CH4 Uptake Rates Research Articles

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.

Greenhouse gas fluxes in a no-tillage chronosequence in Central Ohio

A no-till chronosequence study was conducted to assess the impact of continuous no-till (NT) on greenhouse gases (CO2, CH4 and N2O) emission and the global warming potential (GWP) of agroecosystems. Five paired sites in Central Ohio (USA) under plow till (PT) and NT for 9, 13, 36, 48 and 49 years were selected, and GHG fluxes were measured over a 2-year period. Nearby deciduous forests were included for comparative purposes. Results showed higher CO2 emission under PT than NT (5.74 vs 4.55 Mg CO2-C ha−1). Annual CH4 flux averaged − 0.1 and − 0.07 kg CH4-C ha−1 respectively under NT and PT, and was influenced by location and years under NT (greater rate of CH4 uptake with longer duration of NT). Yet, the rate of CH4 uptake in the agricultural soils was always < 15% of the rate in nearby forest soils (−1.16 kg CH4-C ha−1 y−1). Annual N2O emission was generally higher under PT than NT (6.70 vs 4.68 kg N2O-N ha−1), but an important deviation was observed at one site located on a poorly-drained silt loam soil where N2O emission was 1.8-fold greater under NT than PT, likely due to wet soil conditions and labile organic carbon availability near the soil surface. The GWP of agroecosystems at the study sites averaged 23.1 and 19.9 Mg CO2 equivalents ha−1 y−1 under PT and NT, respectively; N2O emission accounted for 5–60% of the GWP and that contribution increased with NT duration. These results underscore the significance of N2O in defining the climate mitigation potential of agriculture, and also highlight the need for improved N fertilizer management practices (eg. split application, injection) to minimize N2O emission from fields under long-term NT. Even without consideration of agricultural inputs (i.e. fuel, fertilizers, pesticides) and change in soil C storage, the GHG flux data showed that sustained application of NT can help decrease the GWP of agroecosystems, further demonstrating the potential climate mitigation benefits of NT farming.

Grassland restoration measures alter soil methane uptake by changing community phylogenetic structure and soil properties

Grassland restoration can play a critical role in regulating ecosystem carbon cycling, especially for changes in methane (CH4) flux. However, how grassland restoration measures affect soil CH4 flux remains poorly understood. Here, we investigated the relationships between soil CH4 flux and properties of both plant communities and soil under different grassland restoration measures (shallow ploughing, harrowing and natural recovery) in a field experiment conducted during the growing season in a semiarid grassland in northern China using the static chamber method. Soil was a CH4 sink across all three restoration measures. Compared to natural recovery, shallow ploughing significantly enhanced and harrowing significantly decreased soil CH4 uptake. The rates of soil CH4 uptake were positively associated with litter production and belowground biomass and negatively associated with plant taxonomic and phylogenetic diversity. Soil temperature, NH4+-N concentration, and soil organic matter quality had positive effects on CH4 uptake rate, but soil moisture and NO3−-N concentration had negative effects on CH4 uptake rate. Our results elucidate the pathway through which shallow ploughing facilitates soil CH4 uptake in a semiarid grassland and thus highlights its role in sustaining ecosystem multifunctionality as a measure for grassland restoration.

Soil-atmosphere exchange of CH4 in response to nitrogen addition in diverse upland and wetland ecosystems: A meta-analysis

