Soil N2O Fluxes Research Articles

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at −8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH4 uptake and from 70 to 90% WFPS for N2O and CO2 emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N2O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g−1 C. Cumulative soil CO2 emissions and CH4 uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N2O, CO2, and CH4 fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.

Differential Responses and Controls of Soil CO2 and N2O Fluxes to Experimental Warming and Nitrogen Fertilization in a Subalpine Coniferous Spruce (Picea asperata Mast.) Plantation Forest

Emissions of greenhouse gases (GHG) such as CO2 and N2O from soils are affected by many factors such as climate change, soil carbon content, and soil nutrient conditions. However, the response patterns and controls of soil CO2 and N2O fluxes to global warming and nitrogen (N) fertilization are still not clear in subalpine forests. To address this issue, we conducted an eight-year field experiment with warming and N fertilization treatments in a subalpine coniferous spruce (Picea asperata Mast.) plantation forest in China. Soil CO2 and N2O fluxes were measured using a static chamber method, and soils were sampled to analyze soil carbon and N contents, soil microbial substrate utilization (MSU) patterns, and microbial functional diversity. Results showed that the mean annual CO2 and N2O fluxes were 36.04 ± 3.77 mg C m−2 h−1 and 0.51 ± 0.11 µg N m−2 h−1, respectively. Soil CO2 flux was only affected by warming while soil N2O flux was significantly enhanced by N fertilization and its interaction with warming. Warming enhanced dissolve organic carbon (DOC) and MSU, reduced soil organic carbon (SOC) and microbial biomass carbon (MBC), and constrained the microbial metabolic activity and microbial functional diversity, resulting in a decrease in soil CO2 emission. The analysis of structural equation model indicated that MSU had dominant direct negative effect on soil CO2 flux but had direct positive effect on soil N2O flux. DOC and MBC had indirect positive effects on soil CO2 flux while soil NH4+-N had direct negative effect on soil CO2 and N2O fluxes. This study revealed different response patterns and controlling factors of soil CO2 and N2O fluxes in the subalpine plantation forest, and highlighted the importance of soil microbial contributions to GHG fluxes under climate warming and N deposition.

Impact of nitrogen compounds on fungal and bacterial contributions to codenitrification in a pasture soil

Ruminant urine patches on grazed grassland are a significant source of agricultural nitrous oxide (N2O) emissions. Of the many biotic and abiotic N2O production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid N2O, is one of the least understood. Codenitrification forms hybrid N2O via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce N2O. The objective of this study was to assess the relative significance of different N substrates on codenitrification and to determine the contributions of fungi and bacteria to codenitrification. 15N-labelled ammonium, hydroxylamine (NH2OH) and two amino acids (phenylalanine or glycine) were applied, separately, to sieved soil mesocosms eight days after a simulated urine event, in the absence or presence of bacterial and fungal inhibitors. Soil chemical variables and N2O fluxes were monitored and the codenitrified N2O fluxes determined. Fungal inhibition decreased N2O fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N2O flux of the glycine treatment, by 14%. Hydroxylamine (NH2OH) generated the highest N2O fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N2O flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NH2OH substrate treatment followed those of total N2O fluxes (85.7% of total N2O flux). Codenitrification fluxes under non-NH2OH substrate treatments (0.7–1.2% of total N2O flux) were two orders of magnitude lower, and significant decreases in these treatments only occurred with fungal inhibition in the amino acid substrate treatments. These results demonstrate that in situ studies are required to better understand the dynamics of codenitrification substrates in grazed pasture soils and the associated role that fungi have with respect to codenitrification.

