Mean N2O Fluxes Research Articles

GREENHOUSE GAS EMISSIONS IN PERENNIAL BIOENERGY CROPS ON MARGINAL LAND IN SOUTHERN ONTARIO

Knowledge of soil nitrous oxide (N2O) and carbon dioxide (CO2) fluxes in unfertilized perennial bioenergy crops on marginally productive cropland is crucial to understanding their climate mitigation benefits through reduced greenhouse gas (GHG) emissions. The static chamber method was used to quantify and compare N2O and CO2 fluxes in miscanthus (Miscanthus giganteus L.), switchgrass (Panicum virgatum L.), and willow (Salix miyabeana L.) to a successional site over two growing seasons. Mean N2O and CO2 fluxes ranged from –0.02 to 0.09 µg N2O-N m–2 hr–1 and 0.01 to 0.27 mg CO2-C m–2 hr–1, respectively. Whereas mean CO₂ fluxes differed (p<0.05) among land use types and between growing seasons, mean N₂O fluxes did not differ (p≥0.166) among land use types or between growing seasons. Our findings suggest that N₂O fluxes from unfertilized perennial bioenergy crops such as miscanthus, switchgrass, and willow on marginally productive croplands are comparable to those from marginal croplands left to undergo natural succession with regrowth vegetation. This alleviates concerns that cultivating perennial bioenergy crops on marginal croplands could increase GHG emissions, particularly when fertilization is minimized or eliminated, if these results hold true on a global scale. Additionally, variations in soil available nitrogen (N), moisture, and temperature did not correspond with differences in mean CO₂ fluxes during both growing seasons. This suggests that root respiration accounted for most of the CO₂ fluxes, rather than microbial soil respiration.

Improved nitrogen fertilizer management reduces nitrous oxide emissions in a northern Prairie cropland

Arable croplands are a significant source of nitrous oxide (N2O) emissions, largely due to nitrogen (N) fertilizer applications to support crop production. Nevertheless, there is limited research on the N2O dynamics from canola-wheat rotations in the semi-arid northern Prairies, an important agricultural region. Here, we present micrometeorological N2O fluxes measured from January 2021 to April 2024 in Saskatchewan, Canada, to evaluate the impact of N fertilizer management on the year-round N2O emissions from a canola-wheat rotation. A combination of two 4R (Right Source, Right Rate, Right Time, Right Place) N management practices – a reduced N rate and an enhanced efficiency N fertilizer source – was compared to common fertilizer management practices for the region. Two periods at high risk for N2O flux events were identified, after N fertilizer applications and the following spring thaw, with the magnitude of emissions varying over the multi-year period. As for cumulative emissions, the growing season (GS) N2O emissions were 50 % of annual emissions, presenting an opportunity to mitigate N2O emissions through improved N fertilizer management. Indeed, the improved 4R N management reduced N2O emissions by 57 % over the entire study period without impacting yields. The reduction in GS N2O emissions resulted from the 4R N management lowering mean N2O flux at times of high WFPS (>50 %). The non-growing season (NGS) N2O accounted for 11–67 % of annual emissions. Fall soil nitrate levels were a strong explanatory variable of NGS emissions (r2 = 0.69, r2 = 0.39), but the rate of change and magnitude of NGS emissions depended on thawing conditions – lower for drier thaws, higher for wetter thaws. Ultimately, better N fertilizer management reduces cumulative N2O emissions from cropping systems when practiced for several years.

The aeration and dredging stimulate the reduction of pollution and carbon emissions in a sediment microcosm study

Sediment dredging and aeration are used as important technical measures to remediate internal loading of sediment in polluted rivers. However, previous studies have overlooked the impact of dredging and aeration on Greenhouse gases (GHGs) emission. We established three aeration rate(six different aeration intervals), one dredging treatment to investigate the effect of aeration and dredging on pollutant removals and CO2, CH4 and N2O emissions. The results indicated the pollutants and GHGs at 2.4, 3.4, 4.4 L min−1 aeration rates reached collaborative emission reduction after more than 3 h or within 1.5 h. Meanwhile, the GHGs fluxes after aeration decreased with the increasing aeration rate, with the mean CO2, CH4 and N2O fluxes of 69.74, 0.16, 7.53 mg m−2 h−1 and 33.64, 0.09, 4.17 mg m−2 h−1 before and after aeration, respectively. With respect to dredging, the pollutants and N2O reached synergic effects between reduction of pollution and carbon emissions after 1 h dredging. Specifically, the CO2 and CH4 emissions after dredging was lower than those of before dredging, but the N2O emissions was higher than those of before dredging. In addition, our analysis revealed that the dissolved oxygen (DO), oxidation-reduction potential (ORP), available potassium (AK) and ammoniacal nitrogen (NH4+-N) in the sediment influenced GHGs fluxes at the water-air interface in the aeration. Our study indicated moderate aeration and dredging can achieve the synergistic effect in reducing pollution and carbon emissions.

