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Wetland soils are the greatest source of nitrous oxide (N 2 O), a critical greenhouse gas and ozone depleter released by microbes. Yet, microbial players and processes underlying the N 2 O emissions from wetland soils are poorly understood. Using in situ N 2 O measurements and by determining the structure and potential functional of microbial communities in 645 wetland soil samples globally, we examined the potential role of archaea, bacteria, and fungi in nitrogen (N) cycling and N 2 O emissions. We show that N 2 O emissions are higher in drained and warm wetland soils, and are correlated with functional diversity of microbes. We further provide evidence that despite their much lower abundance compared to bacteria, nitrifying archaeal abundance is a key factor explaining N 2 O emissions from wetland soils globally. Our data suggest that ongoing global warming and intensifying environmental change may boost archaeal nitrifiers, collectively transforming wetland soils to a greater source of N 2 O. The wetland soil microbiome has a major impact on greenhouse gas emissions. Here the authors characterize how a group of archaea contribute to N 2 O emissions and find that climate and land use changes could promote these organisms.

One of the characteristics of global climate change is the increase in extreme climate events, e.g., droughts and floods. Forest adaptation strategies to extreme climate events are the key to predict ecosystem responses to global change. Severe floods alter the hydrological regime of an ecosystem which influences biochemical processes that control greenhouse gas fluxes. We conducted a flooding experiment in a mature grey alder ( Alnus incana (L.) Moench) forest to understand flux dynamics in the soil-tree-atmosphere continuum related to ecosystem N 2 O and CH 4 turn-over. The gas exchange was determined at adjacent soil-tree-pairs: stem fluxes were measured in vertical profiles using manual static chambers and gas chromatography; soil fluxes were measured with automated chambers connected to a gas analyser. The tree stems and soil surface were net sources of N 2 O and CH 4 during the flooding. Contrary to N 2 O, the increase in CH 4 fluxes delayed in response to flooding. Stem N 2 O fluxes were lower although stem CH 4 emissions were significantly higher than from soil after the flooding. Stem fluxes decreased with stem height. Our flooding experiment indicated soil water and nitrogen content as the main controlling factors of stem and soil N 2 O fluxes. The stems contributed up to 88% of CH 4 emissions to the stem-soil continuum during the investigated period but soil N 2 O fluxes dominated (up to 16 times the stem fluxes) during all periods. Conclusively, stem fluxes of CH 4 and N 2 O are essential elements in forest carbon and nitrogen cycles and must be included in relevant models.

Northern peatlands constitute a significant source of atmospheric methane (CH4). However, management of undisturbed peatlands, as well as the restoration of disturbed peatlands, will alter the exchange of CH4 with the atmosphere. The aim of this systematic review and meta‐analysis was to collate and analyze published studies to improve our understanding of the factors that control CH4 emissions and the impacts of management on the gas flux from northern (latitude 40° to 70°N) peatlands. The analysis includes a total of 87 studies reporting measurements of CH4 emissions taken at 186 sites covering different countries, peatland types, and management systems. Results show that CH4 emissions from natural northern peatlands are highly variable with a 95% CI of 7.6–15.7 g C m−2 year−1 for the mean and 3.3–6.3 g C m−2 year−1 for the median. The overall annual average (mean ± SD) is 12 ± 21 g C m−2 year−1 with the highest emissions from fen ecosystems. Methane emissions from natural peatlands are mainly controlled by water table (WT) depth, plant community composition, and soil pH. Although mean annual air temperature is not a good predictor of CH4 emissions by itself, the interaction between temperature, plant community cover, WT depth, and soil pH is important. According to short‐term forecasts of climate change, these complex interactions will be the main determinant of CH4 emissions from northern peatlands. Drainage significantly (p < .05) reduces CH4 emissions to the atmosphere, on average by 84%. Restoration of drained peatlands by rewetting or vegetation/rewetting increases CH4 emissions on average by 46% compared to the original premanagement CH4 fluxes. However, to fully evaluate the net effect of management practice on the greenhouse gas balance from high latitude peatlands, both net ecosystem exchange (NEE) and carbon exports need to be considered. CH4 emissions from natural northern peatlands are highly variable with a 95% CI of 7.6–15.7 g C m−2 year−1 for the mean and 3.3–6.3 g C m−2 year−1 for the median. Drainage significantly (p < .05) reduces CH4 emissions to the atmosphere, on average by 84%. Methane emissions are mainly controlled by water table (WT) depth, plant community composition, and soil pH. Temperature is not a good predictor of CH4 emissions by itself, but the interaction between temperature, plant community cover, WT depth, and soil pH is important.