Reactive nitrogen (N) addition may profoundly impact the global CH4 budget through its substantial effects on soil CH4 uptake and emission. However, the magnitude and direction of soil CH4 uptake and emission rates in response to N addition on a global scale are still unclear. Here, to investigate the effects of N addition on soil CH4 uptake and emission rates in various upland and wetland ecosystems, we synthesized a large dataset comprising 878 paired observations from 178 studies. Across these studies, we found that N addition significantly reduced soil CH4 uptake rate in upland ecosystems but significantly increased soil CH4 emission rate in wetland ecosystems. The magnitude of the effects was ecosystem-type dependent. Following N addition, reduction of soil CH4 uptake rate and increase in CH4 emission rate were significantly higher in natural ecosystems (with the exception of grassland ecosystems) than in agricultural ecosystems. However, reduction in soil CH4 uptake rate increased with N addition rate in only natural ecosystems. Moreover, the soil CH4 uptake rate in N-limited ecosystems (cold temperate zone and Tibet Plateau) was less sensitive to N addition compared to N-rich ecosystems (subtropical and tropical zones). Additionally, organic N addition had a lower reduction effect on soil CH4 uptake in upland ecosystems and a lower stimulatory effect on soil CH4 emission rate in wetland ecosystems compared to the addition of inorganic N forms. Overall, our results shed light on the magnitude and direction of the effect of N addition on soil CH4 uptake and emission rates in diverse upland and wetland ecosystems and will help improve ecosystem models for predicting soil CH4 flux caused by N addition.

Biochar dose determines methane uptake and methanotroph abundance in Haplic Luvisol

Biochar promotes C sequestration and improvement of soil properties. Nevertheless, the effects of biochar addition on soil condition are poorly understood, especially with respect to greenhouse gas (GHG) emissions. A large proportion of GHG emissions derive from agriculture and, thus, recognition of the effect of biochar addition to soil on GHG emissions from terrestrial ecosystems is an important issue. The purpose of our study was to evaluate the short- and long-term effects of biochar application on soil in aspects of: GHG exchange (CH4 and CO2), basic physicochemical soil properties and structure of microbial communities in Haplic Luvisol. Soil was collected from fallow fields enriched with three doses of wood offcuts biochar (10, 20 and 30 Mg ha−1) and incubated at two moisture levels (60 and 100% WHC) with the addition of 1% CH4. To evaluate the influence of biochar aging in soil, the samples were analysed directly (short-term response) and five years (long-term response) after amendment. Generally, biochar addition increased soil pH, redox potential (Eh), organic carbon (SOC) and dissolved organic carbon (DOC) contents. Under 60% WHC, direct biochar application to the soil resulted in a clear improvement in the CH4 uptake rate. In contrast to that (at 100% WHC) methane uptake rates were twofold decreased. The positive effect was reduced due to biochar aging in the soil, but five years after application, at 60% WHC and the highest biochar dose (30 Mg ha−1) still significantly enhanced CH4 oxidation. From a short-term perspective, biochar application increased CO2 emissions, but after five years this effect was not observed. Microbial tests confirmed that the improvement in CH4 oxidation was correlated with methanotroph abundance in the soil. Moreover, an increase of Methylocystis abundance in the soil enriched with biochar along with enhanced CH4 uptake rates confirm the positive biochar influence on methanotrophic communities.

Fourier transform infrared spectroscopy and interference of volatile organic compounds on measurements of methane (CH4) fluxes at tree stems – a general phenomenon for plant systems?

Fourier transform infrared spectroscopy and interference of volatile organic compounds on measurements of methane (CH₄) fluxes at tree stems – a general phenomenon for plant systems?

Forest Management Impacts on Greenhouse Gas Fluxes from Riparian Soils Along Headwater Streams

Increasing concentrations of atmospheric greenhouse gases (GHGs; CO2, CH4, N2O) cause climate change. Depending on the conditions, soils have the potential to store carbon or to be a source of GHGs to the atmosphere. Riparian soils in particular have high potential to store carbon, but also to be sources of CH4 and N2O. Headwater streams make up a large proportion of stream length in a drainage network, and their riparian zones have valuable ecosystem functions. In parallel, the riparian zones of headwater streams are particularly vulnerable to forest harvest. Studies of GHG fluxes from these unique ecosystems remain limited. Our objective was to quantify the effects of forestry practices and groundwater discharge (DIS) areas on GHG emissions from riparian forest soils in coastal British Columbia. We compared nine sites with three different forest management protocols: (1) harvesting with a riparian buffer, (2) no buffer, and (3) reference sites without harvesting. We measured gas fluxes, soil temperature, soil moisture and depth to the groundwater table alongside headwater streams monthly over one growing season. We found that CH4 uptake rates were 65% lower at the no buffer sites, and N2O emission rates were 52% lower at the no buffer sites, when compared to the reference sites. Additionally, CH4 uptake was 54% lower at DIS areas than in non-DIS areas. The results of our research help inform forest management by demonstrating that maintaining riparian buffers can be effective in protecting the ecosystem functions contributing to soil GHG fluxes.

