More than 50% of terrestrial soil carbon is stored in permafrost soils. When ice-rich permafrost thaws, soil organic matter may deposit in aerobic or anaerobic environments, depending on local topography and original soil ice content. We tested how the oxygen status and soil substrate quality affect CO2 and CH4 emissions by conducting laboratory soil incubation experiments. We then used the results to estimate global climate forcing from the carbon release in permafrost soils. The soils were incubated in 1L jar under aerobic and anaerobic conditions at 15C for nearly 1 year. We measured CO2 emissions from aerobic incubations, and CO2 and CH4 from anaerobic incubations. Soil %C, %N, pH, and glucosidase activity were analyzed to compare the substrate quality of permafrost soils from Alaska and Siberia. We also measured C and N stable isotopes in bulk soils before the incubation and 13C from CO2 and CH4 emitted during the incubation.
The results show that C mineralization was 4.2 times greater under aerobic conditions than anaerobic conditions after 250 days of soil incubation. However, when global warming potential of CH4 was considered as 25 times that of CO2 in a 100 year time scale, relative climate forcing (ratio between aerobic climate forcing and anaerobic climate forcing) ranged from 0.2 to 7.5. Cumulative carbon mineralization was correlated to substrate quality, including %C, %N, and C to N ratio under both under aerobic and anaerobic conditions. However, relative climate forcing did not show strong correlations to substrate quality. Soil pH showed negative relationship with CO2 emissions under aerobic conditions (p<0.001, R2=0.37) and CO2 (p<0.001, R2=0.39) and CH4 (p<0.001, R2=0.50) emissions under anaerobic conditions. Therefore, we suggest that the fate of permafrost carbon will depend on the oxygen availability status of soils after permafrost thaw, and the organic matter substrate quality.