O) emissions over 1 year from soils of plantations growing acacia, which is a leguminous plant capable of symbiotic nitrogen fixation in root nodules, and secondary forests in Sumatra, Indonesia. N2O emissions from acacia plantation soils fluctuated seasonally, from high in the wetter season to low in the drier season, whereas N2O emissions from secondary forest soils were low throughout the year. Water‐filled‐pore‐space data showed that denitrification contributed substantially to N2O emissions from soils at acacia sites. The average annual N2O flux in acacia plantations was 2.56 kg N ha−1 a−1, which was eight times higher than that from secondary forest soils (0.33 kg N ha−1 a−1). In secondary forests, NH4+ was the dominant form of inorganic nitrogen. However, in acacia plantations, the NH4+: NO3− ratio was relatively lower than that in secondary forests. These results suggest that secondary forests were nitrogen limited, but acacia plantations were less nitrogen limited. Leguminous tree plantations may increase nitrogen cycling, resulting in greater N2O emissions from the soil. However, on a global warming potential basis, N2O emissions from acacia plantation soils accounted for less than 10% of the carbon uptake by plants. Nevertheless, because of the spread of leguminous tree plantations in Asia, the importance of N2O emissions from leguminous tree stands will increase in the coming decades.

Characteristics of CO2, CH4 and N2O emissions from winter-fallowed paddy fields in hilly areas of South China

With closed static chamber and modified gas chromatograph (HP5890 II), the in situ measurements were made on the CO2, CH4 and N2O emissions from winter-fallowed paddy fields in the hilly area of South China. Gas samples were taken simultaneously from the fields with and without rice stubble. The results showed that both of the fields had the peak value of CO2 flux in the late afternoon. In the fields with and without rice stubble, the CH4 flux was positive in the day time while negative in the night, and the N2O flux in the day time was 1.79 and 1.58 times as much as that in the night, respectively. The diurnal average CO2 flux in the field with rice stubble was significantly higher than that in bare field (P < 0.05). Correlation analysis demonstrated that the CO2 flux in winter-fallowed paddy fields had significant correlations with soil temperature, aboveground temperature, and air temperature, suggesting that temperature was the main factor affecting the CO2 emission from rice field after harvesting. During the observation time (from 2003-11-10 to 2004-01-18), the average CO2, CH4 and N2O fluxes in the field with rice stubble were (180.69 +/- 21.21) mg x m(-2) x h(-1), (-0.04 +/- 0.01) mg x m(-2) x h(-1) and (21.26 +/- 19.31) microg x m(-2) x h(-1), respectively. Compared with bare field, the CO2 flux in the field with rice stubble was 13.06% higher, CH4 absorption increased by 50%, while N2O flux was 60.75% lower. It was concluded that the winter fallowed paddy field in hilly area of South China was the source of atmospheric CO2 and N2O, and the sink of atmospheric CH4.