Topography-based statistical modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux

Abstract. Boreal forest soils are globally an important sink for methane (CH4), while these soils are also capable of emitting CH4 under favourable conditions. Soil wetness is a well-known driver of CH4 flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH4 flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH4 flux and soil moisture measurements, complemented by ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a random forest model using digital-elevation-model-derived topographic indices, based on which we upscaled the forest floor CH4 flux. The modelling was performed for two seasons: May–July and August–October. Additionally, we evaluated the number of flux measurement points needed to get an accurate estimate of the flux at the whole study site merely by averaging. Our results demonstrate high spatial heterogeneity in the forest floor CH4 flux resulting from the soil moisture variability as well as from the related ground vegetation. The mean measured CH4 flux at the sample points was −5.07 µmol m−2 h−1 in May–July and −8.67 µmol m−2 h−1 in August–October, while the modelled flux for the whole area was −7.42 and −9.91 µmol m−2 h−1 for the two seasons, respectively. The spatial variability in the soil moisture and consequently in the CH4 flux was higher in the early summer (modelled range from −12.3 to 6.19 µmol m−2 h−1) compared to the autumn period (range from −14.6 to −2.12 µmol m−2 h−1), and overall the CH4 uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH4; however, these wet patches got drier and smaller in size towards the autumn, changing their dynamics to CH4 uptake. The mean values of the measured and modelled CH4 fluxes for the sample point locations were similar, indicating that the model was able to reproduce the results. For the whole site, upscaling predicted stronger CH4 uptake compared to simply averaging over the sample points. The results highlight the small-scale spatial variability of the boreal forest floor CH4 flux and the importance of soil chamber placement in order to obtain spatially representative CH4 flux results. To predict the CH4 fluxes over large areas more reliably, the locations of the sample points should be selected based on the spatial variability of the driving parameters, in addition to linking the measured fluxes with the parameters.

Emissions of CO2, N2O and CH4 From Cultivated and Set Aside Drained Peatland in Central Sweden

Northern peatlands are important carbon (C) reservoirs, storing about one-third of the global terrestrial soil C pool. Anthropogenic influences, such as drainage for agriculture and forestry, lower the originally high groundwater level, leading to peat aeration and decomposition. This is particularly reflected in significant losses of CO2, while fluxes of N2O and CH4 are generally considered of minor importance for the overall greenhouse gas (GHG) balance of cultivated peatlands in Scandinavia. Setting land aside from agricultural production has been proposed as a strategy to reduce GHG emissions from drained peatland, restore natural habitats, and increase C sequestration. However, the evidence for this is rather scarce unless drainage is terminated. In this study, we measured respiration using dark automatic chambers, and CO2, N2O, and CH4 fluxes using manual static chambers, on: 1) cultivated peatland and 2) adjacent set-aside peatland in Central Sweden. The set-aside site was found to be a greater source of respiration than the cultivated site, while higher N2O fluxes and lower CH4 uptake rates were observed for the cultivated site. However, to compare the full GHG balance and assess the abandonment of drained cultivated peatland, additional measures, such as gross primary production (GPP) but also dissolved organic C losses would have to be taken into account.

