CH4 Sink Research Articles

Methane-Oxidizing Communities in Lichen-Dominated Forested Tundra Are Composed Exclusively of High-Affinity USCα Methanotrophs.

Upland soils of tundra function as a constant sink for atmospheric CH4 but the identity of methane oxidizers in these soils remains poorly understood. Methane uptake rates of −0.4 to −0.6 mg CH4-C m−2 day−1 were determined by the static chamber method in a mildly acidic upland soil of the lichen-dominated forested tundra, North Siberia, Russia. The maximal CH4 oxidation activity was localized in an organic surface soil layer underlying the lichen cover. Molecular identification of methanotrophic bacteria based on retrieval of the pmoA gene revealed Upland Soil Cluster Alpha (USCα) as the only detectable methanotroph group. Quantification of these pmoA gene fragments by means of specific qPCR assay detected ~107 pmoA gene copies g−1 dry soil. The pmoA diversity was represented by seven closely related phylotypes; the most abundant phylotype displayed 97.5% identity to pmoA of Candidatus Methyloaffinis lahnbergensis. Further analysis of prokaryote diversity in this soil did not reveal 16S rRNA gene fragments from well-studied methanotrophs of the order Methylococcales and the family Methylocystaceae. The largest group of reads (~4% of all bacterial 16S rRNA gene fragments) that could potentially belong to methanotrophs was classified as uncultivated Beijerinckiaceae bacteria. These reads displayed 96–100 and 95–98% sequence similarity to 16S rRNA gene of Candidatus Methyloaffinis lahnbergensis and “Methylocapsa gorgona” MG08, respectively, and were represented by eight species-level operational taxonomic units (OTUs), two of which were highly abundant. These identification results characterize subarctic upland soils, which are exposed to atmospheric methane concentrations only, as a unique habitat colonized mostly by USCα methanotrophs.

Comparison analysis of global methane concentration derived from SCIAMACHY, AIRS, and GOSAT with surface station measurements

ABSTRACT CH4 (methane) is an important greenhouse gas that has a significant impact on the formation of, and change in, the global climate. Through the development of remote sensing technology, a series of remote sensing detectors to monitor CH4 sources and sinks have been launched into space. Although a series of studies have been conducted around the CH4 concentration retrieved by AIRS (The Atmospheric Infrared Sounder instrument), SCIAMACHY (The Scanning Imaging Absorption spectrometer for Atmospheric Chartography instrument), and GOSAT (The Greenhouse Gases Observing Satellite), there have been few comprehensive comparative studies between these three satellite results and ground-based data. In this article, the correlation coefficient, root-mean-square deviation (RMSD), and bias are used to evaluate the CH4 retrieved from satellite data. The results reveal that: AIRS can reflect the distribution and changes of CH4 all over the world. The space coverage of GOSAT is focused between 60°S to 60°N, and the correlation of GOSAT with land surface stations is generally better than that with ocean surface stations. The space coverage of SCIAMACHY is between GOSAT and AIRS. Though SCIAMACHY data show a poor consistency with surface measurements, SCIAMACHY can also reflect the spatial and temporal dynamic distribution of CH4 around the world to a certain extent.

Greenhouse gas emissions from stormwater bioretention basins

Bioretention basins are frequently subjected to anaerobic conditions, which can create an optimum environment for microbial activities to remove nitrogen (N) and sequester carbon (C) in the below-ground filter media. However, these biological processes are associated with the potential production of greenhouse gas (GHG) emissions that need to be measured. In this study, we quantified nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) fluxes from the soil under a transition period from dry to wet conditions in subtropical Australia. The GHG fluxes were measured on the bed of slow (wet basin) and fast (dry basin) draining basins and their embankment area. In addition, the influence of Carex appressa plant on emissions was investigated. Finally, the denitrification potential of the basin soil and their C and N accumulation (over an 18-month interval) were measured. The dry and the embankment soils were both slight sinks of CH4 (−16 and − 2 μg CH4 − C m−2 h−1) while being a high source of CO2 (> 520 × 103 μg CO2 − C m−2 h−1). In comparison, the wet basin was a source of CO2 and CH4 with a mean value of 123× 103 μg CO2 − C m−2 h−1 and 2405 μg CH4 − C m−2 h−1, respectively. The dry and wet basins were a slight source of N2O emissions and were positively driven by precipitation. The presence of C. appressa plant increased CH4 consumption and N2O generation. The results suggest that adopting a slow-draining design for bioretention systems, such as lowering the hydraulic conductivity and/or provision of a saturation zone higher in the soil profile, can reduce CO2 and N2O fluxes from the soils and potentially improve water quality performance of these basins. However, an increase in CH4 fluxes should be the expected by-product.