Foliar methane and nitrous oxide fluxes in tropical tree species

Methane (CH₄) and nitrous oxide (N₂O) are critical biogenic greenhouse gases (GHGs) with global warming potentials substantially greater than that of carbon dioxide (CO₂). The exchange of these gases in tropical forests, particularly via foliar processes, remains poorly understood. We quantified foliar CH₄ and N₂O fluxes among tropical tree species and examined their potential association with the leaf economics spectrum (LES) traits. Sampling within Lawachara National Park, Bangladesh, we used in-situ measurements of foliar CH₄ and N₂O fluxes employing off-axis integrated cavity output spectroscopy (CH₄, CO₂ and H₂O) and optical feedback–cavity enhanced absorption spectroscopy (N₂O) analyzers. Leaves were measured under dark, low, and high (0, 100, and 1000 μmol·m−2·s−1) light conditions. Surveyed tree species exhibited both net foliar uptake and efflux of CH₄, with a mean flux not different from zero, suggesting negligible net foliar emissions at the stand level. Plant families showed differences in CH₄, but not N₂O fluxes. Consistent efflux was observed for N₂O, with a mean of 0.562 ± 0.060 pmol·m−2·s−1. Pioneer species exhibited a higher mean N₂O flux (0.81 ± 0.17 pmol·m−2·s−1) compared to late-successional species (0.37 ± 0.05 pmol·m−2·s−1). Pioneer species also showed a trend toward a higher mean CH₄ flux (0.24 ± 0.21 nmol·m−2·s−1) compared to mid-successional (−0.01 ± 0.26 nmol·m−2·s−1) and late-successional species (−0.05 ± 0.28 nmol·m−2·s−1). Moreover, among all leaf traits within the leaf economic spectrum, a significant positive relationship was observed between leaf N₂O flux and total leaf nitrogen. Our results suggest that pioneer tree species significantly contribute to net CH₄ and N₂O emissions, potentially counteracting the carbon sequestration benefits in regenerating tropical forests. These findings indicate that accurate GHG budgeting should include direct measurements of foliar CH₄ and N₂O fluxes. Moreover, the results suggest that forest conservation and management strategies that prioritize late successional species will better mitigate GHG emissions.

Methane and nitrous oxide emissions in the rice-shrimp rotation system of the Vietnamese Mekong Delta

Rice-shrimp rotation systems are one of the widespread farming practices in the Vietnamese Mekong Delta coastal areas. However, greenhouse gas (GHG) emissions in the system have remained unclear. This study aimed to examine methane (CH4) and nitrous oxide (N2O) emissions from the system, including (i) land-based versus high-density polyethylene-lined (HDPE) nursery ponds and (ii) conventional versus improved grow-out ponds inoculated with effective microorganisms (EM) bioproducts. The results showed that CH4 flux in land-based and HDPE-lined nursery ponds were 1.04 and 0.25 mgCH4 m−2 h−1, respectively, while the N2O flux was 8.37 and 6.62 μgN2O m−2 h−1, respectively. Global warming potential (GWP) from land-based nursery ponds (18.3 g CO2eq m−2) was approximately 3 folds higher than that of the HDPE-lined nursery pond (6.1 g CO2eq m−2). Similarly, the mean CH4 and N2O fluxes were 15.84 mg CH4 m−2 h−1 and 7.17 μg N2O m−2 h−1 for the conventional ponds, and 10.51 mg CH4 m−2 h−1 and 7.72 μg N2O m−2 h−1 for the improved grow-out ponds. Conventional practices (2388 g CO2eq m−2) had a higher 1.5-fold GWP compared to the improved grow-out pond (1635 g CO2eq m−2). The continuation of the land-based nursery pond and conventional aquacultural farming practices increase CH4 emission and GWP, while applying HDPE-lined nursery ponds combined with improved grow-out ponds could be a promising approach for reducing GHG emissions in rice-shrimp rotation systems. This study recommends further works in the rice-shrimp rotation systems, including (i) an examination of the effects of remaining rice stubbles in the platform on the availability of TOC levels and GHG emissions and (ii) ameliorating dissolved oxygen (DO) concentration on the effectiveness of GHG emission reduction.

