Net CH4 Sink Research Articles

Greenhouse gas emissions and crop-specific emission factors of eight upland crops based on a six-year field experiment in the North China Plain

Nitrous oxide is a significant greenhouse gas (GHG) contributor in crop production, yet detailed documentation, including crop yields and specific emission factors (EFd of N2O), are lacking for the North China Plain. A six-year (2016–2022) diversified crop rotation experiment in this region examined GHG emissions from eight crops (sweet potato, peanut, soybean, spring maize, sweet sorghum, ryegrass, summer maize, and winter wheat) and fallow seasons. The results revealed that summer maize had the highest cumulative soil N2O emissions (5.07 kg N ha−1), with an average flux value of 225.3 μg N2O-N m−2 h−1 during the growing season. Summer maize emitted 16–75 % more N2O than spring crops and winter wheat and 85–86 % more than ryegrass (cover crop) and the winter fallow season. All eight crops acted as weak net CH4 sinks. Annual average CO2-equivalents of N2O and CH4 emissions reflected N2O emissions from all crops and fallow seasons. In addition, GHG footprint metrics (GFa, per unit area; GFy, per kg equivalent yield; GFb, per kg biomass; GFe, per unit economic benefit) based on life cycle assessment thinking showed that intensive cereal crops (winter wheat and summer maize) had the highest GHG footprints, while sweet potato had the lowest due to its greater biomass and lower N input. Other crops had 19–90 % lower GHG footprints than wheat and maize. Furthermore, we also quantified specific-crop EFd of N2O, with ryegrass having the lowest EFd (0.48 ± 0.02 %), followed by winter wheat (0.81 ± 0.43 %), and other crops ranging from 1.00 ± 0.13 % to 2.36 ± 0.85 %. This study provides important emissions data for different crops and winter fallow periods, enhancing our understanding of GHG footprints and emission factors, which are essential for the advancement of sustainable agriculture practices.

White Clover does not Increase Soil N2O Emissions Compared to Ryegrass in Non-Frozen Winter, but Increases CH4 Uptake

As one of the most important forage species in Europe, white clover (Trifolium repens) is a legume that is well recognized for its potential to increase productivity especially under reduced N input. It is hypothesized that legumes have the potential to decrease overwinter soil greenhouse gas (GHG) emissions due to more efficient N recycling as compared to non-legume forbs. We conducted a field experiment recording high-resolution soil nitrous oxide (N2O) and methane (CH4) fluxes during the winter months (December 2019 to March 2020) on a five-year-old grassland in central Germany with white clover, fertilized and unfertilized perennial ryegrass (Lolium perenne), and bare soil. White clover and fertilized ryegrass stimulated soil N2O emissions by 174% and 212% as compared to bare soil, and by 36% and 56% as compared to unfertilized ryegrass, respectively, due to their greater N availability and higher water-filled pore space (WFPS). The estimated cumulative CH4 fluxes under white clover were a net CH4 sink, whereas ryegrass and bare soil were net CH4 sources. Soil N2O fluxes were predominantly regulated by both mineral N and WFPS, while CH4 fluxes were mainly explained by WFPS. N-fertilization during the growing season did not affect off-season N2O and CH4 fluxes in perennial ryegrass plots. The combined non-CO2 global warming potential highlighted the possible mitigation effect of white clover on overwinter GHG emissions. Our findings suggest that GHG emissions from legumes are not offsetting their productive benefits during the non-frozen winter seasons.

Three years of CO2, CH4 and N2O fluxes from different sheepfolds in a semiarid steppe region, China