The pollution of agricultural soils by the heavy metals affects the productivity of the land and has an impact on the quality of the surrounding ecosystems. This study investigated the bacterial community structure in the heavy metal contaminated sites along a smelter and a distantly located paddy field to elucidate the factors that are related to the alterations of the bacterial communities under the conditions of heavy metal pollution. Among the study sites, the bacterial communities in the soil did not show any significant differences in their richness and diversity. The soil bacterial communities at the three study sites were distinct from one another at each site, possessing a distinct set of bacterial phylotypes. Among the study sites, significant changes were observed in the abundances of the bacterial phyla and genera. The variations in the bacterial community structure were mostly related to the general soil properties at the phylum level, while at the finer taxonomic levels, the concentrations of arsenic (As) and lead (Pb) were the significant factors, affecting the community structure. The relative abundances of the genera Desulfatibacillum and Desulfovirga were negatively correlated to the concentrations of As, Pb, and cadmium (Cd) in the soil, while the genus Bacillus was positively correlated to the concentrations of As and Cd. According to the results of the prediction of bacterial community functions, the soil bacterial communities of the heavy metal polluted sites were characterized by the more abundant enzymes involved in DNA replication and repair, translation, transcription, and the nucleotide metabolism pathways, while the amino acid and lipid metabolism, as well as the biodegradation potential of xenobiotics, were reduced. Our results showed that the adaptation of the bacterial communities to the heavy metal contamination was predominantly attributed to the replacement process, while the changes in community richness were linked to the variations in the soil pH values. Bacterial community structure and metabolic potential was affected by higher concentration of heavy metals mainly Arsenic in paddy fields.

Peatland cloud forests, characterized by high altitude and humidity, are among the least-studied tropical ecosystems despite their significance for endemism and the bioavailable nitrogen (N) that can be emitted as N 2 O. While research has mainly focused on soil, the above-ground microbial N cycle remains largely unexplored. We quantified microbial N cycling genes across ecosystem compartments (soil, canopy soil, tree stems, and leaves) in relation to N 2 O and N 2 fluxes and soil physicochemical properties in two peatland cloud forests and a wetland on Réunion Island. Complete denitrification minimized N 2 O emissions and increased N 2 fluxes in wetland soils. In cloud forest soils, archaeal nitrification primarily produced nitrate (NO 3 – ), while low pH potentially slowed denitrification, resulting in minimal N 2 O emissions. Soil N-fixers were more abundant in Erica reunionensis -dominated forests than in mixed forests. Tree stems varied between weak N 2 O sinks and sources, with fluxes unrelated to gene abundances in stems. High prokaryotic and fungal nirK gene abundance in forest canopy soil suggests potential for above-ground denitrification in wet conditions. nosZ -I genes found in forest canopy soil and leaves ( E. reunionensis , Alsophila glaucifolia , and Typha domingensis ) indicate that plants, including forest canopy, may play a significant role in the reduction of N 2 O.

A high degree of uncertainty persists regarding current and future emissions of methane from both natural and constructed wetlands. Part of the problem is the existence of ‘hot spots’ of methane flux, which have not been clearly identified and studied at multiple scales. Methane has a short lifetime compared to carbon dioxide; thus, efforts to avoid methane hot spots from constructed wetlands can promptly decelerate the rate of atmospheric warming. In this study we measured methane fluxes using flux towers in a restored oligohaline wetland in the Sacramento–San Joaquin River Delta, where we previously identified a hot spot of methane flux using footprint-weighed flux maps and chambers. Our main objectives with this study were to determine why this hot spot occurs and what are the biogeochemical and microbiological conditions that lead to these high methane fluxes. We found four main mechanisms that explain the existence of the hot spot. (1) The hot spot was associated with areas where the water level was closer to the surface (2) Methane originated mostly from older unoxidized peat in deeper layers, which had a shorter migration pathway to the atmosphere at the hot spot location due to soil disturbance during wetland construction. (3) Relatively lower methane oxidation in the hot spot in the upper soil layer (10–30 cm under the surface), deduced from isotopic profiles in porewater carbon and upper-level methanotroph abundance. (4) Higher ebullition events at the hot spot that can be related to low water levels and lower bulk density throughout the soil profile. This study thus suggests that mitigating soil disturbances during wetland construction and managing water level can reduce the occurrence and magnitude of hot spots of methane flux in constructed wetlands.

Earth’s climate is tightly connected to carbon and nitrogen exchange between the atmosphere and ecosystems. Wet peatland ecosystems take up carbon dioxide in plants and accumulate organic carbon in soil but release methane. Man-made drainage releases carbon dioxide and nitrous oxide from peat soils. Carbon and nitrous gas exchange and their relationships with environmental conditions are poorly understood. Here, we show that open peatlands in both their wet and dry extremes are greenhouse gas sinks while peat carbon/nitrogen ratios are high and prokaryotic (bacterial and archaeal) abundances are low. Conversely, peatlands with moderate soil moisture levels emit carbon dioxide and nitrous oxide, while prokaryotic abundances are high. The results challenge the current assumption of a uniform effect of drainage on greenhouse gas emissions and show that the peat microbiome of greenhouse-gas sources differs fundamentally from sinks.