“Effects of soil management, rotation and sequence of crops on soil nitrous oxide emissions in the Cerrado: A multi-factor assessment”

The emission of nitrous oxide (N2O), one of the main greenhouse gases, which contributes significantly to global warming, is a major challenge in modern agriculture. The effects of land use systems on N2O emissions are the result of multiple variables, whose interactions need to be better understood. In this sense, this study analyzed the possible effects of different soil managements, crop rotations and sequences, as well as edaphoclimatic factors causing N2O emissions from soils in the Cerrado biome (scrubland). The following four land-use systems were evaluated: 1) No-tillage cultivation with biennial crop rotations and sequences: legume-grass and alternating grass-legume crops in the second season - NT-SS/MP; 2) No-tillage with biennial rotations and sequences: grass-legume and alternating second crop of legume-grass - NT-MP/SS; 3) Conventional planting with disc harrow and biennial legume-grass rotation-CT-S/M; and 4) Native Cerrado (CE), no agricultural land use. The legume and grass species, planted in the two no-tillage treatments were soybean, followed by sorghum BRS3.32 (Sorghum bicolor (L.) Moench) (SS), and maize, followed by pigeon pea (Cajanus cajan) (MP). Nitrous oxide emissions were evaluated for 25 months (October 2013 to October 2015), and the results were grouped in annual, total, growing and non-growing seasons, as well as yield-scaled N2O emissions. The mean N2O fluxes were 24.14, 15.71, 32.49 and 1.87 μg m−2 h−1 in the NT-SS/MP, NT-MP/SS, CT-S/M and Cerrado areas respectively. Cumulative N2O fluxes over the total evaluation period from the systems NT-SS/MP, NT-MP/SS, CT-S/M and CE, respectively, were 3.47, 2.29, 4.87 and 0.26 kg ha−1. A correlation between N2O fluxes and the environmental variables was observed, with the exception of water-filled pore space (WFPS), but N2O peaks were associated with WFPS values of >65%. In the 2014–2015 growing season, yield-scaled N2O emissions from NT-MP/SS were lower than from CT-S/M. A multi-factor approach indicated that conventional management with main season soybean or maize and no alternating crop sequence intensifies soil N2O emissions in the Cerrado.

Greenhouse gas fluxes from different types of permafrost regions in the Daxing'an Mountains, Northeast China.

Global warming will increase the greenhouse gas (GHG) fluxes of permafrost regions. However, little is known about the difference in GHG fluxes among different types of permafrost regions. In this study, we used the static opaque chamber and gas chromatography techniques to determine the fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in predominantly continuous permafrost (PCP), predominantly continuous and island permafrost (PCIP), and sparsely island permafrost (SIP) regions during the growing season. The main factors causing differences in GHG fluxes among three types of permafrost regions were also analyzed. The results showed mean CO2 fluxes in SIP were significantly higher than that in PCP and PCIP, which were 342.10 ± 11.46, 105.50 ± 10.65, and 127.15 ± 14.27 mg m-2 h-1, respectively. This difference was determined by soil temperature, soil moisture, total organic carbon (TOC), nitrate nitrogen (NO3--N), and ammonium nitrogen (NH4+-N) content. Mean CH4 fluxes were -26.47 ± 48.83 (PCP), 118.35 ± 46.93 (PCIP), and 95.52 ± 32.86 μg m-2 h-1 (SIP). Soil temperature, soil moisture, and TOC content were the key factors to determine whether permafrost regions were CH4 sources or sinks. Similarly, PCP behaved as the sink of N2O, PCIP and SIP behaved as the source of N2O. Mean N2O fluxes were -3.90 ± 1.71, 0.78 ± 1.55, and 3.78 ± 1.59 μg m-2 h-1, respectively. Soil moisture and TOC content were the main factors influencing the differences in N2O fluxes among the three permafrost regions. This study clarified and explained the differences in GHG fluxes among three types of permafrost regions, providing a data basis for such studies.