To better assess greenhouse gas (GHG) emissions from livestock folds in semi-arid steppe zones and reduce uncertainties in regional and national GHG emission inventories, we measured the fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from sheepfolds under contrasting management regimes (i.e., summer sheepfolds under continuous and rotational grazing strategies and the winter sheepfold) for 3 consecutive years. Our results showed that these GHG fluxes had high intra-annual and interannual variations, emphasizing the importance of multi-year measurement for achieving temporally representative annual budgets. Sheep presence and temperature appeared to be the key factors driving CH4, CO2 and N2O fluxes from sheepfolds, e.g., higher GHG emissions usually occurred in seasons with sheep presence. However, the sheepfold type exerted a distinct influence on the temperature sensitivity of GHG fluxes, i.e., the Q10 values for GHG fluxes were generally higher in summer sheepfolds than in winter sheepfold. The annual CH4, CO2 and N2O emissions for the 3 sheepfolds were estimated to be 1.5–16.5 kg C ha−1 yr−1 (or 1.9–2.6 g C yr−1sheep−1), 8.6–16.0 t C ha−1 yr−1 (or 5.1–6.6 kg C yr−1sheep−1) and 28.3–41.9 kg N ha−1 yr−1 (or 19.0–26.8 g N yr−1sheep−1), respectively. Averaging across the 3 years, the annual net GHG emissions (CH4 + CO2 + N2O) for all sheepfolds ranged from 47 to 71 t CO2-eq ha−1 yr−1 (or 27–36 kg CO2-eq yr−1 sheep−1), of which CO2 and N2O emissions contributed the most; moreover, the annual net GHG emissions had no significant differences between sheepfold types or grazing strategies. Given that local steppe soils have a lower magnitude of soil respiration (CO2) and N2O emissions and are also net sink for atmospheric CH4, the sheepfold sites in this region are undoubtedly one of the significant hotspots for GHG emissions and could be key areas to focus mitigation action.

Water table depth and plant species determine the direction and magnitude of methane fluxes in floodplain meadow soils.

Methane (CH4) is a powerful greenhouse gas with ongoing efforts aiming to quantify and map emissions from natural and managed ecosystems. Wetlands play a significant role in the global CH4 budget, but uncertainties in their total emissions remain large, due to a combined lack of CH4 data and fuzzy boundaries between mapped ecosystem categories. European floodplain meadows are anthropogenic ecosystems that originated due to traditional management for hay cropping. These ecosystems are seasonally inundated by river water, and straddle the boundary between grassland and wetland ecosystems; however, an understanding of their CH4 function is lacking. Here, we established a replicated outdoor floodplain-meadow mesocosm experiment to test how water table depth (45, 30, 15 cm below the soil surface) and plant composition affect CH4 fluxes over an annual cycle. Water table was a major controller on CH4, with significantly higher fluxes (overall mean 9.3 mg m-2 d-1) from the high (15 cm) water table treatment. Fluxes from high water table mesocosms with bare soil were low (mean 0.4 mg m-2 d-1), demonstrating that vegetation drove high emissions. Larger emissions came from high water table mesocosms with aerenchymatous plant species (e.g. Alopecurus pratensis, mean 12.8 mg m-2 d-1), suggesting a role for plant-mediated transport. However, at low (45 cm) water tables A. pratensis mesocosms were net CH4 sinks, suggesting that there is plasticity in CH4 exchange if aerenchyma are present. Plant cutting to simulate a hay harvest had no effect on CH4, further supporting a role for plant-mediated transport. Upscaling our CH4 fluxes to a UK floodplain meadow using hydrological modelling showed that the meadow was a net CH4 source because oxic periods of uptake were outweighed by flooding-induced anoxic emissions. Our results show that floodplain meadows can be either small sources or sinks of CH4 over an annual cycle. Their CH4 exchange appears to respond to soil temperature, moisture status and community composition, all of which are likely to be modified by climate change, leading to uncertainty around the future net contribution of floodplain meadows to the CH4 cycle.

Natural grassland conversion to agriculture or pine plantations: Effects on soil methane uptake

AbstractUpland soils are the only known biological sink for methane (CH4) by methanotrophic bacteria consumption. This process is mainly limited by the diffusion processes related to the soil's physical characteristics, which can be modified because of changes in land use depending on the soil type, the original system and the new land use converted. Our study focused on determining the differences in soil CH4 uptake because of changes in land use (from natural grassland to agricultural land and two Pinus radiata afforestation, differing in thinning management) and on determining which are the main drivers that control CH4 uptake in the studied soil type (Hapludoll), with focus on the diffusion process. CH4 fluxes were measured 12 times with the static chamber technique between October 2015 and April 2019. Also, CH4 gradient concentration in the soil profile and physical and chemical variables were measured on the same dates. All land uses studied acted as net CH4 sinks. Land‐use change from grassland to agriculture decreased soil CH4 uptake (~37% ± 19), whereas afforestation increased (~85% ± 73) this environmental service related to natural grassland. We found that the main drivers that control CH4 uptake in this soil are water and air‐filled pore space, variables that govern soil CH4 diffusion; they are mostly related to differences in bulk density (compaction) among land uses. Organic matter was also an important driver, mainly related to soil structure. Land‐use change affected all of these drivers. CH4 concentration presented differences at deeper soil layers only in the two afforestations, which differed in management (pruning and thinning vs. no management). However, CH4 uptake did not present significant differences between them, suggesting that there is no need for a high tree cover to increase the CH4 sink of the soil. This mixed tree and herbaceous cover may result in a similar environmental service output, increasing the options of land uses.

