Sunlight stimulates methane uptake and nitrous oxide emission from the High Arctic tundra

Unique high Arctic methane metabolizing community revealed through in situ 13CH4-DNA-SIP enrichment in concert with genome binning

Greenhouse gas (GHG) emissions from Arctic permafrost soils create a positive feedback loop of climate warming and further GHG emissions. Active methane uptake in these soils can reduce the impact of GHG on future Arctic warming potential. Aerobic methane oxidizers are thought to be responsible for this apparent methane sink, though Arctic representatives of these organisms have resisted culturing efforts. Here, we first used in situ gas flux measurements and qPCR to identify relative methane sink hotspots at a high Arctic cytosol site, we then labeled the active microbiome in situ using DNA Stable Isotope Probing (SIP) with heavy 13CH4 (at 100 ppm and 1000 ppm). This was followed by amplicon and metagenome sequencing to identify active organisms involved in CH4 metabolism in these high Arctic cryosols. Sequencing of 13C-labeled pmoA genes demonstrated that type II methanotrophs (Methylocapsa) were overall the dominant active methane oxidizers in these mineral cryosols, while type I methanotrophs (Methylomarinovum) were only detected in the 100 ppm SIP treatment. From the SIP-13C-labeled DNA, we retrieved nine high to intermediate quality metagenome-assembled genomes (MAGs) belonging to the Proteobacteria, Gemmatimonadetes, and Chloroflexi, with three of these MAGs containing genes associated with methanotrophy. A novel Chloroflexi MAG contained a mmoX gene along with other methane oxidation pathway genes, identifying it as a potential uncultured methane oxidizer. This MAG also contained genes for copper import, synthesis of biopolymers, mercury detoxification, and ammonia uptake, indicating that this bacterium is strongly adapted to conditions in active layer permafrost and providing new insights into methane biogeochemical cycling. In addition, Betaproteobacterial MAGs were also identified as potential cross-feeders with methanotrophs in these Arctic cryosols. Overall, in situ SIP labeling combined with metagenomics and genome binning demonstrated to be a useful tool for discovering and characterizing novel organisms related to specific microbial functions or biogeochemical cycles of interest. Our findings reveal a unique and active Arctic cryosol microbial community potentially involved in CH4 cycling.

N2O emissions from plants are reduced under photosynthetic activity

New plant functions in the exchange of greenhouse gases between ecosystems and atmosphere have recently been discovered. We tested whether photosynthetic activity has an effect on N₂O emission rates from incubated plant-soil systems.Two laboratory experiments were performed. One to unravel possible effect of photosynthetic activity on the net N₂O ecosystem exchange for two species (beech and ash saplings). The other to account for possible effects from rhizosphere and aboveground plant parts separately (ash sapling only).Total N₂O emissions from both plant and plant-soil systems were significantly lower under light than in darkness (31%-65%). The photosynthetic effect only applied to the aboveground plant parts.Underlying processes have now to be unraveled to improve our understanding of ecosystem functioning. This will improve modeling and budgeting of greenhouse gas exchanges between ecosystems and the atmosphere.

Effects of warming on N2O fluxes in a boreal peatland of Permafrost region, Northeast China

Climate warming is expected to increasingly influence boreal peatlands and alter their greenhouse gases emissions. However, the effects of warming on N₂O fluxes and the N₂O budgets were ignored in boreal peatlands. Therefore, in a boreal peatland of permafrost zone in Northeast China, a simulated warming experiment was conducted to investigate the effects of warming on N₂O fluxes in Betula. Fruticosa community (B. Fruticosa) and Ledum. palustre community (L. palustre) during the growing seasons from 2013 to 2015. Results showed that warming treatment increased air temperature at 1.5m aboveground and soil temperature at 5cm depth by 0.6°C and 2°C, respectively. The average seasonal N₂O fluxes ranged from 6.62 to 9.34μgm^-2h^-1 in the warming plot and ranged from 0.41 to 4.55μgm^-2h^-1 in the control plots. Warming treatment increased N₂O fluxes by 147% and transformed the boreal peatlands from a N₂O sink to a source. The primary driving factors for N₂O fluxes were soil temperature and active layer depth, whereas soil moisture showed a weak correlation with N₂O fluxes. The results indicated that warming promoted N₂O fluxes by increasing soil temperature and active layer depth in a boreal peatland of permafrost zone in Northeast China. Moreover, elevated N₂O fluxes persisted in this region will potentially drive a noncarbon feedback to ongoing climate change.

Potential effects of ultraviolet radiation reduction on tundra nitrous oxide and methane fluxes in maritime Antarctica

Stratospheric ozone has begun to recover in Antarctica since the implementation of the Montreal Protocol. However, the effects of ultraviolet (UV) radiation on tundra greenhouse gas fluxes are rarely reported for Polar Regions. In the present study, tundra N2O and CH4 fluxes were measured under the simulated reduction of UV radiation in maritime Antarctica over the last three-year summers. Significantly enhanced N2O and CH4 emissions occurred at tundra sites under the simulated reduction of UV radiation. Compared with the ambient normal UV level, a 20% reduction in UV radiation increased tundra emissions by an average of 8 μg N2O m-2 h-1 and 93 μg CH4 m-2 h-1, whereas a 50% reduction in UV radiation increased their emissions by an average of 17 μg N2O m-2 h-1 and 128 μg CH4 m-2 h-1. No statistically significant correlation (P > 0.05) was found between N2O and CH4 fluxes and soil temperature, soil moisture, total carbon, total nitrogen, NO3--N and NH4+-N contents. Our results confirmed that UV radiation intensity is an important factor affecting tundra N2O and CH4 fluxes in maritime Antarctica. Exclusion of the effects of reduced UV radiation might underestimate their budgets in Polar Regions with the recovery of stratospheric ozone.