Soil moisture and activity of nitrite- and nitrous oxide-reducing microbes enhanced nitrous oxide emissions in fallow paddy soils

Although cumulative N2O emissions are greater in the winter fallow season than in the rice-growing period, the mechanisms by which the emissions affect fallow paddy fields remain unclear. We aimed to identify N2O flux characteristics and illustrate how key nirS-, nirK- and nosZ-containing denitrifiers affect N2O emission levels in acidic fallow paddy soil. Five water-filled pore space (WFPS) levels were set at 25%, 50%, 75%, 100% and 125%, respectively. During the 48-h-long, high-flux incubation period, the N2O flux was the highest in soil samples with 75% WFPS, followed by those with 100% WFPS. The size of nirS-containing denitrifier community was more sensitive to the shifts in soil moisture and showed a stronger correlation with N2O flux than that of nirK-containing denitrifiers, whereas higher N2O concentrations induced an increase in the levels of nosZ-containing bacteria. After incubation for 48 h, nirK- and nosZ-denitrifying bacterial composition varied remarkably under 50%, 75%, and 100% WFPS treatments. However, the composition of nirS-containing denitrifying bacterial community gradually varied with an increase in soil moisture from 25% to 100% WFPS. Certain dominant OTUs of nirK- nirS- and nosZ-containing denitrifiers were highly abundant, especially under treatments of 50%, 75% and 100% WFPS, which were closely associated with the N2O flux. Thus, nirK, nirS and nosZ-containing denitrifiers respond to soil moisture differently, and enriched species might mainly be involved in controlling N2O flux in fallow paddy soils via denitrification, while the abundance of nirS-containing denitrifiers might affect N2O emission levels more significantly than that of nirK-containing denitrifiers.

N2O Emissions from a Tea Field with Deep Application of Nitrogen Fertilizer and Intercropping with White Clover

In recent years, the area of tea fields in China has expanded. The application rate of nitrogen fertilizer is usually high in tea fields, which causes high N2O emissions. Tea fields are important sources of N2O emissions; thus, it is necessary to research N2O emission reduction in tea fields. A three-year field study was conducted to investigate soil N2O emissions and influencing factors under different fertilization measurements in a typical tea field in a subtropical hilly region of China. Three treatments-conventional fertilization, deep application of nitrogen fertilizer, and intercropping with clover-were studied to measure the soil N2O fluxes and the related soil and environmental properties. The results showed that the subtropical hilly tea field had high N2O emissions, and the cumulative annual emissions of N2O-N were as high as 5.1-10.1 kg·hm-2. The N2O emissions occurred mainly in spring and summer. When the soil temperature was lower than 15℃, the N2O flux shown mainly a positive correlation with the soil temperature. However, when the soil temperature was higher than 15℃, the positive correlation was mainly with the soil water, soil NH4+-N, and NO3--N contents. Compared with conventional fertilization, the intercropping of white clover did not significantly reduce N2O emissions, although deep application of fertilizer increased annual N2O emissions when the rainfall was high. Neither intercropping of white clover or deep application of fertilizer affected the tea yield or the yield-scaled N2O emissions compared with conventional fertilization. Our study indicates that both intercropping of white clover and deep application of fertilization without reducing the nitrogen application rate did not reduce the soil N2O emissions in subtropical tea fields. Further studies are needed to determine the effects of deep fertilization application combined with a reduction in the nitrogen application rate on N2O emissions from tea fields.

Denitrification Rate and Its Potential to Predict Biogenic N2O Field Emissions in a Mediterranean Maize-Cropped Soil in Southern Italy