Spatiotemporal patterns and drivers of methane uptake across a climate transect in Inner Mongolia Steppe

Steppe soils are important biological sinks for atmospheric methane (CH4), but the strength of CH4 uptake remains uncertain due to large spatiotemporal variation and the lack of in situ measurements at regional scale. Here, we report the seasonal and spatial patterns of CH4 uptake across a 1200 km transect in arid and semi-arid steppe ecosystems in Inner Mongolia, ranging from meadow steppe in the east plain to typical and desert steppes on the west plateau. In general, seasonal patterns of CH4 uptake were site specific, with unimodal seasonal curves in meadow and typical steppes and a decreasing seasonal trend in desert steppe. Soil moisture was the dominant factor explaining the seasonal patterns of CH4 uptake, and CH4 uptake rate decreased with an increase in soil moisture. Across the transect, CH4 uptake showed a skewed unimodal spatial pattern, with the peak rate observed in the typical steppe sites and with generally higher uptake rates in the west plateau than in the east plain. Soil moisture, together with soil temperature, soil total carbon, and aboveground plant biomass, were the main drivers of the regional patterns of CH4 uptake rate. These findings are important for model development to more precisely estimate the soil CH4 sink capacity in arid and semi-arid regions.

Effects of Increased Precipitation and Nitrogen Deposition on Methane Uptake of Alpine Meadow in Qinghai-Tibet Plateau: in situ Experiments

The methane (CH4) fluxes in grasslands are sensitive to changes in precipitation and soil nitrogen concentrations, which was poorly understood, especially on the Tibetan Plateau. Therefore, an experiment was conducted from May 2012 to October 2013, by using the static chamber technique and gas chromatography method. We set up five treatments: an increased 2 g m-2 NH4NO3 treatment (2gN); an increased 4 g m-2 NH4NO3 treatment (4gN); and treatment with precipitation increased by 20% (Pre), and added 2 g m-2 NH4NO3 and precipitation (20%) treatment (N+Pre), and a control treatment (CK). The five treatments showed decreasing CH4 uptake rates in the following order: CK (71.66 ± 6.6 µg m-2 h-1) > N+Pre (58.57 ± 3.7 µg m-2 h-1) > Pre (52.66 ± 2.3 µg m-2 h-1) > 2gN (47.63 ± 3.1 µg m-2 h-1) > 4gN (39.12 ± 3.3 µg m-2 h-1). The 2gN and 4gN treatment resulted in 33.5% and 45.4% lower CH4 uptake than the CK treatment, respectively. The path analysis indicated that the above-ground biomass and litter fall were the most important factor promoting and limiting the CH4 uptake rate of alpine meadow, respectively.

Asymmetric response of soil methane uptake rate to land degradation and restoration: Data synthesis.

Land degradation and restoration profoundly affect soil CH4 uptake capacity in terrestrial ecosystems. However, a comprehensive assessment of the response of soil CH4 uptake to land degradation and restoration at global scale is not available. Here, we present a global meta-analysis with a database of 228 observations from 83 studies to investigate the effects of land degradation and restoration on the capacity of soil CH4 uptake. We found that land degradation significantly decreased the capacity of soil CH4 uptake, except the conversion of pasture to cropland where the soil CH4 uptake rate showed no response. In contrast, all types of land restoration significantly increased the capacity of soil CH4 uptake. Interestingly, the response of soil CH4 uptake rate to land degradation and restoration was asymmetric: the increased soil CH4 uptake rate in response to the land restoration was smaller compared to the decrease in CH4 uptake rate induced by the land degradation. The effect of land degradation on soil CH4 uptake rate was not dependent on the time since land use change, but the CH4 sink strength increased with the time since land restoration. The response of soil CH4 uptake rate to both land degradation and restoration was predominantly regulated by changes in the soil water-filled pore space, soil bulk density, and pH, whereas alterations in the substrate quantity and quality had negligible effect. Additionally, the effects of land degradation and restoration on soil CH4 uptake were strongly related to the mean annual precipitation and soil texture. Overall, our results provide novel insights for understanding of how land degradation and restoration can affect the CH4 sink strength of upland soils, and more importantly, our findings are beneficial to take measures to enhance the potential of soil CH4 uptake in response to global land use change.