Comparative accounting of methane and nitrous oxide fluxes with related soil parameters of degraded mangrove wetlands and adjacent rice fields in Sundarban, India

In Sundarban, around 10.5% of mangroves are primarily converted to rice-based system by anthropogenic activities during last 80 years. Both mangrove wetlands and rice fields, typically providing a passage for transporting the greenhouse gases (methane and nitrous oxide) from soil systems to atmosphere. In this article, we attempted to quantify the seasonal methane (CH4) and nitrous oxide (N2O) fluxes from six different degraded mangroves and adjacent rice fields to know how the land use change affects the fluxes. In mangroves, the average CH4 fluxes were higher in monsoon compared to pre-monsoon, winter, and summer season. Methane emissions were also higher in monsoon and summer in rice than winter and pre-monsoon. Overall, the mean CH4 fluxes were more in rice (2665 μg m−2 h−1) than mangrove (159 μg m−2 h−1). However, the average N2O fluxes were higher in summer followed by monsoon in mangroves, while, in rice, it was higher in monsoon. Like CH4 the higher mean N2O flux was observed in rice (72 μg m−2 h−1) over mangrove (69 μg m−2 h−1). The soil labile carbon pools were higher in mangroves than rice, but the enzymatic activities were more in rice. The fluxes were positively correlated with the soil enzymatic activities, whereas those were negatively correlated with soil pH, EC, and salinity. So, we found shifting of mangrove to rice resulting in higher CH4 and N2O fluxes from surface soils. Therefore, regeneration and protection of mangroves should be given priority in upper Sundarban where anthropogenic activities are prevailed to mitigate climate change.

Nitrous Oxide Dynamics in the Southern Benguela Upwelling System

AbstractSt Helena Bay in the southern Benguela upwelling system (SBUS) is characterized by seasonal upwelling, water mass and nutrient retention, and persistent cyclonic circulation that limits oxygen exchange, creating ideal conditions for subsurface N2O production and subsequent ventilation at the ocean‐atmosphere interface. However, due to a paucity of observations, little is known about N2O dynamics in the SBUS. Here we use a coupled physical‐biogeochemical model and new observations to investigate the magnitude and seasonality of the N2O source and ocean‐atmosphere flux terms along a transect from St Helena Bay into offshore waters. Both the model and observations indicate that significant N2O production occurs at depth in nearshore waters, with ΔN2O concentrations (i.e., the difference between the simulated or observed concentration of N2O and the equilibrium value) exceeding 14 μmol m−3. Equatorward advection of this N2O occurs at a maximum rate of 4.50 μmol N2O m−2 s−1 in the upper 200 m. By contrast, the SBUS poleward undercurrent hosts low N2O concentrations on the shelf slope. Modeled fluxes range from −0.02 to 0.2 nmol m−2 s−1, consistent with reports from other upwelling systems. The ocean‐atmosphere N2O flux reaches 0.21 g N m−2 yr−1 in nearshore St Helena Bay, and follows a distinct seasonal cycle driven by ΔN2O disequilibrium in winter and prevailing south‐easterly winds and associated upwelling in spring and summer. We calculate a mean N2O flux for the whole SBUS of 4 ± 2 × 10−3 Tg N yr−1, representing 0.1% of the estimated global ocean annual flux.

Modeling N2O emissions of complex cropland management in Western Europe using DayCent: Performance and scope for improvement

Under the United Nations Framework Convention on Climate Change (UNFCCC), industrialized countries and countries with economies in transition (so called Annex 1 countries) are encouraged to move towards more sophisticated approaches for national greenhouse gas reporting. To develop a model-based approach for estimating nitrous oxide (N2O) emissions from agricultural soils, model calibration is one of the first important steps. Extensive multisite field observations are necessary for this purpose, as agricultural management in Western Europe is complex (e.g., diverse crop rotations, different types of fertilizer and soil tillage). In the present study, we used ca. 24,000 daily N2O flux observations from six cropland sites, two in France and four in Switzerland, to conduct an automatic data-driven calibration of the biogeochemical model DayCent. This model is planned to be used for greenhouse gas reporting in the entire European Union as well as in Switzerland. After a site-specific calibration, a leave-one-out (LOO) cross-evaluation was conducted to assess the model’s ability to predict N2O emissions for sites it was not calibrated for. Mean observed N2O fluxes for 54 interactions of crop cycles, field studies and treatments were used to evaluate the model. The LOO cross-evaluation resulted in a R2 of 0.63 for the prediction of mean N2O fluxes per crop cycle, compared to an R2 of 0.51 obtained with default parameterization. Our results showed that the improvement in N2O predictions was associated with the adjustment of only seven parameters controlling the N cycle in soil (e.g., the maximum daily nitrification amount and the inflection point for the effect of water-filled pore space on denitrification) out of several hundred parameters. These parameters showed a wide range of values between sites, revealing an important challenge for calibration-based improvement of N2O simulations. Despite the remaining uncertainty, our model-based estimates of N2O emission per crop cycle (2.64 kg N ha-1) were clearly closer to measurements (2.67 kg N ha-1) than commonly used emission factor approaches (1.60–1.71 kg N ha-1). Based on extensive field observations, our results suggest that, after data-driven calibration of only few N cycle parameters, DayCent simulations are useful for reporting N2O emissions of complex cropland management. These model based-estimates were more accurate, because they consider key drivers that are disregarded by simpler approaches. Moving towards more complex methods of N2O reporting, is therefore expected to improve the accuracy and additionally allows to assess mitigation options.