The denitrification rate in C2H2-amended intact soil cores and soil N2O fluxes in closed static chambers were monitored in a Mediterranean irrigated maize-cropped field. The measurements were carried out during: (i) a standard fertilization management (SFM) activity and (ii) a manipulation experimental (ME) test on the effects of increased and reduced application rates of urea at the late fertilization. In the course of the SFM, the irrigations following early and late nitrogen fertilization led to pulses of denitrification rates (up to 1300 μg N2O-N m−2 h−1) and N2O fluxes (up to 320 μg N2O-N m−2 h−1), thanks to the combined action of high soil temperatures and not limiting nitrates and water filled pore space (WFPS). During the ME, high soil nitrates were noted in all the treatments in the first one month after the late fertilization, which promoted marked N-losses by microbial denitrification (from 500 to 1800 μg N2O-N m−2 h−1) every time the soil WFPS was not limiting. At similar maize yield responses to fertilizer treatments, this result suggested no competition for N between plant roots and soil microbial community and indicated a probable surplus of nitrogen fertilizer input at the investigated farm. Correlation and regression analyses (CRA) on the whole set of data showed significant relations between both the denitrification rates and the N2O fluxes with three soil physical-chemical parameters: nitrate concentration, WFPS and temperature. Specifically, the response functions of denitrification rate to soil nitrates, WFPS and temperature could be satisfactorily modelled according to simple Michaelis-Menten kinetic, exponential and linear functions, respectively. Furthermore, the CRA demonstrated a significant exponential relationship between N2O fluxes and denitrification and simple empirical functions to predict N2O emissions from the denitrification rate appeared more fitting (higher concordance correlation coefficient) than the predictive empirical algorithm based on soil nitrates, WFPS and temperature. In this regard, the empirically established relationships between the denitrification rate on intact soil cores under field conditions and the soil variables provided local-specific threshold values and coefficients which may effectively work to calibrate and adapt existing N2O process-based simulation models to the local pedo-climatic conditions.

Impacts of crop rotational diversity and grazing under integrated crop-livestock system on soil surface greenhouse gas fluxes.

Integrated crop-livestock (ICL) system is beneficial in enhancing soil organic carbon and nutrient cycling. However, the benefits of the ICL system on mitigation of GHG emissions are poorly understood. Thus, the present study was initiated in 2011 to assess the effect of crop rotation diversity and grazing managed under the ICL system on GHG emissions. The cropping system investigated here included spring wheat grown continuously for five years and a 5-yr crop rotation (spring wheat-cover crops-corn-pea/barley-sunflower). Each phase was present each year. Yearling steers grazed only the pea/barley, corn and cover crops plots in 2016 and 2017. Exclusion areas avoided the grazing in these crops to compare the GHG fluxes under grazed vs. non-grazed areas. The GHG fluxes were measured weekly from all crop phases during the growing season for both years using a static chamber. Cumulative CO2 and CH4 fluxes were similar from all crop phases over the study period. However, continuous spring wheat recorded higher cumulative N2O fluxes (671 g N ha-1) than that under spring wheat in rotation (571 g N ha-1). Grazing decreased cumulative CO2 fluxes (359 kg C ha-1) compared to ungrazed (409 kg C ha-1), however, no effect from grazing on cumulative CH4 and N2O fluxes over the study period were found. The present study shows that grazing and crop rotational diversity affected carbon and nitrogen inputs, which in turn affected soil CO2 and N2O fluxes. Long-term monitoring is needed to evaluate the response of soil GHG emissions to grazing and crop rotation interactions under the ICL system.

Modelling land–atmosphere daily exchanges of NO, NH<sub>3</sub>, and CO<sub>2</sub> in a semi-arid grazed ecosystem in Senegal

Abstract. Three different models (STEP–GENDEC–NOflux, Zhang2010, and Surfatm) are used to simulate NO, CO2, and NH3 fluxes at the daily scale for 2 years (2012–2013) in a semi-arid grazed ecosystem at Dahra (15∘24′10′′ N, 15∘25′56′′ W, Senegal, Sahel). Model results are evaluated against experimental results acquired during three field campaigns. At the end of the dry season, when the first rains re-wet the dry soils, the model STEP–GENDEC–NOflux simulates the sudden mineralization of buried litter, leading to pulses in soil respiration and NO fluxes. The contribution of wet season fluxes of NO and CO2 to the annual mean is respectively 51 % and 57 %. NH3 fluxes are simulated by two models: Surfatm and Zhang2010. During the wet season, air humidity and soil moisture increase, leading to a transition between low soil NH3 emissions (which dominate during the dry months) and large NH3 deposition on vegetation during wet months. Results show a great impact of the soil emission potential, a difference in the deposition processes on the soil and the vegetation between the two models with however a close agreement of the total fluxes. The order of magnitude of NO, NH3, and CO2 fluxes is correctly represented by the models, as well as the sharp transitions between seasons, specific to the Sahel region. The role of soil moisture in flux magnitude is highlighted, whereas the role of soil temperature is less obvious. The simultaneous increase in NO and CO2 emissions and NH3 deposition at the beginning of the wet season is attributed to the availability of mineral nitrogen in the soil and also to microbial processes, which distribute the roles between respiration (CO2 emissions), nitrification (NO emissions), volatilization, and deposition (NH3 emission/deposition). The objectives of this study are to understand the origin of carbon and nitrogen compounds exchanges between the soil and the atmosphere and to quantify these exchanges on a longer timescale when only a few measurements have been performed.

