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

Many environmental factors affecting methane (CH4 ) and nitrous oxide (N2 O) fluxes have been investigated during the processes of carbon and nitrogen transformation in the boreal tundra. However, effects of sunlight on CH4 and N2 O fluxes and their budgets were neglected in the boreal tundra. Here, summertime CH4 and N2 O fluxes in the presence and total absence of sunlight were investigated at the six tundra sites (DM1-DM6) on Ny-Ålesund in the High Arctic. The mean CH4 fluxes at the tundra sites ranged from -4.7 to -158.6μg CH4 m-2 h-1 in the presence of light, indicating that a large CH4 sink occurred in the tundra soils. However, enhanced CH4 emission in total absence of light occurred at all the tundra sites. The mean N2 O fluxes ranged from 7.4 to 14.6μg N2 O m-2 h-1 in the presence of light, whereas in the absence of light all the tundra sites generally released less N2 O, and even significant N2 O uptake occurred there. Soil temperature, chamber temperature and soil moisture showed no significant correlations with tundra CH4 and N2 O flux. The presence of sunlight increased tundra CH4 uptake by 114.2μg CH4 m-2 h-1 and N2 O emission by 10.9μg N2 O m-2 h-1 compared with total absence of light. Overall our results showed that tundra ecosystem switched from CH4 sink and N2 O emission source in the presence of light to CH4 emission source and N2 O sink in the absence of light. Therefore sunlight had an important effect on CH4 and N2 O budgets in the High Arctic tundra. The exclusion of sunlight might overestimate CH4 budgets, but underestimate N2 O budgets in the Arctic tundra ecosystem.