Riparian forests are known as hot spots of nitrogen cycling in landscapes. Climate warming speeds up the cycle. Here we present results from a multi-annual high temporal-frequency study of soil, stem, and ecosystem (eddy covariance) fluxes of N 2 O from a typical riparian forest in Europe. Hot moments (extreme events of N 2 O emission) lasted a quarter of the study period but contributed more than half of soil fluxes. We demonstrate that high soil emissions of N 2 O do not escape the ecosystem but are processed in the canopy. Rapid water content change across intermediate soil moisture was a major determinant of elevated soil emissions in spring. The freeze-thaw period is another hot moment. However, according to the eddy covariance measurements, the riparian forest is a modest source of N 2 O. We propose photochemical reactions and dissolution in canopy-space water as reduction mechanisms.

Cloud forests are unique yet understudied ecosystems regarding CH 4 exchange despite their significance in carbon storage. We investigated CH 4 fluxes in peat soil and tree stems of two tropical cloud forests on Réunion Island, one featuring Erica reunionensis and the second a mix of E. reunionensis and Alsophila glaucifolia . The study examined microbiomes across below-ground (soil) and above-ground (canopy soil, leaves, and stems) forest compartments. Metagenomics and qPCR analyses targeted key genes in methanogenesis and methanotrophy in soil and above-ground samples, alongside soil physicochemical measurements. CH 4 fluxes from peat soil and tree stems were measured using gas chromatography and portable trace gas analyzers. Peat soil in both forests acted as a CH 4 sink (− 23.8 ± 4.84 µg C m − 2 h − 1 ) and CO 2 source (55.5 ± 5.51 µg C m − 2 h − 1 ), with higher CH 4 uptake in sites dominated by endemic tree species E. reunionensis . In forest soils, a high abundance of n-DAMO 16 S rRNA gene (3.42 × 10 7 ± 7 × 10 6 copies/g dw) was associated with nitrate levels and higher rates of CH 4 uptake and CO 2 emissions. NC-10 bacteria (0.1–0.3%) were detected in only the Erica forest soil, verrucomicrobial methanotrophs (0.1–3.1%) only in the mixed forest soil, whereas alphaproteobacterial methanotrophs (0.1–3.3%) were present in all soils. Tree stems in both forests were weak sinks of CH 4 (-0.94 ± 0.4 µg C m − 2 h − 1 ). The canopy soil hosted verrucomicrobial methanotrophs (0.1–0.3%). The leaves in both forests exhibited metabolic potential for CH 4 production, e.g., exhibiting high mcrA copy numbers (3.5 × 10 5 ± 2.3 × 10 5 copies/g dw). However, no CH 4 -cycling functional genes were detected in the stem core samples. Tropical cloud forest peat soils showed high anaerobic methanotrophy via the n-DAMO process, while aerobic methanotrophs were abundant in canopy soils. Leaves hosted methanotrophs but predominantly methanogens. These results highlight the significant differences between canopy and soil microbiomes in the CH 4 cycle, emphasizing the importance of above-ground microbiomes in forest CH 4 gas budgets.

The aim of this study is to estimate wintertime emissions of greenhouse gases CO2, N2O and CH4 in two abandoned peat extraction areas (APEA), Ess-soo and Laiuse, and in two Oxalis site-type drained peatland forests (DPF) on nitrogen-rich sapric histosol, a Norway spruce and a Downy birch forest, located in eastern Estonia. According to the long-term study using a closed chamber method, the APEAs emitted less CO2 and N2O, and more CH4 than the DPFs. Across the study sites, CO2 flux correlated positively with soil, ground and air temperatures. Continuous snow depth > 5 cm did not influence CO2, but at no snow or a thin snow layer the fluxes varied on a large scale (from −1.1 to 106 mg C m−2 h−1). In all sites, the highest N2O fluxes were observed at a water table depth of −30 to −40 cm. CH4 was consumed in the DPFs and was always emitted from the APEAs, whereas the highest flux appeared at a water table >20 cm above the surface. Considering the global warming potential (GWP) of the greenhouse gas emissions from the DPFs in the wintertime, the flux of N2O was the main component of warming, showing 3–6 times higher radiative forcing values than that of CO2 flux, while the role of CH4 was unimportant. In the APEAs, CO2 and CH4 made up almost equal parts, whereas the impact of N2O on GWP was minor.