The effect of land-use systems on greenhouse gas production and crop yields in Wakiso District, Uganda

Land-use systems are among the leading sources of anthropogenic GHG into the atmosphere; driving global warming, climate change, and associated extreme weather events. To estimate GHGs from the land-use systems, we measured fluxes (positive/negative emissions per unit of time) of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) from the cropping and adjacent non-cropping systems in Wakiso District, Uganda. The study comprised of 9 treatments with 2 inter-cropping systems (banana-coffee and maize-beans), 4 sole-cropped systems (banana, maize, beans, and sweet potato), and 3 non-cropping systems (fallow lands, pasture lands, and grasslands). Four management practices: mulching with no-tillage (mulched-NT); non-mulching with reduced-tillage (non-mulched-RT); non-mulching with deep-tillage (non-mulched-DT); compositing with reduced-tillage (compost-RT) were embedded in the cropping systems. The study also assessed the effect of the cropping systems and their management practices on crop yields. The GHG fluxes were monitored using the static chamber technique and analysed using a gas chromatography (G.C.). The results showed that all the land-use systems were sources of CO2 flux with sole-cropped sweet potato under non-mulched-DT (148.12 ± 3.4 μg C m−2 h−1) as the most substantial source, and the grasslands (33.03 ± 1.7 μg C m−2 h−1) as the weakest source. Similarly, all the land-use systems were sources of N2O flux with sole-cropped beans under non-mulched-RT (14.21 ± 0.4 μg N m−2 h−1) as the strongest source, and grasslands (3.43 ± 0.4 μg N m−2 h−1) as the weakest source. The management practices were sinks (any system that absorbs more GHGs than it releases) of CH4 except mulched-NT in the maize-beans intercrop, sole-cropped beans, and sole-cropped maize. The sole-cropped sweet potato under mulched-RT (0.3 ± 0.01 μg C m−2 h−1) was the strongest source of CH4, while the grasslands (−0.67 ± 0.01 μg C m−2 h−1) were the strongest sink. The intercropped systems significantly reduced GHG fluxes and had a yield advantage relative to the sole-cropped systems. Our results suggested that intercropping in central Uganda could be a more sustainable and eco-friendly approach concerning GHG fluxes.

Nitrous oxide and methane emissions from beef cattle excreta deposited on feedlot pen surface in tropical conditions

As feedlot beef production systems are expanding into tropical climates, considerably less is known on their environmental impact specific to these regions. Our objective was to investigate the dynamics of nitrous oxide (N2O) and methane (CH4) fluxes in a feedlot system representative of Brazil, in order to aggregate new emission data for the characterization of intensive cattle raising in tropical regions. Excreta was obtained from Nellore steers in a feedlot (n = 25, BW = 393 ± 31 kg). Urine (1.3 L) and feces (1.3 kg) were applied separately in plots on an open-lot feedlot pen surface at the beginning of the trial and fluxes were monitored 92 d with static chambers. Although CH4 fluxes were variable, all treatments were a sink for CH4 as the average of the monitoring period (−8.9, −3.1, and −15.4 μg C m−2 h−1 for feces, urine, and control, respectively). The N2O fluxes were higher in the urine affected area, which was related to the higher soil moisture and mineral N availability in comparison to the area with feces. The occurrence of rainfall from 67 to 70 DAA was determinant of very high N2O fluxes either for urine or feces. Hence, GHG emissions from individual excreta were characterized by a period of small but significant fluxes, followed by a period of indistinguishable fluxes at the background level. A third period after rainfall portrayed the large impact of excreta on GHG emissions from the feedlot. The direct N2O emission factor (EF) for the N in urine was significantly higher than for the N in feces (2.83 vs. 0.32%, respectively, P < 0.0001), and combining both resulted in an EF for the N in excreta of 1.83%, which is 8.5% less than IPCC default EF for dry lot systems. From the calculated direct emissions of N2O, gas monitoring of urine and feces in separate should be also considered for the development of emission factors for inventory purposes in tropical environments. For the regions where feedlots are used in the dry season for finishing cattle, cleaning pens regularly can contribute substantially to mitigate N2O emissions as suggested by the results of the present study.