Maize/Peanut Intercropping Reduces Carbon Footprint Size and Improves Net Ecosystem Economic Benefits in the Huang-Huai-Hai Region: A Four-Year Study

The dual challenges of global climate change and reductions in the amount of arable land represent growing threats to the stability of global human populations. Efforts to further optimize cropping systems to maximize yields while minimizing greenhouse gas emissions in limited land areas have thus emerged as a focus in modern agriculture. Cereal-intercropping management strategies may represent a promising approach to simultaneously addressing both of these challenges in China. We aimed at comprehensively assessing changes in yield, carbon footprint, and net ecosystem economic benefit when transitioning from maize/peanut monoculture to intercropping in a field-scale study in an effort to aid in the development of low-carbon intercropping systems that do not have an adverse impact on Chinese grain yields. Beginning in June of 2018, a randomized complete block design with three treatments was used to initiate this study: (1) peanut monoculture (P), (2) maize monoculture (M), and (3) maize/peanut intercropping (MP). We compared yield, greenhouse gas emissions, carbon footprint and net ecosystem economic benefit. Results over four years showed that the land equivalent ratio associated with MP was greater than 1. All three of these cropping systems were net CO2 and N2O sources as well as net CH4 sinks, with MP generating significantly (p < 0.05) lower N2O and CO2 flux as well as smaller seasonal N2O and CO2 emissions relative to M. MP additionally reduced the carbon footprint associated with this cropping system by 11.11–31.65% and 30.37–43.62% relative to M and P, respectively. Consistently, MP treatment resulted in respective 70.69% and 26.25% net ecosystem economic benefit (NEEB) increases relative to the M and P conditions while simultaneously enhancing energy use efficiency. In summary, MP systems have potential economic benefit with lower environmental risk alternative to traditional peanut or maize monocropping systems. Converting from peanut or maize monocropping systems to MP systems practices contributed to improved farmland use efficiency, clean production and increased farmers’ income in an agricultural system.

Tree stems are a net source of CH4 and N2O in a hemiboreal drained peatland forest during the winter period

Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH4) and sources of nitrous oxide (N2O). However, further attention is needed on CH4 and N2O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH4 and N2O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH4 and N2O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH4 and N2O concentrations were measured using an automated dynamic soil chamber system. Tree stems were a net source of CH4 and N2O during the winter period. The origin of stem CH4 emissions was unclear, as stem and soil CH4 fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N2O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N2O emissions and environmental parameters. Soil was a net sink for CH4, largely determined by changes in soil temperature. Soil N2O dynamics were characterised by hot moments—short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH4 sink by 14% and added 2% to forest floor N2O emissions. Therefore, CH4 and N2O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem.

Methane emission from stems of European beech (Fagus sylvatica) offsets as much as half of methane oxidation in soil.

Trees are known to be atmospheric methane (CH4 ) emitters. Little is known about seasonal dynamics of tree CH4 fluxes and relationships to environmental conditions. That prevents the correct estimation of net annual tree and forest CH4 exchange. We aimed to explore the contribution of stem emissions to forest CH4 exchange. We determined seasonal CH4 fluxes of mature European beech (Fagus sylvatica) stems and adjacent soil in a typical temperate beech forest of the White Carpathians with high spatial heterogeneity in soil moisture. The beech stems were net annual CH4 sources, whereas the soil was a net CH4 sink. High CH4 emitters showed clear seasonality in their stem CH4 emissions that followed stem CO2 efflux. Elevated CH4 fluxes were detected during the vegetation season. Observed high spatial variability in stem CH4 emissions was neither explicably by soil CH4 exchange nor by CH4 concentrations, water content, or temperature studied in soil profiles near each measured tree. The stem CH4 emissions offset the soil CH4 uptake by up to 46.5% and on average by 13% on stand level. In Central Europe, widely grown beech contributes markedly to seasonal dynamics of ecosystem CH4 exchange. Its contribution should be included into forest greenhouse gas flux inventories.