Invited Review: Methane sources, quantification, and mitigation in grazing beef systems

ABSTRACT Purpose The purpose of the review was to examine enteric methane emissions, quantification methods, and mitigation strategies in grazing beef systems. Sources Peer-reviewed literature and conference abstracts were the main sources of information for this review. Synthesis Methane emissions (CH4) can be reduced by improving forage quality by including more cool-season forages and legumes and rotationally grazing animals. Including forages with beneficial secondary compounds such as condensed tannins and saponins also has CH4-mitigation potential. Providing nutritional supplements that improve the nutritional status of the animal and the efficiency of feed energy use has the potential to reduce CH4 emissions from grazing cattle. Genetic selection has shown some viability in reducing herd emissions, but heritability estimates are low for CH4 yield. More research is needed to understand the potential. Soil methanotrophy may partially offset CH4 emissions when animals are stocked moderately but soil CH4 uptake rates are relatively low in most grazing ecosystems. A new metric to quantify the global warming potential of CH4, GWP*, may allow future models to more appropriately consider the behavior and effects of CH4 in the atmosphere. Conclusions and Applications Methane mitigation strategies in grazing environments are limited, but producer decisions that improve the nutritional status of animals, the quality of the forage base, and supplementation of known CH4-mitigation compounds can reduce CH4 production. Now that less expensive, easier to use quantification tools exist, researchers need to conduct more long-term monitoring experiments and focus on reducing CH4 production of grazing animals where potential for reduction is largest.

Simulated nitrogen deposition influences soil greenhouse gas fluxes in a Mediterranean dryland

Soil nitrogen (N) availability is a key driver of soil-atmosphere greenhouse gas (GHG) exchange, yet we are far from understanding how increases in N deposition due to human activities will influence the net soil-atmosphere fluxes of the three most important GHGs: nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2). We simulated four levels of N deposition (10, 20 and 50 kg N ha−1 yr−1, plus unfertilised control) to evaluate their effects on N2O, CH4 and CO2 soil fluxes in a semiarid shrubland in central Spain. After 8 years of experimental fertilisation, increasing N availability led to a consistent increase in N2O emissions, likely due to simultaneous increases in soil microbial nitrification and/or denitrification processes. However, only intermediate levels of N fertilisation reduced CH4 uptake, while increasing N fertilisation had no effects on CO2 fluxes, suggesting complex interactions between N deposition loads and GHG fluxes. Our study provides novel insight into the responses of GHGs to N deposition in drylands, forecasting increases in N2O emissions, and decreases in CH4 uptake rates, with likely consequences to the on-going climate change.

Soil properties and not high CO2 affect CH4 production and uptake in periodically waterlogged arable soils

Several studies related to CH4 cycling focus on the effect of elevated atmospheric CO2 levels on soil methanogenesis and methanotrophy. However, periodically waterlogged soils are characterized by much higher CO2 concentrations, while aggregates forming in arable soils due to traditional cultivation may be a natural CH4 source for low-affinity methanotrophs. We present a comprehensive laboratory investigation into both methanogenesis and methanotrophy in three different soils under ambient and high CO2 concentrations. The tested materials were agricultural soils widely distributed in Europe: Eutric Cambisol, Haplic Podzol and Mollic Gleysol with determined properties: Corg, Ntot, clay, silt, sand, pH, NO3−, NH4+. We conducted two independent experiments on methanogenesis (flooded conditions) and methanotrophy (non-flooded conditions). All samples for both studies were divided into three sets which differed in the initial CO2 concentration in headspace: ambient CO2 (0.03% v/v ± 0.005), and two high CO2 levels (5% ± 0.4 and 10% ± 0.5 v/v CO2). Moreover, 1% (v/v) of CH4 was added to all sets prepared for the methanotrophy test since low-affinity methanotrophy in the tested soils had been reported previously. Gas concentrations (CH4, CO2, O2) in headspace were measured using gas chromatography method. We observed that the examined soils differed in their ability to produce CH4 as follows: Haplic Podzol = Eutric Cambisol > Mollic Gleysol, as well as to consume CH4: Mollic Gleysol > Haplic Podzol > Eutric Cambisol. The CH4 emissions started faster in Gleysol, but the final CH4 concentration and CH4 production rate in this soil were significantly lower than in the two other soils. The CH4 uptake rate significantly differed among the tested soils. The MANOVA confirmed the significance of the soil-type factor in determining both process rates, whereas the effect of CO2 level and the interaction between both factors were not significant. The amount of consumed O2 during CH4 oxidation was also characteristic for examined soil, but did not differ significantly between CO2 variants. CH4 production and consumption in three different soils (Eutric Cambisol, Haplic Podzol and Mollic Gleysol) collected from periodically waterlogged arable fields were more strongly affected by soil properties than by high CO2 concentration. In contrast, the high CO2 level significantly decreased CO2 production accompanying tested the CH4-related processes.