Effects of nitrogen and water addition on N2O emissions in temperate grasslands, northern China

Temperate grasslands are considered an important natural source of the trace greenhouse gas N2O, which has attracted much attention in recent years. Increased annual precipitation and N deposition have been observed in temperate grasslands in northern China owing to climate change and anthropogenic activities. Although the individual effects of increased precipitation and N deposition on N2O emissions have been extensively reported, their interactive effects remain unclear. An in situ experiment with two levels of water supply (ambient and +15% precipitation levels) and four levels of N application (0, 25, 50, and 100 kg N ha−1 yr−1) was conducted in a semi-arid temperate grassland from 2016 to 2018 to quantify the effects of additional precipitation and N on N2O emissions in temperate grasslands. The results showed that temperate grasslands are a small source of N2O with negative N2O fluxes during the growing seasons. Water addition amplified the variability in N2O fluxes by lowering the mean N2O fluxes and strengthening N2O emission pulses triggered by precipitation. The trade-off between increased N2O emission pulses and decreased mean fluxes resulted in decreased growing season cumulative N2O emissions. The stimulated N2O emissions by N addition were linearly related to N application rates. However, this linear correlation shifted to similar N2O emissions among the N application rates by water addition. In addition, N2O emissions were negatively correlated with soil moisture and microbial biomass carbon (MBC) but positively correlated with soil dissolved organic carbon (DOC), NH4+, and NO3−. Our results suggest that (1) water addition increases the sensitivity of N2O emissions to N addition and limits the loss of additional N as N2O. (2) In addition to directly increasing N2O emissions, N addition increases N2O emissions indirectly by increasing soil DOC, NH4+, and NO3− levels, whereas water addition decreases N2O emissions indirectly by increasing MBC and soil moisture. (3) The ability of soil properties to explain the variability in N2O fluxes was limited, whereas the ability of soil properties to explain the interannual variability in cumulative N2O emissions was highly reliable.

Effects of water and nitrogen management on N2O emissions and NH3 volatilization from a vineyard in North China

Agricultural soils are a major source of anthropogenic N2O emissions and NH3 volatilization because of the large input of nitrogen (N) via fertilizers. Chinese vineyards commonly receive excessive water and N applications, but the response of gaseous losses to these management practices are not well documented. In this study, a field experiment was conducted to measure N2O emissions and NH3 volatilization from a typical table grape (Vitis vinifera L.) vineyard in North China. Three water and N regulation management strategies were applied and compared with the local farmer's traditional water and N management (TWN, traditional flooding irrigation and traditional N application rate, flood irrigation was carried out after the fertilizers furrowing-applied) as a control, mobile water and fertilization (MWF, 62% tradition N application rate and 60% traditional irrigation amount, flood irrigation was carried out after the dissolved fertilizers injected into rhizosphere (20 cm deep) with a liquid-jet gun), optimum water and N (OWN, 69% tradition N application rate and 70% traditional irrigation amount, the fertilization and irrigation method were the same as those in the TWN treatment) and optimum water and N combined with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) (OWN+DMPP, 1% DMPP was applied on the basis of the OWN treatment). The MWF, OWN and OWN+DMPP treatments significantly (P < 0.05) reduced the mean N2O fluxes with little effect on the NH3 volatilization rate. However, over the whole grape-growing season, the cumulative N2O total and yield-scaled N2O emissions in the three treatment groups were significantly reduced by 27.44–29.23% and 34.66–43.71%, respectively. In addition, the cumulative NH3 and yield-scaled NH3 volatilization were reduced by 4.13–6.61% and 21.81–26.15%, respectively. Notably, N2O emissions were significantly positively correlated with soil temperature, WFPS and NH4-N nitrogen contents, suggesting that these are the most important factors controlling N2O emissions in table grape plantations. Additionally, NH3 volatilization was closely related with soil temperature. Comprehensive evaluation showed that, based on the traditional water and N application rate, 38% N reduction combined with mobile water and fertilizer (MWF) effectively reduced the risk of N gaseous losses to the environment and saves irrigation water while maintaining grape yield.