Divergent drivers of the microbial methane sink in temperate forest and grassland soils.

Aerated topsoils are important sinks for atmospheric methane (CH4 ) via oxidation by CH4 -oxidizing bacteria (MOB). However, intensified management of grasslands and forests may reduce the CH4 sink capacity of soils. We investigated the influence of grassland land-use intensity (150 sites) and forest management type (149 sites) on potential atmospheric CH4 oxidation rates (PMORs) and the abundance and diversity of MOB (with qPCR) in topsoils of three temperate regions in Germany. PMORs measurements in microcosms under defined conditions yielded approximately twice as much CH4 oxidation in forest than in grassland soils. High land-use intensity of grasslands had a negative effect on PMORs (-40%) in almost all regions and fertilization was the predominant factor of grassland land-use intensity leading to PMOR reduction by 20%. In contrast, forest management did not affect PMORs in forest soils. Upland soil cluster (USC)-α was the dominant group of MOBs in the forests. In contrast, USC-γ was absent in more than half of the forest soils but present in almost all grassland soils. USC-α abundance had a direct positive effect on PMOR in forest, while in grasslands USC-α and USC-γ abundance affected PMOR positively with a more pronounced contribution of USC-γ than USC-α. Soil bulk density negatively influenced PMOR in both forests and grasslands. We further found that the response of the PMORs to pH, soil texture, soil water holding capacity and organic carbon and nitrogen content differ between temperate forest and grassland soils. pH had no direct effects on PMOR, but indirect ones via the MOB abundances, showing a negative effect on USC-α, and a positive on USC-γ abundance. We conclude that reduction in grassland land-use intensity and afforestation has the potential to increase the CH4 sink function of soils and that different parameters determine the microbial methane sink in forest and grassland soils.

Oceanic and atmospheric methane cycling in the cGENIE Earth system model – release v0.9.14

Abstract. The methane (CH4) cycle is a key component of the Earth system that links planetary climate, biological metabolism, and the global biogeochemical cycles of carbon, oxygen, sulfur, and hydrogen. However, currently lacking is a numerical model capable of simulating a diversity of environments in the ocean, where CH4 can be produced and destroyed, and with the flexibility to be able to explore not only relatively recent perturbations to Earth's CH4 cycle but also to probe CH4 cycling and associated climate impacts under the very low-O2 conditions characteristic of most of Earth's history and likely widespread on other Earth-like planets. Here, we present a refinement and expansion of the ocean–atmosphere CH4 cycle in the intermediate-complexity Earth system model cGENIE, including parameterized atmospheric O2–O3–CH4 photochemistry and schemes for microbial methanogenesis, aerobic methanotrophy, and anaerobic oxidation of methane (AOM). We describe the model framework, compare model parameterizations against modern observations, and illustrate the flexibility of the model through a series of example simulations. Though we make no attempt to rigorously tune default model parameters, we find that simulated atmospheric CH4 levels and marine dissolved CH4 distributions are generally in good agreement with empirical constraints for the modern and recent Earth. Finally, we illustrate the model's utility in understanding the time-dependent behavior of the CH4 cycle resulting from transient carbon injection into the atmosphere, and we present model ensembles that examine the effects of atmospheric pO2, oceanic dissolved SO42-, and the thermodynamics of microbial metabolism on steady-state atmospheric CH4 abundance. Future model developments will address the sources and sinks of CH4 associated with the terrestrial biosphere and marine CH4 gas hydrates, both of which will be essential for comprehensive treatment of Earth's CH4 cycle during geologically recent time periods.