Effects of artificial nitrogen addition and reduction in precipitation on soil CO2 and CH4 effluxes and composition of the microbial biomass in a temperate forest

AbstractTo understand the effects of artificially increased atmospheric nitrogen (N) and decreased precipitation on the soil microbial community and soil CO2 and CH4 efflux, a two‐factorial field experiment was carried out with the following treatments: (a) control, (b) N addition (+50 kg N ha−1 year−1), (c) decreased precipitation (−30% of throughfall) and (d) combined N addition and decreased precipitation (+50 kg N ha−1 year−1, −30% of throughfall). The N addition and decreased precipitation treatments had a positive effect on mean CO2 efflux (2.61 μmol CO2 m−2 s−1 for control, 4.26 μmol CO2 m−2 s−1 for N addition, 3.62 μmol CO2 m−2 s−1 for reduction in precipitation) and mean rates of CH4 uptake (0.00088 μmol CH4 m−2 s−1 for control, 0.00154 μmol CH4 m−2 s−1 for N addition, 0.00151 μmol m−2 s−1 for reduction in precipitation) during the growing season. The combined N addition and reduction in precipitation also led to increased soil CO2 efflux and CH4 uptake, but the combined response was weaker than the response to single variable treatments (3.10 μmol CO2 m−2 s−1 and 0.00130 μmol CH4 m−2 s−1 interaction), indicating antagonistic effects of the combined treatment. At the seasonal scale, changes in soil CO2 and CH4 efflux were complex, especially in the spring freezing and thawing period, during which pulses of CO2 efflux and CH4 uptake occurred. Decreased precipitation altered the composition of the bacterial community in the winter and growing seasons, and composition of the microbial community also showed a seasonal change. However, changes in microbial community structure under N addition, precipitation reduction and their interaction might be important factors leading to an increase or decrease in soil carbon efflux.Highlights How global changes (N addition and decreased precipitation) affect soil C flux. Seasonal effects of global changes on soil C flux were analysed quantitatively. Decreased precipitation and N‐addition increased soil CO2 flux and decreased soil CH4 flux. Global changes regulate soil C flux by altering soil water, microbial and nutrient status.

Moderate grazing increases the abundance of soil methane-oxidizing bacteria and CH4 uptake rate in a typical steppe of Inner Mongolia, China.