Joint control by soil moisture, functional genes and substrates on response of N2O flux to climate extremes in a semiarid grassland

Nitrous oxide (N2O), the third most important greenhouse gas, contributes to the increasing frequency and severity of climate extremes. Disentangling feedbacks of climate extremes on terrestrial N2O emission is important for forecasting future climate changes. Here, we experimentally imposed extreme drought and heat wave events during three years in a semiarid grassland to investigate the responses of N2O flux. We identified that N2O flux suppression during droughts was mediated by soil water content (SWC), microbial biomass carbon (MBC), soil inorganic nitrogen (SIN) and dissolved organic carbon (DOC) contents, and the abundance of archaeal amoA, nirK, and narG. However, bacterial amoA, nirS, and nosZ remained stable. Upon rewetting following droughts, the SWC, SIN, DOC, archaeal amoA, nirK, narG, and resultant N2O fluxes recovered to the magnitude of the ambient control. In contrast, heat waves alone or in combination with drought did not impact N2O fluxes or the underlying physical, chemical and microbial states. Stepwise multiple linear regression suggested that SWC, DOC, and MBC were the key factors regulating immediate responses of N2O flux to climate extremes while the major factors regulating seasonal mean N2O flux in response to climate extremes were archaeal amoA abundance, nirK abundance, and MBC. Our results suggest that N2O fluxes were sensitive to droughts but insensitive to heat waves. Soil moisture induced changes in substrate availability, and the community size of total and functional microorganisms in soil jointly regulated N2O responses to climate extremes. The relative importance of regulating factors shifted at different timescales.

Nitrous oxide flux observed with tall-tower eddy covariance over a heterogeneous rice cultivation landscape

Although croplands are known to be strong sources of anthropogenic N2O, large uncertainties still exist regarding their emission factors, that is, the proportion of N in fertilizer application that escapes to the atmosphere as N2O. In this study, we report the results of an experiment on the N2O flux in a landscape dominated by rice cultivation in the Yangtze River Delta, China. The observation was made with a closed-path eddy covariance system on a 70-m tall tower from October 2018 to December 2020 (27 months). Temperature and precipitation explained 78% of the seasonal and interannual variability in the observed N2O flux. The growing season (May to October) mean flux (1.14 nmol m−2 s−1) was much higher than the median flux found in the literature for rice paddies. The mean N2O flux during the observational period was 0.90 ± 0.71 nmol m−2 s−1, and the annual cumulative N2O emission was 7.6 and 9.1 kg N2O-N ha−1 during 2019 and 2020, respectively. The corresponding landscape emission factor was 3.8% and 4.6%, respectively, which were much higher than the IPCC default direct (0.3%) and indirect emission factors (0.75%) for rice paddies.

Soil Moisture but Not Warming Dominates Nitrous Oxide Emissions During Freeze–Thaw Cycles in a Qinghai–Tibetan Plateau Alpine Meadow With Discontinuous Permafrost

Large quantities of organic matter are stored in frozen soils (permafrost) within the Qinghai–Tibetan Plateau (QTP). The most of QTP regions in particular have experienced significant warming and wetting over the past 50 years, and this warming trend is projected to intensify in the future. Such climate change will likely alter the soil freeze–thaw pattern in permafrost active layer and toward significant greenhouse gas nitrous oxide (N2O) release. However, the interaction effect of warming and altered soil moisture on N2O emission during freezing and thawing is unclear. Here, we used simulation experiments to test how changes in N2O flux relate to different thawing temperatures (T5–5°C, T10–10°C, and T20–20°C) and soil volumetric water contents (VWCs, W15–15%, W30–30%, and W45–45%) under 165 F–T cycles in topsoil (0–20 cm) of an alpine meadow with discontinuous permafrost in the QTP. First, in contrast to the prevailing view, soil moisture but not thawing temperature dominated the large N2O pulses during F–T events. The maximum emissions, 1,123.16–5,849.54 μg m–2 h–1, appeared in the range of soil VWC from 17% to 38%. However, the mean N2O fluxes had no significant difference between different thawing temperatures when soil was dry or waterlogged. Second, in medium soil moisture, low thawing temperature is more able to promote soil N2O emission than high temperature. For example, the peak value (5,849.54 μg m–2 h–1) and cumulative emissions (366.6 mg m–2) of W30T5 treatment were five times and two to four times higher than W30T10 and W30T20, respectively. Third, during long-term freeze–thaw cycles, the patterns of cumulative N2O emissions were related to soil moisture. treatments; on the contrary, the cumulative emissions of W45 treatments slowly increased until more than 80 cycles. Finally, long-term freeze–thaw cycles could improve nitrogen availability, prolong N2O release time, and increase N2O cumulative emission in permafrost active layer. Particularly, the high emission was concentrated in the first 27 and 48 cycles in W15 and W30, respectively. Overall, our study highlighted that large emissions of N2O in F–T events tend to occur in medium moisture soil at lower thawing temperature; the increased number of F–T cycles may enhance N2O emission and nitrogen mineralization in permafrost active layer.