Effect of Compost Derived from Urban Waste on Chard (Beta vulgaris L., var cycla) Yield and Soil GHG Fluxes in a Mediterranean Agricultural System

The use of recycled materials as soil amendments and fertilizers is an important priority in the agronomic sector to valorize waste from a circular economic perspective and reduce waste disposal, reduce dependence on external inputs, and provide better climate change mitigation options. In this study, we evaluated the agroecological performance of compost derived from recycled organic matrices of urban waste (mixed composted amendment, MCA) of the metropolitan area of Rome. MCA is available in big quantities and might represent an interesting option to substitute conventional mineral nitrogen fertilizer (CF). The effect of MCA, CF, and a combination of both (MIX 1:1) was tested on crop yield and greenhouse gas emissions in a field trial on a common Mediterranean crop (Swiss chart, one season, two crop cycles). The MCA effect on crop yield was positive and comparable to CF and MIX treatments, while MCA treatment showed the lower soil mineral nitrogen (N) content. GHG emissions in the MCA treatment were comparable to those observed in CF and MIX, being overall quite low. The soil acted as a weak net CH4 sink in all treatments (−12.6 ± 6.1 μg CH4 m−2 h−1); no differences in CO2 emissions between MCA and CF or MIX treatments were observed (range 0.1−0.2 g m−2 h−1). The N2O emission intensity of MCA was slightly lower than MIX and CF treatments (0.09, 0.011, and 0.011 g N2O kg−1 crop dry weight, respectively). Overall, MCA seemed a valid alternative to CF for the tested agro-environmental indicators in the spring/summer Mediterranean conditions.

Long-term tillage, residue management and crop rotation impacts on N2O and CH4 emissions from two contrasting soils in sub-humid Zimbabwe

The respective contribution of conservation agriculture (CA) principles (no-tillage, permanent soil cover/mulch and crop rotations) on greenhouse gas (GHG) emissions is still unclear. This study was conducted at two long-term experimental sites established in 2013 in Zimbabwe, on an abruptic Lixisol at Domboshava Training Center (DTC) and on a xanthic Ferralsol at the University of Zimbabwe Farm (UZF). The purpose of the study was to unravel the individual and combined effects of tillage, mulching and rotation on N2O and CH4 emissions in low nitrogen (N) input maize-based cropping systems (< 60 kg N ha−1) and to compare emissions within maize rows and between maize rows. We hypothesised that integrating no tillage, mulch and cereal-legume rotation would enhance N2O emissions. Six treatments, replicated four times were investigated: conventional tillage, conventional tillage with rotation, no-tillage, no-tillage with mulch, no-tillage with rotation, no-tillage with mulch and rotation. The main crop was maize (Zea mays L.) and treatments with rotation included cowpea (Vigna unguiculate L. Walp.). Gas samples were regularly collected using the static chamber method in the maize row and inter-row spaces during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season. Soil moisture and mineral N were measured in the 0–20 cm soil depth. In 2019/20, cumulative total N2O emissions were significantly higher in mulch treatments at DTC, while at UZF N2O emissions were higher with cowpea rotation. Cumulative total N2O emissions ranged from 215 to 496 g N2O-N ha−1 yr−1 and from 226 to 395 g N2O-N ha−1 yr−1, at DTC and UZF, respectively. In 2020/21, N2O emissions were much lower and no differences were found between treatments on both sites (145 to 179 g N2O-N ha−1 yr−1 and 83 to 136 g N2O-N ha−1 yr−1 at DTC and UZF, respectively). A significant relationship was found between soil nitrate and daily N2O emissions. At UZF, highest N2O emissions were observed at a water-filled pore space of 60–70%. There were no significant differences in yield-scaled N2O emissions between treatments at both sites for the two seasons. DTC was a net source of CH4 (694 g CH4-C ha−1 yr−1 on average), while UZF was a net sink of CH4 (−494 g CH4-C ha−1 yr−1 on average). No evidence was found for in situ CH4 production at DTC, and an external source is most likely. Our study indicates that for low N input cropping systems in the sub-humid tropics, N loss through N2O is low.