Microbial oxidation is the only biological sink of atmospheric methane (CH4). It is essential to understand the variation of CH4 fluxes among different grassland use types for developing low-emission management system. Here, we measured the CH4 flux and the soil methane-oxidizing bacteria abundance in a typical steppe under grazing, mowing and fencing management in central Inner Mongolia, with the aims to determine the effects of these grassland use types on CH4 flux, and to test the hypothesis that pmoA functional gene abundance regulates CH4 fluxes. The measurements were conducted on the experimental grassland that had experienced four grassland use treatments over five years. The treatments were whole growing season grazing from May to September (T1), spring and summer grazing (twice in May and July)(T2), autumn mowing (T3) and enclosure (T0). We measured CH4 flux using static chamber method, and quantified the abundance of pmoA functional genes using molecular techniques. Moreover, we measured plant biomass and soil physicochemical properties. The results showed that moderate grazing significantly enhanced CH4 uptake rate and the methane-oxidizing bacteria abundance (i.e., the pmoA gene copy number per gram of dry soil). The pmoA gene copy number ranged from 6.9×104 to 3.9×105 per gram of dry soil in growing season. The CH4 uptake rate was (68.21±3.01) μg·m-2·h-1 under T1, which was 22.1%, 37.5% and 30.9% higher than that under T2, T3 or T0 , respectively. The CH4 uptake rate was positively correlated with abundance of CH4 oxidizing bacteria and soil sand content, but negatively correlated with soil silt content, soil moisture, NH4+-N and NO3--N content, and plant biomass. These results suggested that the steppe ecosystem is a CH4 sink under all land-use types in central Inner Mongolia, and that moderate grazing would enhance methane-oxidizing bacteria abundance and CH4 uptake by improving soil sand content, reducing soil mineral nitrogen content and plant production in the typical steppe ecosystem. These results were of significance for the development of low-emission grassland management system.

Soil net methane uptake rates in response to short-term litter input change in a coniferous forest ecosystem of central China

The uptake of CH4 by well-drained soil plays a vital role in mitigating the atmospheric CH4, but the impacts of shift in plant litter input on uptake of CH4 and the underlying mechanism are not fully understood. Here, we conducted in situ measurements of soil CH4 flux rates monthly throughout the year after the short-term litter input manipulations (i.e. Detritus Input and Removal Treatment-DIRT: control, CK; double litter, DL; no litter, NL; no roots, NR; and no aboveground litter and no roots, NRNL) in a coniferous forest (Platycladus orientalis (Linn.) Franco) ecosystem in subtropical China. The associated microclimates, soil properties and microbial PLFAs were also measured. Our results showed that soils acted as CH4 sink in all litter manipulation treatments, and the CH4 sink capacity significantly differed under litter manipulation treatments. Based on annual average values, net CH4 uptake rates decreased by 37.7 ± 4.9% and 41.7 ± 5.8% in the NL and NRNL treatments (i.e. litter layer removal), respectively, compared to the CK treatment. Thus, the net CH4 uptake induced by litter layer approximately accounted for 37.7 ± 4.9% of the total net CH4 uptake rate. The net CH4 uptake rate was not significantly influenced by the root exclusion (NR) treatment. In contrast, the effect of litter addition on net CH4 uptake rate was strongly depended on soil water content. During the dry season, litter addition did not significantly affect net CH4 uptake rate. In contrast, during the wet season, the net CH4 uptake rate decreased by 47.1 ± 4.9% in the DL treatment compared to the CK treatment. There was no significant difference in net CH4 uptake rate between dry and wet season under other litter input manipulation treatments. The net CH4 uptake rate was positively correlated with the abundance of methanotrophic bacteria across all litter input manipulation treatments, whereas the significant negative relationship between net CH4 uptake rate and water filled pore space (WFPS) was only found in the DL treatment. Overall, our results suggest that aboveground organic layer (i.e. litter) is more important in regulating the soils acting as atmospheric CH4 sink than roots, while the regulating function primarily depends on soil dry/wet conditions and the abundance of methanotrophic bacteria.

