Greenhouse gas emission from the cold soils of Eurasia in natural settings and under human impact: Controls on spatial variability
The annual balance of biogenic greenhouse gases (GHGs; carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) in the atmosphere is well studied. However, the contributions of specific natural land sources and sinks remain unclear, and the effect of different human land use activities is understudied. A simple way to do this is to evaluate GHG soil emissions. For CO2, it usually comprises 60–75% of gross respiration in natural terrestrial ecosystems, while local human impact can increase this share to almost 100%. Permafrost-affected soils occupying 15% of the land surface mostly in the Eurasia and North America contain approximately 25% of the total terrestrial carbon. The biogenic GHG soil emissions from permafrost are 5% of the global total, which makes these soils extremely important in the warming world. Measurements of CO2, methane, and nitrous oxide, from eighteen locations in the Arctic and Siberian permafrost, across tundra, steppe, and north taiga domains of Russia and Svalbard, were conducted from August to September during 2014–2017 in 37 biotopes representing natural conditions and different types of human impact. We demonstrate that land use caused significant alteration in soil emission and net fluxes of GHGs compared to natural rates, regardless of the type and duration of human impact and the ecosystem type. The cumulative effect of land use factors very likely supported an additional net-source of CO2 into the atmosphere because of residual microbial respiration in soil after the destruction of vegetation and primary production under anthropogenic influence. Local drainage effects were more significant for methane emission. In general, land use factors enforced soil emission and net-sources of CO2 and N2O and weakened methane sources. Despite the extended heat supply, high aridity caused significantly lower emissions of methane and nitrous oxide in ultra-continental Siberian permafrost soils. However, these climatic features support higher soil CO2 emission rates, in spite of dryness, owing to the larger phytomass storage, presence of tree canopies, thicker active layer, and greater expressed soil fissuring. Furthermore, the “Birch effect” was much less expressed in ultra-continental permafrost soils than in permafrost-free European soils. Models and field observations demonstrated that the areal human footprint on soil CO2 fluxes could be comparable to the effect of climate change within a similar timeframe. Settlements and industrial areas in the tundra function as year-round net CO2 sources, mostly owing to the lack of vegetation cover. As a result, they could compensate for the natural C-balance on significantly larger areas of surrounding tundra.