Biochar decreases soil N2O emissions in Moso bamboo plantations through decreasing labile N concentrations, N-cycling enzyme activities and nitrification/denitrification rates

Biochar application has been shown to be effective in mitigating soil N2O emission in agricultural soils. However, the effect of biochar on soil N2O emission in subtropical plantations and the underlining mechanisms are poorly understood. The objective of this study was to investigate the effect of biochar application rates (0, 5 and 15 t ha−1) on soil properties and N2O emissions within a Moso bamboo (Phyllostachys edulis) plantation in a 24-month field trial. Biochar application significantly (P < 0.05) decreased soil N2O emission, with the reduction rate positively related to the biochar application rate. The reduction in N2O emission by biochar application diminished over time. Biochar did not affect soil temperature (at 5 cm depth), and there were no significant relationships between N2O emission and either soil moisture content or soil microbial biomass C. However, biochar application significantly (P < 0.05) increased the water soluble organic C but decreased soil NH4+-N, NO3−-N and water soluble organic N (WSON) concentrations, soil urease and protease activities, and soil gross nitrification and denitrification rates. Soil N2O flux was positively (P < 0.05) correlated with soil NH4+-N, NO3−-N and WSON concentrations, soil urease and protease activities, and soil gross nitrification and denitrification rates, regardless of the treatment. Structural equation modeling suggests that biochar application decreased soil N2O emissions directly by decreasing soil gross nitrification and denitrification rates, and indirectly by decreasing concentrations of NH4+-N, NO3−-N and WSON, and the activities of urease and protease. We conclude that biochar application decreased soil N2O emission in the Moso bamboo plantation by decreasing labile N concentrations, N-cycling enzyme activities and nitrification/denitrification rates. Our findings suggest that biochar application can be an effective method to mitigate soil N2O emission in subtropical plantations.

Contrasting effects of nitrogen and phosphorus additions on soil nitrous oxide fluxes and enzyme activities in an alpine wetland of the Tibetan Plateau.

Alpine wetlands are important ecosystems, but an increased availability of soil nutrients may affect their soil nitrous oxide (N2O) fluxes and key enzyme activities. We undertook a 3-year experiment of observing nitrogen (N) and/or phosphorus (P) addition to alpine wetland soils of the Tibetan Plateau, China, with measurements made of soil extracellular enzyme activities and soil N2O fluxes. Our study showed that soil N2O flux was significantly increased by 72% and 102% following N and N+P additions, respectively. N addition significantly increased acid phosphatase (AP) and β-1, 4-N-acetyl-glucosaminidase (NAG) activities by 32% and 26%, respectively. P addition alone exerted a neutral effect on soil AP activities, while increasing NAG activities. We inferred that microbes produce enzymes based on ‘resource allocation theory’, but that a series of constitutive enzymes or the treatment duration interfere with this response. Our findings suggest that N addition increases N- and P-cycling enzyme activities and soil N2O flux, whereas P addition exerts a neutral effect on P-cycling enzyme activities and N2O flux after 3 years of nutrient applications to an alpine wetland.

Effects of harvest on greenhouse gas emissions from forested swamp during non-growing season in Xiaoxing'an Mountains of China.

