Elevated soil moisture amplified the effects of freeze–thaw cycles on soil CO2 and CH4 fluxes in subalpine forests

Soil carbon emissions from subalpine ecosystems have been demonstrated to be influenced by freeze–thaw cycles (FTCs). Under climate change, moisture and number of FTCs have been altered significantly in subalpine regions. Thus, we selected a typical subalpine forest in Northwest China and conducted an incubation study to explore the effects of various soil moisture (SM) levels and numbers of FTCs on soil CO2 and CH4 fluxes during nine FTCs. Our results revealed that the soil CO2 emissions and CH4 uptakes had significant responses to changes in SM (FCO2 = 2327.32, p < 0.001; FCH4 = 353.51, p < 0.001) and the number of FTCs (FCO2 = 2506.45, p < 0.001; FCH4 = 60.85, p < 0.001). Specifically, CO2 emissions in the thawing phases and CH4 uptakes in the freezing phases were largest in the first FTC and then gradually decreased and stabilized with an increase in the number of FTCs. Regarding SM, the soil CO2 emissions and CH4 uptakes at 60 and 90 % water-filled pore space (WFPS) at the same number of FTCs were significantly higher than those at 30 % WFPS. Moreover, the interactive effects of the number of FTCs and SM on soil CO2 emissions and CH4 uptakes were significant (FCO2 = 279.70, p < 0.001; FCH4 = 17.76, p < 0.001). Especially in the first FTC, SM dramatically amplified the effects of FTCs on CO2 emissions and CH4 uptakes. A partial least squares path model further confirmed that the number of FTCs had significant negative effects while SM had significant positive effects on CO2 emissions and CH4 uptakes. In addition, SM explained more of the variation than the number of FTCs in CO2 emissions and CH4 uptakes. CO2 emissions were modulated primarily by substrate accessibility and nutrient availability, and for CH4 uptakes, microbial properties also played a substantial role in addition to substrate accessibility and nutrient availability. We conclude that the increases in the number of FTCs and SM due to climate warming and humidification may trigger the potentially substantial increases in soil CO2 emissions and CH4 uptakes.