Resilience of methane cycle and microbial functional genes to drought and flood in an alkaline wetland: A metagenomic analysis

Alkaline wetlands distributed in arid or semi-arid areas are hotspots of methane (CH4) emissions. Periods of drought and flood, although regular, are stressful events encountered by methanogenic anaerobes in alkaline wetlands. To investigate the response of the CH4 cycle of alkaline wetlands to such stresses, we take Zhalong wetland as an example, then determined the CH4 flux and soil microbiomes in the wetland during wet, dry, and flooded periods. The in-situ CH4 flux in the wet period was 9.55–17.29 mg‧m−2‧h−1, but sharply degraded to 3.37–6.61 mg‧m−2‧h−1 in the dry period. It resumed to 4.51–20.80 mg‧m−2‧h−1 when the wetland was flooded again, which indicated that methanogenesis is quite resilient to drought. Syntrophic acetogenesis, and subsequently aceticlastic methanogenesis, were the dominant methanogenic pathways and resisted drought. Members belonging to Syntrophobacterales were the dominant syntrophic acetogens. They enter a viable but non-culturable (VBNC) state to resist drought. The dominant Methanosarcinales have the ability to repair reactive oxygen species damage during dry periods. The community of CH4 sink was governed by anaerobic methanotrophs, which entered a VBNC state or used repair systems to survive dry periods. This study revealed the responses of the CH4 cycle and microbial functional genes to drought and flood in alkaline wetlands.

Trees as net sinks for methane (CH4 ) and nitrous oxide (N2 O) in the lowland tropical rain forest on volcanic Réunion Island.

Summary Trees are known to emit methane (CH4) and nitrous oxide (N2O), with tropical wetland trees being considerable CH4 sources. Little is known about CH4 and especially N2O exchange of trees growing in tropical rain forests under nonflooded conditions.We determined CH4 and N2O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400‐yr‐old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows.The stems studied were net sinks for atmospheric CH4 and N2O, as were cryptogams, which seemed to be co‐responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH4. Their N2O fluxes were negligible.Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH4 and N2O. The volcanic tropical lowland rain forest appears to be an important CH4 sink, as well as a possible N2O sink.

Microbial Activity in Subterranean Ecosystems: Recent Advances

Of the several critical challenges present in environmental microbiology today, one is the assessment of the contribution of microorganisms in the carbon cycle in the Earth-climate system. Karstic subterranean ecosystems have been overlooked until recently. Covering up to 25% of the land surface and acting as a rapid CH4 sink and alternately as a CO2 source or sink, karstic subterranean ecosystems play a decisive role in the carbon cycle in terms of their contribution to the global balance of greenhouse gases. Recent data indicate that microbiota must play a significant ecological role in the biogeochemical processes that control the composition of the subterranean atmosphere, as well as in the availability of nutrients for the ecosystem. Nevertheless, there are still essential gaps in our knowledge concerning the budgets of greenhouse gases at the ecosystem scale and the possible feedback mechanisms between environmental-microclimatic conditions and the rates and type of activity of microbial communities in subterranean ecosystems. Another challenge is searching for bioactive compounds (antibiotics) used for treating human diseases. At present, there is a global health emergency and a strong need for novel biomolecules. In recent decades, great research efforts have been made to extract antibiotics from marine organisms. More recently, caves have been receiving considerable attention in search of novel antibiotics. Cave methanotrophic and heterotrophic bacteria are producers of bioactive compounds and may be potential sources of metabolites with antibacterial, antifungal or anticancer activities of interest in pharmacological and medical research, as well as enzymes with a further biotechnological use. Here we also show that bacteria isolated from mines, a still unexplored niche for scientists in search of novel compounds, can be a source of novel secondary metabolites.

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.

How does replacing natural forests with rubber and oil palm plantations affect soil respiration and methane fluxes?