Soil greenhouse gas emission during non-growing season plays an important role in ecosystem carbon and nitrogen cycling in mid and high latitude regions. However, the effects of harvest on greenhouse gas emission during non-growing remain unclear. We measured the fluxes of CO2, CH4 and N2O and environmental factors (soil temperature, moisture, soil organic carbon and total nitrogen etc.) during non-growing season from four kinds of forested swamps (Alnus sibirica swamp, Betula platyphylla swamp, Larix gmelinii-Carex schmidti swamp, L. gmelinii-moss swamp) under different harvest disturbances for 10 years, including control (no cutting), 45% selective cutting, clear cutting, by using static chamber technique and gas chromatography in Xiaoxing'an Mountains, Northeast China. The aim of this study was to reveal the effects of harvest on greenhouse gas emission from temperate forested swamp during non-growing season and the main controlling factors. The results showed that the average fluxes of CO2, CH4 and N2O from four kinds of swamps distributed in 53.08-81.31 mg·m-2·h-1, 0.09-3.07 mg·m-2·h-1 and 4.07-8.83 μg·m-2·h-1, respectively. Clear cutting significantly increased the fluxes of CO2, CH4, and N2O from A. sibirica swamp and L. gmelinii-moss swamp. Selective cutting significantly increased CO2 fluxes from B. platyphylla swamp and L. gmelinii-moss swamp and decreased CO2 flux from A. sibirica swamp. Selective cutting significantly decreased CH4 fluxes from all the four forested swamps and N2O flux from Larix gmelinii-Carex schmidti swamp. The CO2 fluxes from natural forested swamps were strongly influenced by soil temperature, soil organic carbon and C/N. CH4 fluxes were influenced by soil temperature, soil organic carbon. N2O fluxes were affected by air temperature and soil pH. Harvesting increased the correlation between soil CO2 flux and air temperature, soil moisture and snow depth, the correlation between soil CH4 flux and air temperature, soil moisture and C/N, as well as the correlation between soil N2O flux and soil total nitrogen and C/N. The annual cumulative contribution of CO2, CH4 and N2O emission from natural forested swamp during non-growing season were 33.2%-46.5%, 6.3%-9.1% and 61.5%-68.3%, respectively. The clear cutting increased the annual cumulative contribution of CO2 from B. platyphylla swamp and L. gmelinii-moss swamp and that of N2O from other swamps except L. gmelinii-moss swamp. The selective cutting increased the annual cumulative contribution of CO2, CH4 and N2O from L. gmelinii-C. schmidti swamp and L. gmelinii-moss swamp, but decreased that from B. platyphylla swamp. The annual cumulative contributions of N2O and CO2 during non-growing season were relatively high from temperate natural forested swamps, and clear cutting further increased their contribution, while the selective cutting just increased that of CH4 during non-growing season.

Soil N2O, CH4, and CO2 Fluxes in Forest, Grassland, and Tillage/No-Tillage Croplands in French Guiana (Amazonia)

The agricultural landscape of French Guiana (Amazonia) is expected to undergo substantial change as a result of rapid population growth in the region. Such changes in the landscape will lead to the conversion of tropical forests into land destined for agricultural use. Little information is available on the effect of different agricultural systems on greenhouse gas (GHG) emissions in French Guiana. For our experiment, two hectares of forest were cleared, without the use of fire, at the Combi experimental site (sandy-clayey Ferralsol) at the end of 2008. After one year with legume and grass cover, the site was modified to include the following three fertilized agricultural systems: (1) Grassland (Brachiaria ruziziensis, mowed), (2) cropland (maize/soybean rotation) with disc tillage, and (3) cropland (maize/soybean rotation) with no-tillage in direct seeding. Soil N2O, CH4, and CO2 fluxes were measured with dark chambers from May 2011 to November 2014. Our results show that grassland was a significantly lower emitter of N2O but a significantly higher emitter of CH4 compared to the two cropland systems studied. We did not observe significant differences between the two cropland systems for N2O and CH4 fluxes. Measurements of the net ecosystem CO2 exchange would be useful to better compare the role of different agricultural systems as a source of GHGs.