Methane dynamics in the Hailuogou Glacier forefield, Southwest China

Soils in glacier forefields have a significant capacity for atmospheric CH4 uptake, but this pattern could be changed by high soil water content (SWC). The Hailuogou Glacier in SW China is a typical temperate monsoon glacier on siliceous bedrock, where a forefield soil chronosequence has developed with progressive glacier recession. To understand CH4 dynamics and their potential regulatory factors, we measured the concentrations and stable carbon (C) isotope compositions of CH4 and CO2, soil physicochemical properties, and perfromed a high-throughput sequencing. Among nine sampling sites, soil CH4 concentrations of six sites were below atmospheric levels and δ13C–CH4 values were similar to atmospheric levels. The average value was approximately −48.6‰ and without obvious fractionation. The soil CH4 concentrations exceeded atmospheric levels for the remaining three sites, and the δ13C–CH4 values were more enriched with increasing soil CH4 concentration. We calculated the soil-atmosphere CH4 flux (Jatm) using the concentration gradient method based on the soil CH4 concentration, sampling depth, and soil porosity. Jatm ranges from −0.08 to −0.52 mg m−2 d−1, acting as an atmospheric CH4 sink. It also shows that the correlation with soil exposure age or vegetation succession was insignificant. But the CH4 emission shows a larger variation changing from 0.05 to 1.8 mg m−2 d−1, which could result from local CH4 production differences catalyzed by aceticlastic methanogens. The results showed that not all sites acted as a net CH4 sink. SWC may have an important influence on CH4 dynamics in the Hailuogou Glacier forefield (HGF).

Linking transcriptional dynamics of CH4-cycling grassland soil microbiomes to seasonal gas fluxes

Soil CH4 fluxes are driven by CH4-producing and -consuming microorganisms that determine whether soils are sources or sinks of this potent greenhouse gas. To date, a comprehensive understanding of underlying microbiome dynamics has rarely been obtained in situ. Using quantitative metatranscriptomics, we aimed to link CH4-cycling microbiomes to net surface CH4 fluxes throughout a year in two grassland soils. CH4 fluxes were highly dynamic: both soils were net CH4 sources in autumn and winter and sinks in spring and summer, respectively. Correspondingly, methanogen mRNA abundances per gram soil correlated well with CH4 fluxes. Methanotroph to methanogen mRNA ratios were higher in spring and summer, when the soils acted as net CH4 sinks. CH4 uptake was associated with an increased proportion of USCα and γ pmoA and pmoA2 transcripts. We assume that methanogen transcript abundance may be useful to approximate changes in net surface CH4 emissions from grassland soils. High methanotroph to methanogen ratios would indicate CH4 sink properties. Our study links for the first time the seasonal transcriptional dynamics of CH4-cycling soil microbiomes to gas fluxes in situ. It suggests mRNA transcript abundances as promising indicators of dynamic ecosystem-level processes.

Increased annual methane uptake driven by warmer winters in an alpine meadow

Pronounced nongrowing season warming and changes in soil freeze-thaw (F-T) cycles can dramatically alter net methane (CH4 ) exchange rates between soils and the atmosphere. However, the magnitudes and drivers of warming impacts on CH4 uptake in different stages of the F-T cycle are poorly understood in cold alpine ecosystems, which have been found to be a net sink of atmospheric CH4 . Here, we reported a year-round ecosystem daily CH4 uptake in an alpine meadow on the Qinghai-Tibetan Plateau after a 5-year warming experiment that included a control, a low-level warming treatment (+2.4℃ at 5cm soil depth), and a high-level warming treatment (+4.5℃ at 5cm soil depth). We found that warming shortened the F-T cycle under the low-level warming and soils did not freeze under the high-level warming. Although both warming treatments increased the mean CH4 uptake rate, only the high-level warming significantly increased annual CH4 uptake compared to the control. The warming-induced stimulation of CH4 uptake mainly occurred in the cold season, which was mostly during spring thaw under low-level warming and during the frozen winter under high-level warming due to a longer period with thawed soil. We also found that warming significantly stimulated daily CH4 uptake mainly by reducing near-surface soil water content in the warm season, whereas both soil water content and temperature controlled daily CH4 uptake in different ways during the autumn freeze, frozen winter, and spring thaw periods of the control. Our study revealed a strong warming effect on CH4 uptake during the entire F-T cycle in the alpine meadow, especially the unfrozen winter. Our results also suggested the important roles of soil pH, available phosphorus, and methanotroph abundance in regulating annual CH4 uptake in response to warming, which should be incorporated into biogeochemical models for accurately forecasting CH4 fluxes under future climate scenarios.