Soil greenhouse gas emissions from a sisal chronosequence in Kenya

Sisal (Agave sisalana) is a climate-resilient crop grown on large-scale farms in semi-arid areas. However, no studies have investigated soil greenhouse gas (GHGs: CO2, N2O and CH4) fluxes from these plantations and how they relate to other land cover types. We examined GHG fluxes (Fs) in a sisal chronosequence at Teita Sisal Estate in southern Kenya. The effects of stand age on Fs were examined using static GHG chambers and gas chromatography for a period of one year in seven stands: young stands aged 1–3 years, mature stands aged 7–8 years, and old stands aged 13–14 years. Adjacent bushland served as a control site representing the surrounding land use type. Mean CO₂ fluxes were highest in the oldest stand (56 ± 3 mg C m-2 h-1) and lowest in the 8-year old stand (38 ± 3 mg C m-2 h-1), which we attribute to difference in root respiration between the stand. All stands had 13–28% higher CO₂ fluxes than bushland (32 ± 3 mg C m-2 h-1). CO2 fluxes in the wet season were about 70% higher than dry season across all sites. They were influenced by soil water content (WS) and vegetation phenology. Mean N2O fluxes were very low (<5 µg N m-2 h-1) in all sites due to low soil nitrogen (N) content. About 89% of CH4 fluxes were below the detection limit (LOD ± 0.02 mg C m-2 h-1). Our results imply that sisal plantations have higher soil CO2 emissions than the surrounding land use type, and the seasonal emissions were largely driven by WS and the vegetation status. Methane and nitrous oxide are of minor importance. Thus, soil GHG fluxes from sisal plantations are a minor contributor to agricultural GHG emissions in Kenya.

Effect of Grazing Intensities on Soil N2O Emissions from an Alpine Meadow of Zoige Plateau in China

The alpine meadow of Zoige Plateau plays a key role in local livestock production of cattle and sheep. However, it remains unclear how animal grazing or its intensity affect nitrous oxide (N2O) emissions, and the main driving factors. A grazing experiment including four grazing intensities (G0, G0.7, G1.2, G1.6 yak ha−1) was conducted between January 2013 and December 2014 to evaluate the soil nitrous oxide (N2O) fluxes under different grazing intensities in an alpine meadow on the eastern Qinghai–Tibet Plateau of China. The N2O fluxes were examined with gas collected by the static chamber method and by chromatographic concentration analysis. N2O emissions in the growing seasons (from May to September) were lower than that in non-growing seasons (from October to April) in 2013, 1.94 ± 0.30 to 3.37 ± 0.56 kg N2O ha−1 yr−1. Annual mean N2O emission rates were calculated as 1.17 ± 0.50 kg N2O ha−1 yr−1 in non-grazing land (G0) and 1.94 ± 0.23 kg N2O ha−1 yr−1 in the grazing land (G0.7, G1.2, and G1.6). The annual mean N2O flux showed no significant differences between grazing treatments in 2013. However, there were significantly greater fluxes from the G0.7 treatment than from the G1.6 treatment in 2014, especially in the growing season. Over the two years, the soil N2O emission rate was significantly negatively correlated with soil water-filled pore space (WFPS) and dissolved organic carbon (DOC) content as well as positively correlated with soil available phosphorus (P). No relationship was observed between soil N2O emission rate and temperature or rainfall. Our results showed that the meadow soils acted as a source of N2O for most periods and turned into a weak sink of N2O later during the sampling period. Our results highlight the importance of proper grazing intensity in reducing N2O emissions from alpine meadow. The interaction between grazing intensity and N2O emissions should be of more concern during future management of pastures in Zoige Plateau.