Modelling Soil Greenhouse Gas Fluxes from a Broad-Leaved Korean Pine Forest in Changbai Mountain: Forest-DNDC Model Validation

Fluctuations in soil greenhouse gas (GHG) are an important part of the terrestrial ecosystem carbon-nitrogen cycle, but uncertainties remain about the dynamic change and budget assessment of soil GHG flux. Using high frequency and consecutive soil GHG fluxes measured with an automatic dynamic chamber system, we tested the applicability of the current Forest-DNDC model in simulating soil CH4, CO2 and N2O fluxes in a temperate broad-leaved Korean pine forest at Changbai Mountain. The results showed that the Forest-DNDC model reproduced general patterns of environmental variables, however, simulated seasonal variation in soil temperature, snow melt processes and soil moisture partly deviated from measured variables, especially during the non-growing season. The modeled CH4 flux was close to the field measurement and co-varied mainly with soil temperature and snowpack. The modeled soil CO2 flux had the same seasonal trend to that of the observation along with variation in temperature, however, simulated CO2 flux in the growing season was underestimated. The modeled N2O flux attained a peak in summer due to the influence of temperature, which was apparently different from the observed peak of N2O flux in the freeze-thaw period. Meanwhile, both modeled CO2 flux and N2O flux were dampened by rainfall events. Apart from consistent estimation of annual soil CH4 flux, the annual accumulation of CO2 and N2O was underestimated. It is still necessary to further optimize model parameters and processes using long-term high-frequency observation data, especially transference of heat and water in soil and GHG producing mechanism. Continues work will improve modeling, ecosystem carbon-nitrogen budget assessment and estimation of soil GHGs flux from the site to the region.

Hidden contribution of shrimp farming effluents to greenhouse gas emissions from mangrove soils

In the present study, we evaluated CO2, CH4, and N2O fluxes in mangrove soils that receive shrimp farming effluents. Soil and gas samples were collected from a mangrove that receives the discharge of shrimp pond effluents and another mangrove forest free of the effluent impacts. CO2 and N2O emissions were significantly higher in the mangrove soil affected by the shrimp effluents (51.4 mg m2 h−1 and 90.9 μg m−2 h−1, respectively) than in the mangrove soil free of the effluents (26.5 mg m−2 h−1 and 40.4 μg m−2 h−1, respectively). No significant differences in CH4 emissions were observed between the two mangroves soils (mean values ranging from 0.2 to 0.8 mg m−2 h−1). Suboxic conditions govern nitrification-denitrification and mineralization processes and control CO2 and N2O emissions from the soil; however, CH4 fluxes were not affected. The highest emissions from the impacted soil (93% higher for CO2 and 125% higher for N2O) were attributable to the high content of organic carbon, total nitrogen, and potentially mineralizable nitrogen, which reduce the role of the mangrove in mitigating greenhouse gas emissions.

Contribution and Driving Mechanism of N2O Emission Bursts in a Chinese Vegetable Greenhouse after Manure Application and Irrigation

Solar greenhouse vegetable fields have been found to be hotspots of nitrous oxide (N2O) emissions in China, mainly due to excessive manure application and irrigation. Pulses of N2O emissions have been commonly reported by field monitoring works conducted in greenhouse fields, though their significance regarding total N2O emissions and the driving mechanism behind them remain poorly understood. N2O fluxes were monitored in situ using a static opaque chamber method in a typical greenhouse vegetable field. Then, laboratory incubations were conducted under different soil moisture and manure application gradients to monitor nitrous oxide emissions and related soil properties, using a robotized incubation system. Field monitoring showed that the occurrence of clear N2O emission bursts closely followed fertilization and irrigation events, accounting for 76.7% of the annual N2O efflux. The soil N2O flux increased exponentially with the water-filled pore space (WFPS), causing extremely high N2O emissions when the WFPS was higher than 60%. During the lab incubation, emission bursts led to N2O peaks within 40 h, synchronously changing with the transit soil NO2−. An integrated analysis of the variations in the gas emission and soil properties indicated that the denitrification of transit NO2− accumulation was the major explanation for N2O emission bursts in the greenhouse filed. Nitrous oxide emission bursts constituted the major portion of the N2O emissions in the Chinese greenhouse soils. Nitrite (NO2−) denitrification triggered by fertilization and irrigation was responsible for these N2O emission pulses. Our results clarified the significance and biogeochemical mechanisms of N2O burst emissions; this knowledge could help us to devise and enact sounder N2O mitigation measures, which would be conducive to sustainable development in vegetable greenhouse fields.