Long-term dynamics of soil, tree stem and ecosystem methane fluxes in a riparian forest

The carbon (C) budgets of riparian forests are sensitive to climatic variability. Therefore, riparian forests are hot spots of C cycling in landscapes. Only a limited number of studies on continuous measurements of methane (CH4) fluxes from riparian forests is available. Here, we report continuous high-frequency soil and ecosystem (eddy-covariance; EC) measurements of CH4 fluxes with a quantum cascade laser absorption spectrometer for a 2.5-year period and measurements of CH4 fluxes from tree stems using manual chambers for a 1.5 year period from a temperate riparian Alnus incana forest.The results demonstrate that the riparian forest is a minor net annual sink of CH4 consuming 0.24 kg CH4-C ha−1 y−1. Soil water content is the most important determinant of soil, stem, and EC fluxes, followed by soil temperature. There were significant differences in CH4 fluxes between the wet and dry periods. During the wet period, 83% of CH4 was emitted from the tree stems while the ecosystem-level emission was equal to the sum of soil and stem emissions. During the dry period, CH4 was substantially consumed in the soil whereas stem emissions were very low. A significant difference between the EC fluxes and the sum of soil and stem fluxes during the dry period is most likely caused by emission from the canopy whereas at the ecosystem level the forest was a clear CH4 sink. Our results together with past measurements of CH4 fluxes in other riparian forests suggest that temperate riparian forests can be long-term CH4 sinks.

Annual greenhouse gas emissions from sheepfolds and cattle sheds

AbstractLivestock sheds are local and regional hotspots of greenhouse gases (GHG) emissions, but only very few studies analyse the intensities of GHG emissions from this source. The objective of this study was to quantify annual CH4, CO2 and N2O emissions from inside and outside of sheepfolds and summer cattle sheds in a typical agro‐pastoral ecotone using static chamber technique. Both sheepfolds and cattle shed functioned as huge net sources of CH4 and N2O at annual scale. Animal presence increased CH4, CO2 and N2O effluxes for up to 1100 times compared to the animal sheds without animals. N2O emissions boosted for 160%–280% during and after rainfall and spring‐thaw events. The CH4 and CO2 fluxes increased exponentially with faeces temperature for the outside sheepfold and summer cattle shed. The annual GHG emissions from both sheepfolds and summer cattle shed were 56 t CO2 equivalents ha−1, of which N2O contributed to 94%. Sheepfold dominated the total GHG emissions from animal sheds and accounted for 83% of the annual GHG flux. Annual emission on a per animal basis was 15, 0.2 and 28 kg CO2 eq year−1 sheep−1 and 26, 10 and 140 kg CO2 eq year−1 cattle−1 for N2O, CH4 and CO2, respectively. The annual N2O emissions from animal sheds were 70–250 times larger than nearby grassland soils, which were also net sink for atmospheric CH4. Concluding, animal sheds are very intensive local hotspots of GHG emissions, which should be considered at the local and regional scales.