AbstractReplacement of forest by agricultural systems is a major factor accelerating the emissions of greenhouse gases; however, related field studies in the tropics are very scarce. To evaluate the impact of forest transition to plantations on soil methane (CH4) and respiration (CO2) fluxes, we conducted measurements in an undisturbed forest, a disturbed forest, young and old rubber plantations, and an oil palm plantation on mineral soil in Jambi, Sumatra, Indonesia. Methane fluxes and their controlling variables were monitored monthly over fourteen months; soil respiration was measured less frequently. All of the plantations were managed by smallholders and had never been fertilized. To assess the effect of common management practices in oil palm plantations, we added urea at a rate of 33.3 kg N/ha and thereafter monitored intensively soil CH4 fluxes. The soil acted as a sink for CH4 (kg CH4‐C·ha−1·yr−1) in the undisturbed forest (−1.4 ± 1.0) and young rubber plantation (−1.7 ± 0.7). This was not the case in the other land‐use systems which had fluxes similar to fluxes in the undisturbed forest, with 0.4 ± 0.9, −0.2 ± 0.3, and 0.2 ± 0.7 kg·ha−1·yr−1 in the disturbed forest, old rubber plantation, and oil palm plantation, respectively. In the oil palm plantation, there was no inhibitory effect of nitrogenous fertilizer on methanotrophy. Annual soil respiration (Mg CO2‐C·ha−1·yr−1) was higher in the oil palm plantation (17.1 ± 1.9) than in the undisturbed forest (13.9 ± 1.2) while other land‐use systems respired at a similar level to the undisturbed forest (13.1 ± 1.4, 15.9 ± 1.7, and 14.1 ± 1.0 in the disturbed forest, young, and old rubber plantations, respectively). Substrate (litterfall and soil) availability and quality exerted a strong control over annual fluxes of both gases along the land‐use gradient. Temporal variation in CH4 was extremely high and in respiration fluxes was moderate, but was not specifically linked to seasonal variation. Further comprehensive and long‐term research is critically needed to determine more thoroughly the direction and magnitude of changes in soil trace gas emissions as affected by forest‐to‐plantation conversion in the tropics.

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.

Interactive effects of dung deposited onto urine patches on greenhouse gas fluxes from tropical pastures in Kenya

Dung and urine patches on grasslands are hotspots of greenhouse gas (GHG) emissions in temperate regions, while its importance remains controversial for tropical regions as emissions seem to be lower. Here we investigated N2O, CH4 and CO2 emissions from urine and dung patches on tropical pastures in Kenya, thereby disentangling interactive and pure water, dung or urine effects. GHG fluxes were monitored with automated chambers for 42–59 days covering three seasons (short rainy season, long rainy season, dry season) for six treatments (Control; +1 L water; +1 kg dung; 1 L urine; 1 L water +1 kg dung; 1 L urine +1 kg dung). Cumulative CO2 emissions did not differ among treatments in any of the seasons. Water or urine addition alone did not affect CH4 fluxes, but these were elevated in all dung-related treatments. Scaled up on the total area covered, dung patches halve the CH4 sink strength of tropical pastures during the dry season, while during the rainy season they may turn tropical pastures into a small CH4 source. For N2O, both dung and urine alone and in combination stimulated emissions. While the N2O emission factor (EFN2O) from dung being constant across seasons, the EFN2O for urine was greater during the short rainy season than during the dry season. Combined application of urine + dung was additive on EFN2O. While the mean dung EFN2O in our study (0.06%) was similar to the IPCC Guidelines for National GHG Inventories EFN2O for dry climate (0.07%), the urine EFN2O we measured (0.03–0.25%) was lower than the IPCC value (0.32%). In addition, the IPCC Guidelines assume a urine-N: dung-N ratio of 0.66:0.34, which is higher than found for SSA (<0.50:0.50). Consequently, IPCC Guidelines still overestimate N2O emissions from excreta patches in SSA.

Comparison of Soil Greenhouse Gas Fluxes during the Spring Freeze–Thaw Period and the Growing Season in a Temperate Broadleaved Korean Pine Forest, Changbai Mountains, China

Soils in mid-high latitudes are under the great impact of freeze–thaw cycling. However, insufficient research on soil CO2, CH4, and N2O fluxes during the spring freeze–thaw (SFT) period has led to great uncertainties in estimating soil greenhouse gas (GHG) fluxes. The present study was conducted in a temperate broad-leaved Korean pine mixed forest in Northeastern China, where soils experience an apparent freeze–thaw effect in spring. The temporal variations and impact factors of soil GHG fluxes were measured during the SFT period and growing season (GS) using the static-chamber method. The results show that the soil acted as a source of atmospheric CO2 and N2O and a sink of atmospheric CH4 during the whole observation period. Soil CO2 emission and CH4 uptake were lower during the SFT period than those during the GS, whereas N2O emissions were more than six times higher during the SFT period than that during the GS. The responses of soil GHG fluxes to soil temperature (Ts) and soil moisture during the SFT and GS periods differed. During the SFT period, soil CO2 and CH4 fluxes were mainly affected by the volumetric water content (VWC) and Ts, respectively, whereas soil N2O flux was influenced jointly by Ts and VWC. The dominant controlling factor for CO2 was Ts during the GS, whereas CH4 and N2O were mainly regulated by VWC. Soil CO2 and N2O fluxes accounted for 97.3% and 3.1% of the total 100-year global warming potential (GWP100) respectively, with CH4 flux offsetting 0.4% of the total GWP100. The results highlight the importance of environmental variations to soil N2O pulse during the SFT period and the difference of soil GHG fluxes between the SFT and GS periods, which contribute to predicting the forest soil GHG fluxes and their global warming potential under global climate change.