Soil carbon dioxide and methane fluxes from forests and other land use types in an African tropical montane region

In the last 40 years, large areas of the Mau forest, the largest contiguous tropical montane forest in East Africa, have been cleared for agriculture. To date, there are no empirical data on how this land use change affects carbon dioxide (CO2) fluxes from soil respiration and soil methane (CH4) fluxes. This study reports measured annual soil CO2 and CH4 fluxes from the native Mau forest and previously forested lands converted to smallholder grazing land, smallholder and commercial tea plantations and eucalyptus plantations. Fluxes were measured weekly from August 2015 to August 2016 using the static chamber method. Grazing lands had the highest (p = 0.028) cumulative respiratory CO2 fluxes (25.6 ± 2.9 Mg CO2–C ha−1 year−1), whereas lowest fluxes were observed in commercial tea plantations (5.6 ± 0.5 Mg CO2–C ha−1 year−1). Soil respiratory CO2 fluxes were positively correlated with soil pH, but negatively correlated with soil C:N ratio. Annual soil fluxes were explained by soil pH, bulk density and the interaction between soil pH and C:N ratio. Most soils were sinks for atmospheric CH4 across all land use types. Methane uptake was highest for native forest sites (− 3.08 ± 0.35 to − 5.84 ± 0.61 kg CH4–C ha−1 year−1) and for eucalyptus plantations (− 3.43 ± 0.19 kg CH4–C ha−1 year−1). Uptake decreased significantly with increasing land use intensity (smallholder tea plantations: − 1.42 ± 0.09 kg CH4–C ha−1 year−1, commercial tea plantations: − 1.44 ± 0.29 kg CH4–C ha−1 year−1). Soils of smallholder grazing lands had the lowest CH4 uptake rates (− 0.36 ± 0.25 kg CH4–C ha−1 year−1). Annual CH4 uptake was negatively correlated with mean annual soil water-filled pore space (p < 0.01) and bulk density (p = 0.003) and decreased with increasing soil inorganic NH4+ concentrations (p = 0.03). Annual soil CH4 can be explained by mainly soil water content and bulk density and these factors are related to gas diffusion. Our study shows that converting tropical montane forests to managed land use types affects soil CO2 and CH4 fluxes. Specifically, the CH4 sink strength in managed land use types of these montane tropical soils was reduced to less than half of the sink strength in the native forest. Soil respiratory CO2 fluxes were also altered by land use with grazing lands emitting 3–4 times more CO2 than the other land use types.

Afforestation and deforestation enhanced soil CH4 uptake in a subtropical agricultural landscape: Evidence from multi-year and multi-site field experiments

The impact of afforestation and deforestation on the carbon cycle and carbon sequestration in agricultural landscape has been well studied, while the direction and magnitude of the effects on soil CH4 fluxes remain uncertain in particular in the subtropical region. Thus, multi-site and multi-year field experiments were conducted to measure soil CH4 fluxes from an afforestation chronosequence (cropland [wheat-maize rotation], 15-year old forest, 20-year old forest and 30-year forest) and a deforestation chronosequence (secondary forest, grassland, cropland without fertilization and cropland with fertilization [wheat-maize rotation]) in a subtropical agricultural landscape from 2012 to 2017. The soil at all land uses functioned exclusively as a sink for atmospheric CH4 through the whole experimental years. Soil CH4 uptakes showed great seasonal and inter-annual variations along with those of temporal patterns of soil environmental variables. At the afforestation chronosequence, annual CH4 uptake rates averaged 1.37, 1.68, 1.80 and 2.97 kg C ha−1 yr−1 for cropland, 15-year old forest, 20-year old forest and 30-year old forest. Compared to cropland, afforestation increased annual CH4 uptake by 23 to 117%. Soil CH4 uptake decreased with increasing soil content, soil NH4+ content and soil NO3− content but increased with increasing soil DOC content at the afforestation chronosequence (P < 0.05). At the deforestation chronosequence, annual CH4 uptake rates were 1.37, 1.70, 1.77 and 2.01 kg C ha−1 yr−1 for secondary forest, grassland, cropland without fertilization and cropland with fertilization. Compared to secondary forest, deforestation increased annual CH4 uptake by 24 to 47%. Soil CH4 uptakes were negatively correlated with soil water content and positively correlated with soil NO3− content. We conclude that both afforestation and deforestation have the potential to increase the sink capacities of atmospheric CH4 in the subtropical agricultural landscape and consequently provide the negative feedbacks to climate system.