Impacts of burning on soil trace gas fluxes in two wooded savanna sites in Burkina Faso

In this study, we report the impacts woodland savanna burning has on the soil trace gas fluxes in seasonally dry woodland savannas in Burkina Faso. We measured nitrous oxide (N2O), nitric oxide (NO), carbon dioxide (CO2) and methane (CH4) fluxes at two ongoing long-term experimental sites (Tiogo and Dindéresso), each included three fire-exclusion plots and four annually burnt plots (0.24–0.25 ha plot size). We measured soil trace gas fluxes using chamber methods before and after fire during the dry season and at the start of and during the wet season. There was considerable volatilization of aboveground biomass C and N during fire, but soil N2O, NO, CO2 and CH4 fluxes were not significantly influenced following fire events. Instead, soil moisture dynamics were more important in regulating soil CO2 and N2O emissions. The onset of the wet season induced a short-term pulse of N2O emissions, when water-filled pore space exceeded 60%. We also detected a short-term pulse of NO emissions, lasting only four hours, following fire. The low soil N-oxide fluxes reflected the sites’ extremely low inorganic N levels in the soil.

The Effect of Land-Use Change on Soil CH4 and N2O Fluxes: A Global Meta-Analysis

Land-use change is a prominent feature of the Anthropocene. Transitions between natural and human-managed ecosystems affect biogeochemical cycles in many ways, but soil processes are among the least understood. We used a global meta-analysis (62 studies, 1670 paired comparisons) to examine effects of land conversion on soil–atmosphere fluxes of methane (CH4) and nitrous oxide (N2O) from upland soils, and determine soil and environmental factors driving these effects. Conversion from a natural ecosystem to any anthropogenic land use increased soil CH4 and N2O fluxes by 234 kg CO2-equivalents ha−1 y−1, on average. Reversion of managed ecosystems to that resembling natural ecosystems did not fully reverse those effects, even after 80 years. In general, neither the type of ecosystem converted, nor the type of subsequent anthropogenic land use, affected the magnitude of increase in soil emissions. Land-use changes in wetter ecosystems resulted in greater increases in CH4 fluxes, but reduced N2O fluxes. An interacting suite of soil variables influenced CH4 and N2O fluxes, with availability of inorganic nitrogen (that is, extractable ammonium and nitrate), pH, total carbon, and microclimate being strong mediators of effects of land-use change. In addition, time after a change in land use emerged as a critical factor explaining the effects of land-use change—with increased emissions of both greenhouse gases diminishing rapidly after conversion. Further research is needed to elucidate complex biotic and abiotic mechanisms that drive land-use change effects on soil greenhouse gas emissions, but particularly during this initial disturbance when emissions are greatest relative to native vegetation. Efforts to mitigate emissions will be severely hampered by this gap in knowledge.

Water table management and fertilizer application impacts on CO2, N2O and CH4 fluxes in a corn agro-ecosystem

Water table management with controlled drainage and subsurface-irrigation (SI) has been identified as a Beneficial Management Practice (BMP) to reduce nitrate leaching in drainage water. It has also been shown to increase crop yields during dry periods of the growing season, by providing water to the crop root zone, via upward flux or capillary rise. However, by retaining nitrates in anoxic conditions within the soil profile, SI could potentially increase greenhouse gas (GHG) fluxes, particularly N2O through denitrification. This process may be further exacerbated by high precipitation and mineral N-fertilizer applications very early in the growing season. In order to investigate the effects of water table management (WTM) with nitrogen fertilization on GHG fluxes from corn (Zea mays) agro-ecosystems, we conducted a research study on a commercial farm in south-western Quebec, Canada. Water table management treatments were: free drainage (FD) and controlled drainage with subsurface-irrigation. GHG samples were taken using field-deployed, vented non-steady state gas chambers to quantify soil CO2, N2O and CH4 fluxes weekly. Our results indicate that fertilizer application timing coinciding with intense (≥24 mm) precipitation events and high temperatures (>25 °C) triggered pulses of N2O fluxes, accounting for up to 60% of cumulative N2O fluxes. Our results also suggest that splitting bulk fertilizer applications may be an effective mitigation strategy, reducing N2O fluxes by 50% in our study. In both seasons, pulse GHG fluxes mostly occurred in the early vegetative stages of the corn, prior to activation of the subsurface-irrigation. Our results suggest that proper timing of WTM mindful of seasonal climatic conditions has the potential to reduce GHG emissions.