Inorganic nitrogen fertilizer and high N application rate promote N2O emission and suppress CH4 uptake in a rotational vegetable system

Understanding the influence of management practices on greenhouse gas (GHG) emissions from greenhouse cropping is of great importance for assessing the environmental impacts of the greenhouse cropping industry and improving its sustainability. A tomato–cucumber–tomato rotation experiment was carried out in a typical solar greenhouse in northwest China with four treatments including two irrigation levels (regular (RI) and low (LI)) and two fertilizer types (organic fertilizer (N1) and inorganic fertilizer (N2)). Greenhouse gas fluxes (nitrous oxide, N2O, and methane, CH4) were measured regularly using the closed chamber method during the three growing seasons, along with soil water-filled pore space (WFPS), temperature, mineral-N concentration and nitrobacteria, nitrosomonas and denitrifying bacteria abundance. Over the rotation, the soil acted as a source for N2O and a sink for CH4, with the mean fluxes of 0.12 mg N2O-N m−2 h−1 and -0.31 mg CH4-C m−2 h−1, respectively. The stepwise multiple linear regressions indicated that WFPS and soil temperature accounted for significant portion of N2O emission and CH4 uptake variations, respectively for both fertilizer types. Fertilization rate and type resulted in much greater difference of cumulative GHG emission between treatments than the irrigation level. Inorganic fertilizer with higher nitrogen application rate usually resulted in higher cumulative N2O emission and lower CH4 uptake than organic fertilizer application. Over the rotation, total greenhouse emission (GHGt) and greenhouse emission intensity (GHGI) on average followed the same order of RIN2 > LIN2 > LIN1 > RIN1 with N2O emission as the dominant component for each treatment. Overall, organic fertilizer with proper water application under drip irrigation can effectively mitigate greenhouse gas emissions and maintain relatively high and stable vegetable yields in solar greenhouse cropping in northwest China.

Patterns and isotopic composition of greenhouse gases under ice in lakes of interior Alaska

Arctic and boreal lake greenhouse gas emissions (GHG) are an important component of regional carbon (C) budgets. Yet the magnitude and seasonal patterns of lake GHG emissions are poorly constrained, because sampling is limited in these remote landscapes, particularly during winter and shoulder seasons. To better define patterns of under ice GHG content (and emissions potential at spring thaw), we surveyed carbon dioxide (CO2) and methane (CH4) concentrations and stable isotopic composition during winter of 2017 in 13 lakes in the arid Yukon Flats Basin of interior Alaska, USA. Partial pressures of CO2 and CH4 ranged over three orders of magnitude, were positively correlated, and CO2 exceeded CH4 at all but one site. Shallow, organic matter-rich lakes located at lower elevations tended to have the highest concentrations of both gases, though CH4 content was more heterogeneous and only abundant in oxygen-depleted lakes, while CO2 was negatively correlated to oxygen content. Isotopic values of CO2 spanned a narrow range (−10‰ to −23‰) compared to CH4, which ranged over 50‰ (−19‰ to −71‰), indicating CH4 source pathways and sink strength varied widely between lakes. Miller-Tans and Keeling plots qualitatively suggested two groups of lakes were present; one with isotopically enriched source CH4 possibly more dominated by acetoclastic methanogenesis, and one with depleted signatures suggesting a dominance of the hydrogenotrophic production. Overall, regional lake differences in winter under ice GHG content appear to track landscape position, oxygen, and organic matter content and composition, causing patterns to vary widely even within a relatively small geographic area of interior Alaska.