Warming and drought increase but wetness reduces the net sink of CH4 in alpine meadow on the Tibetan Plateau

Alpine meadows are significant CH4 sink. However, the response of soil CH4 uptake to future climate change in the alpine meadows remains poorly understood. We carried out an alpine meadow field experiments to determine how warming and altered precipitation (drought and wetness) affect CH4 uptake, as well as its underlying mechanisms. Our results showed that this mesic alpine meadow was a CH4 sink under all treatments (ranging from −311.4 to −487.1 mg m−2 CH4). Warming significantly increased CH4 uptake by an average of 18.14% of the annual cumulative CH4 uptake and by 48.68% of the non-growing season cumulative CH4 uptake. Drought enhanced the positive warming effect on CH4 uptake by decreasing soil water content, but wetness weakened this effect. Moreover, using high-throughput sequencing and GeoChip approaches, we found that warming stimulated CH4 uptake not only by increasing CH4 oxidation but also by decreasing CH4 production through changing the methanotroph community compositional structure. Further analysis revealed that the dynamics of daily CH4 uptake were mainly regulated by soil water content during the growing season, whereas daily CH4 uptake was strongly correlated with soil temperature during the non-growing season. Warming and altered precipitation influence soil CH4 uptake in different ways during the growing and non-growing seasons. Based on our results, we predict that Tibetan Plateau mesic alpine meadows will remain a net sink of CH4 under future climate warming and drought scenarios, but soil CH4 uptake will decrease if precipitation increases in future.

Watershed scale patterns and controlling factors of ecosystem respiration and methane fluxes in a Tibetan alpine grassland

Ecosystem respiration (Re) and methane (CH4) fluxes are two important soil-atmosphere carbon exchange processes that have been well documented at the local scale. However, the spatial patterns and controlling factors of these processes remain unclear in the Tibetan alpine permafrost region at the watershed scale. We conducted a two-year field monitoring study of Re and CH4 fluxes at three altitudinal positions on shady and sunny slopes in a Tibetan alpine grassland to determine the spatial variability in the two processes and to identify their underlying mechanisms. The microbial factor had a controlling effect on Re spatial variability in alpine grassland watersheds. The lower soil temperature and soil organic matter content at the upper slope positions on the shady slopes inhibited Re because they reduced the soil microbial activity. We found that the alpine grassland to be a net sink of atmospheric CH4, and the average CH4 flux rates exhibited large spatial variance ranging from −1.60 to −10.48 µg CH4 m−2 h−1 within the watersheds. The spatial variability in soil volumetric water content explained 76% of the variation in CH4 fluxes within the watersheds. We suggest that the influence of permafrost on hydrologic conditions may increase the spatial variability of soil moisture (measured as soil volumetric water content and water-filled pore space) in alpine grassland watersheds, and generally form poorly drained landscape at the lower slope position where Re and CH4 uptake are inhibited. Our results highlight the indirect effects of terrain and permafrost on Re and CH4 fluxes through their effects on biophysiochemical factors. We recommend that the spatial variability in Re and CH4 fluxes at the watershed scale of Tibetan alpine grassland should be given more attention in earth system models, especially the variability of CH4 fluxes with altitudinal position.

Topography-related controls on N2O emission and CH4 uptake in a tropical rainforest catchment

Forest soils in the warm-humid tropics significantly contribute to the regional greenhouse gas (GHG) budgets. However, spatial heterogeneity of GHG fluxes is often overlooked. Here, we present a study of N2O and CH4 fluxes over 1.5 years, along a topographic gradient in a rainforest catchment in Xishuangbanna, SW China. From the upper hillslope to the foot of the hillslope, and further to the flat groundwater discharge zone, we observed a decrease of N2O emission associated with an increase of soil water-filled-pore-space (WFPS), which we tentatively attribute to more complete denitrification to N2 at larger WFPS. In the well-drained soils on the hillslope, denitrification at anaerobic microsites or under transient water-saturation was the potential N2O source. Negative CH4 fluxes across the catchment indicated a net soil CH4 sink. As the oxidation of atmospheric CH4 is diffusion-limited, soil CH4 consumption rates were negatively related to WFPS, reflecting the topographic control. Our observations also suggest that during dry seasons N2O emission was significantly dampened (<10 μg N2O-N m−2 h−1) and CH4 uptake was strongly enhanced (83 μg CH4-C m−2 h−1) relative to wet seasons (17 μg N2O-N m−2 h−1 and 56 μg CH4-C m−2 h−1). In a post-drought period, several rain episodes induced exceptionally high N2O emissions (450 μg N2O-N m−2 h−1) in the groundwater discharge zone, likely driven by flushing of labile organic carbon accumulated during drought. Considering the global warming potential associated with both GHGs, we found that N2O emissions largely offset the C sink contributed by CH4 uptake in soils (more significant in the groundwater discharge zone). Our study illustrates important topographic controls on N2O and CH4 fluxes in forest soils. With projected climate change in the tropics, weather extremes may interact with these controls in regulating forest GHG fluxes, which should be accounted for in future studies.