Response of Soil N2O Emissions to Soil Microbe and Enzyme Activities with Aeration at Two Irrigation Levels in Greenhouse Tomato (Lycopersicon esculentum Mill.) Fields

Aerated irrigation is proven to increase soil N2O emissions; however, the mechanisms of N2O release are still unknown. A field experiment for two consecutive greenhouse tomato-growing seasons, from August 2016 to July 2017, was carried out to examine (1) the differences of aeration and irrigation on soil N2O emissions with a static chamber GC technique, and on soil physical and biotic parameters, and (2) the response of soil N2O emissions to soil physical and biotic parameters. Two irrigation levels were included: 60% (low irrigation) and 100% (high irrigation) of the full irrigation amount. Each irrigation level contained aeration and control, totaling four treatments. During the two growing seasons, soil N2O emissions with aeration were 4.5% higher than the control (p &gt; 0.05). Soil N2O emissions under the high irrigation were 13.8% greater than under the low irrigation, and the difference was significant in 2017 (p &lt; 0.05). Aeration and irrigation had positive effects on the mean soil nitrifier abundance and mean soil urease activity, and the impact of irrigation on urease was significant in 2016 (p = 0.001). In addition, aeration negatively influenced the mean soil denitrifier abundance, while irrigation positively influenced the mean soil denitrifier abundance. Regression analysis showed that the soil water-filled pore space, temperature, and denitrifier abundance were primary factors influencing soil N2O fluxes. This study provides a further understanding of the processes affecting soil N2O emissions and N dynamics, which may assist in developing mitigation strategies to reduce N2O emissions.

Variation in N2O emission and N2O related microbial functional genes in straw- and biochar-amended and non-amended soils

Reducing mineral fertilizer application is an important management practice for mitigating nitrous oxide (N2O) emissions. Crop straw and biochar are two farmer-friendly residues that can be used for reducing the application of mineral fertilizers. However, different effect of straw- and biochar-amended and non-amended soils on N2O emissions is not well understood. Here, we conducted a mesocosm experiment in a vegetable field in Southwestern China using four treatments: control (CT, no addition), solely mineral fertilizers at the recommended dosage (F), maize straw with mineral fertilizers (SF), and biochar with mineral fertilizers (BF), based on the same total nitrogen, phosphorus and potassium contents. Soil N2O fluxes, cumulative N2O emission, soil chemical parameters, the abundance of N2O related microbial functional genes (amoA, nirS, nirK and nosZ) and microbial community structure were monitored and analyzed. Compared to F, the SF treatment increased the cumulative N2O emission. However, no significant difference was observed in cumulative N2O emission between F and BF treatments. Covariation in cumulative N2O emission and soil dissolved organic nitrogen (DON) content indicated that heterotrophic ammonia oxidation fueled by soil DON might be the driving force for N2O production. Compared to F, both SF and BF treatments increased the amoA gene copy numbers of ammonia oxidizing bacteria (AOB) but reduced those of ammonia oxidizing archaea (AOA); nirS and nosZ gene copy numbers were also reduced under SF and BF treatments. Compared to SF, the diversity of nirK and nosZ communities under the BF treatment was increased, whereas the diversity of nirS community was decreased. Lower cumulative N2O emission under BF than that under SF may be attributed to lower N2O production from heterotrophic ammonia oxidation fueled by soil DON and increased N2O consumption mediated by nosZ-harboring denitrifiers.