Belowground changes to community structure alter methane-cycling dynamics in Amazonia

Amazonian rainforest is undergoing increasing rates of deforestation, driven primarily by cattle pasture expansion. Forest-to-pasture conversion has been associated with increases in soil methane (CH4) emission. To better understand the drivers of this change, we measured soil CH4 flux, environmental conditions, and belowground microbial community structure across primary forests, cattle pastures, and secondary forests in two Amazonian regions. We show that pasture soils emit high levels of CH4 (mean: 3454.6 ± 9482.3 μg CH4 m−2 d−1), consistent with previous reports, while forest soils on average emit CH4 at modest rates (mean: 9.8 ± 120.5 μg CH4 m−2 d−1), but often act as CH4 sinks. We report that secondary forest soils tend to consume CH4 (mean: −10.2 ± 35.7 μg CH4 m−2 d−1), demonstrating that pasture CH4 emissions can be reversed. We apply a novel computational approach to identify microbial community attributes associated with flux independent of soil chemistry. While this revealed taxa known to produce or consume CH4 directly (i.e. methanogens and methanotrophs, respectively), the vast majority of identified taxa are not known to cycle CH4. Each land use type had a unique subset of taxa associated with CH4 flux, suggesting that land use change alters CH4 cycling through shifts in microbial community composition. Taken together, we show that microbial composition is crucial for understanding the observed CH4 dynamics and that microorganisms provide explanatory power that cannot be captured by environmental variables.

Environmental and anthropogenic drivers of soil methane fluxes in forests: Global patterns and among-biomes differences.

Forest soils are the most important terrestrial sink of atmospheric methane (CH4 ). Climatic, soil and anthropogenic drivers affect CH4 fluxes, but it is poorly known the relative weight of each driver and whether all drivers have similar effects across forest biomes. We compiled a database of 478 in situ estimations of CH4 fluxes in forest soils from 191 peer-reviewed studies. All forest biomes (boreal, temperate, tropical and subtropical) but savannahs act on average as CH4 sinks, which presented positive fluxes in 65% of the sites. Mixed effects models showed that combined climatic and edaphic variables had the best support, but anthropogenic factors did not have a significant effect on CH4 fluxes at global scale. This model explained only 19% of the variance in soil CH4 flux which decreased with declines in precipitation and increases in temperature, and with increases in soil organic carbon, bulk density and soil acidification. The effects of these drivers were inconsistent across biomes, increasing the model explanation of observed variance to 34% when the drivers have a different slope for each biome. Despite this limited explanatory value which could be related to the use of soil variables calculated at coarse scale (~1km), our study shows that soil CH4 fluxes in forests are determined by different environmental variables in different biomes. The most sensitive system to all studied drivers were the temperate forests, while boreal forests were insensitive to climatic variables, but highly sensitive to edaphic factors. Subtropical forests and savannahs responded similarly to climatic variables, but differed in their response to soil factors. Our results suggest that the increase in temperature predicted in the framework of climate change would promote CH4 emission (or reduce CH4 sink) in subtropical and savannah forests, have no influence in boreal and temperate forests and promote uptake in tropical forests.

Long term effects of fire on the soil greenhouse gas balance of an old-growth temperate rainforest

Forest fires can cause great changes in the composition, structure and functioning of forest ecosystems. We studied the effects of a fire that occurred >50 years ago in a temperate rainforest that caused flooding conditions in a Placic Andosol to evaluate how long these effects last; we hypothesized that the effects of fire on the soil greenhouse gas (GHG) balance could last for many years. We made monthly measurements of fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) during two years of soils in an unburned forest (UF) and a nearby site that burned >50 years ago (BS). Our results show that CO2 emissions from soils were higher in the UF than in the BS, and positively correlated with temperature and negatively with soil water content at both sites. Both sites were net CH4 sinks (higher in the UF) and fluxes correlated positively with soil water content and negatively with temperature (stronger relation in the BS). Emissions of N2O were low at both sites and showed correlation with friction velocity at the UF site. The soil GHG balance showed that the UF emitted about 80% more than the BS (5079 ± 1772 and 2815 ± 1447 g CO2-eq m−2 y−1, respectively). Combining our measured fluxes with data of CO2 net ecosystem exchange, we estimated that at the ecosystem level, the UF was a GHG sink while the BS was a source, showing a long-lasting effect of the fire and the importance of preserving these forest ecosystems.