Optimizing tillage method and irrigation schedule for greenhouse gas mitigation, yield improvement, and water conservation in wheat–maize cropping systems

Excessive agricultural inputs meet food demand only at a huge environmental cost, and optimized management strategies are needed to promote the balanced development of food security, water conservation, and environmental sustainability. Here, we conducted a two-year field experiment to investigate annual greenhouse gas (GHG) emission, yield, water use efficiency (WUE), global warming potential (GWP), and greenhouse gas intensity (GHGI) from 2016 to 2018 in a typical wheat–maize cropping system in the North China Plain. Two tillage methods (T1, rotary tillage; T2, subsoiling) and four irrigation schedules (W1, pre-planting irrigation; W2, pre-planting + jointing irrigation; W3, pre-planting + anthesis irrigation; W4, pre-planting + jointing + anthesis irrigation) were used for winter wheat, and conventional practices were adopted for summer maize. In the winter wheat–summer maize season, the soil acted as a net sink for CH4 but as a source for CO2 and N2O in all treatments, and CO2 accounted for the highest proportion of the GWP. The inappropriate irrigation period in the W3 treatment caused greater GHG release and reduced crop yield, whereas the excessive irrigation in the W4 treatment led to low water productivity. Although the use of irrigation water increased annual GHG emissions, an appropriate irrigation schedule could significantly mitigate the GHGI. Compared with the W3 and W4 treatments, the W2 treatment increased yield by an average of 7.56–10.58% and 2.06–2.68%, improved WUE by 9.95–17.83% and 11.29–22.84%, reduced GWP by 3.70–5.10% and 0.65–2.25%, and decreased GHGI by 10.66–14.26% and 3.05–4.86%, respectively. The T2 treatment resulted in high GHG emissions and was accompanied by low yield and water productivity. Relative to the T2 treatment, the annual yield and WUE in the T1 treatment increased by an average of 4.41–15.15% and 8.12–12.76%, and the annual GWP and GHGI decreased by 3.97–4.62% and 8.80–16.93%. Therefore, rotary tillage combined with supplementary irrigation at the jointing stage can mitigate GHG emissions and improve yield and water productivity, making it an environmentally friendly agricultural practice.

Soil Is a Net Source of Methane in Tropical African Forests

Research Highlights: Monitoring of soil CH4 fluxes in African tropical forest conducted run for almost two years, contributing to the scant information on greenhouse gas (GHG) fluxes from forests available from this region. Data showed that the forest soil acted as a net yearly source of CH4. Hotspots of CH4 emissions were measured both in upland and lowland areas of the forest, and on an annual basis they overcame the soil CH4 sink during drier periods or in well-drained areas. Background and Objectives: Atmospheric studies indicate that tropics are a strong CH4 source. Regional budgets attribute the majority of this source to wetland ecosystems and flooded lowland forests, whereas un-flooded forests are considered net CH4 sinks, although few studies in tropical forests, in particular in Africa, are available. The present work aims to contribute to this knowledge gap. Materials and Methods: Monitoring campaigns were conducted along the year in the tropical forest of the Ankasa National Park, Ghana, in two contrasting environments, uphill and downhill, using close static chambers coupled with gas chromatography. Results: The uphill area was a net weak CH4 sink with mean daily fluxes ranging from −1.29 to 0.44 mg CH4 m−2 d−1. The downhill area was a significant CH4 source with mean daily fluxes ranging from −0.67 to 188.09 mg CH4 m−2 d−1 and with peaks up to 1312 mg CH4 m−2 d−1 in the wet season. Conclusions: The net annual soil CH4 budget for the Ankasa Park, normalizing the proportion of downhill areas over the whole park surface, was a source of about 3.3 kg CH4 ha−1 yr−1. Overlooking such areas might lead to underestimates of the total CH4 source strength of forested areas.