Earthworms did not increase long-term nitrous oxide fluxes in perennial forage and riparian buffer ecosystems

Many laboratory and mesocosm studies have demonstrated that earthworms influence nitrogen (N) cycling reactions and produce nitrous oxide (N2O) in well-aerated soils, but whether earthworms can stimulate N2O fluxes in realistic field conditions remains to be determined. We conducted two field experiments, in perennial forage agroecosystems for 2 yr and agriculture riparian buffers for 1 yr, to compare N2O fluxes from enclosures with ambient and artificially elevated earthworm populations. Despite a short-term (< 3 month) increase in mean N2O fluxes from the perennial forage enclosures with artificially elevated earthworm populations, this effect disappeared within 1 yr, with no significant difference (p> 0.05) in mean N2O flux from enclosures in either field experiment. The elevated earthworm populations declined and stabilized at the same level as the ambient earthworm populations within 1–2 yr after the field experiments began. The homeostatic regulation of earthworm populations under field conditions could be due to inter- and intra-specific competition, related to limitation in the food supply and habitat preferred by earthworms. Mean N2O fluxes in the perennial forage fields were negatively correlated with soil moisture, but not related to earthworm populations. In the riparian buffers, the average N2O flux was negatively correlated with vegetation cover, and positively correlated with soil moisture and the size of the earthworm population at the end of the study. Our results suggest that the effects of earthworm addition on N2O emissions in laboratory studies can not necessarily be extrapolated to field settings. Earthworm field experiments that continue in the longer-term and in a variety of ecosystems should improve our understanding of the seasonal and environmental variability in earthworm activity and N2O production under field conditions.

CH4 and N2O fluxes from planted forests and native Cerrado ecosystems in Brazil

Forest soils are N2O sources and commonly act as CH4 sinks. This study evaluated the dynamics of the CH4 and N2O fluxes of soils under Eucalyptus plantations and native Cerrado vegetation, as well as possible interactions between environmental factors and fluxes. The study was carried out in the Distrito Federal, Brazil, during 26 months, in three areas: in two stands of the hybrid Eucalyptus urophylla × Eucalyptus grandis, planted in 2011 (E1), and in 2009 (E2) and native Cerrado vegetation (CE). Measurements to determine the fluxes in a closed static chamber were carried out from Oct 2013 to Nov 2015. Soil and climate factors were monitored. During the study period, the mean CH4 fluxes were –22.48, –8.38 and –1.31 μg CH4 m–2 h–1 and the mean N2O fluxes 5.45, 4.85 and 3.85 μg N2O m–2 h–1 from E1, E2 and CE, respectively. Seasonality affected plantations in the studied sites. Cumulative CH4 influxes were calculated (year-1: –1.86 to -0.63 kg ha–1 yr–1; year-2: –1.85 to –1.34 kg ha–1 yr–1). Cumulative N2O fluxes in the three sites were ≤ 0.85 kg ha–1 yr–1. The change in land use from Cerrado to Eucalyptus plantations did not significantly changed regarding greenhouse gases (GHG), compared to the native vegetation. Flux rates of both gases (N2O and CH4) were low. Temporal variations in GHG fluxes and different ages of the stands did not cause significant differences in cumulative annual fluxes.

Reduced tillage with residue retention and nitrogen application rate increase N2O fluxes from irrigated wheat in a subtropical floodplain soil

Nitrous oxide (N2O) emissions were measured in irrigated wheat (Triticum aestivum) in an annual wheat- mungbean (Vigna radiata)-rice (Oryza sativa L) rotation that had been running for seven consecutive years. Effect of two soil disturbance levels (strip vs. conventional tillage; ST vs. CT both with 30 % residue retention) and three nitrogen (N) fertilizer rates (60, 100 and 140 % of the recommended N fertilizer dose, RD; called hereafter 60RD, 100RD and 140RD, respectively) were assessed. Mean N2O fluxes were about 110 % higher in ST than in CT. The rate of N fertilizer application influenced the mean and cumulative N2O fluxes with significantly higher fluxes in ST than in CT. Based on the respective maximum grain yields (CT: 140RD, 3.52 t ha−1; ST: 60RD, 3.19 t ha−1) yield-scaled N2O emissions were higher in ST than those in CT. However, tillage vs. N rate interactions showed both the highest and lowest yield-scaled N2O fluxes in ST with 140RD and 60RD, respectively. Soil microbial biomass carbon (MBC), organic carbon (SOC), total N (TN), nitrate (NO3- -N), aggregate mean weight diameter (MWD) and larger aggregate size classes (2.0‒0.85 and >2.0 mm) were significantly higher in ST and positively correlated with N2O fluxes. Our results highlight that, despite increased N2O emissions, ST with residue can trade-off emissions to improve soil macro aggregation, C sequestration and retention of N and crop yield with the lower N fertilizer and other energy inputs. Reduction of the recommended N fertilization rate could be considered if ST is adopted.