Controls of greenhouse gases production and soil nutrients dynamics in the Lena River Delta, Northeastern Siberia

Abstract

The permafrost stores vast amounts of organic carbon. As permafrost thaws due to global warming, previously frozen organic matter become accessible to microbial decomposition. This process releases greenhouse gases like carbon dioxide (CO2) and methane (CH4) creating a positive feedback loop, where increased greenhouse gas emissions lead to further warming and subsequent thawing, a process known as the permafrost carbon-climate feedback. In this thesis two key research gaps in the understanding of the permafrost carbon-climate feedback are addressed with studies from sites in the Lena River Delta, Northeastern Siberia, Russia. The second chapter assessed the determining factors for the ratio between in situ CO2 to CH4 production in the polygonal tundra. This is an important topic about the permafrost carbon-climate feedback, since CH4 has a 28-fold higher global warming potential than CO2 and it is crucial to determine the factors modulating the partitioning between CO2 and CH4 fluxes from organic matter decomposition in the Arctic. In this study I quantified the CO2:CH4 production ratios of soil organic matter decomposition in wet and dry tundra soils in Samoylov Island, Lena River Delta, by using CO2 fluxes from clipped plots and in situ CH4 fluxes from vegetated plots. The results show that active layer depth and soil temperature were the main factors controlling these ratios, which decreased towards the end of the end of the growing season, when the active layer was deep and warm enough for methanogenesis. CH4 production was associated with subsoil (40 cm) temperature, while heterotrophic respiration was related to topsoil (5 cm) temperatures, mainly due to the mostly oxic environment of topsoil, inducing aerobic CO2 production, and the anoxic environment of the subsoil, inducing CH4 production. The third chapter assessed the effect of thawing and warming on the availabilities of phosphorus (P) and potassium (K) in incubated soils of Kurungnakh Island, Lena River Delta. The fate of soil nutrients in permafrost affected soils with warming, will determine if there will be significant constraints to enhanced primary productivity in the Arctic. The tundra vegetation is currently limited in P, thus the effect of climate change on soil P availability might alleviate P limitation and increase primary productivity and C sequestration, or might magnify P limitation, restricting primary productivity, and increasing the net greenhouse gas emissions of permafrost ecosystems. However, this relationship between P and the tundra C balance is further complicated by the consumption of P by microbes, which in turn might make it less straightforward. P and K nutrients were immobilized during the experiment, instead of mineralized, regardless of the temperature treatment. P and K concentrations and turnover rates had a high correlation with C turnover, and samples that emitted more CO2 were also the ones that immobilized more P, which was demonstrated by the correlation found between CO2 production rates and P turnover rates. The results of this thesis suggest that more CH4 is expected to be produced in permafrost ecosystems stemming from deeper and warmer active layers. And that P limitation is likely to persist with short-term soil warming, potentially imposing significant constraints on the tundra vegetation capacity for C sequestration.