Different responses of CO2 and CH4 to freeze-thaw cycles in an alpine forest ecosystem in northwestern China

The frequency and intensity of freeze-thaw cycles (FTCs) at high latitudes and altitudes are expected to increase with climate change, with likely effects on soil carbon turnover and CO2 and CH4 fluxes. However, the effects of FTCs on CO2 and CH4 fluxes remain unexplored, especially in alpine forest ecosystems. Here, we conducted an incubation experiment using intact soil cores to explore the effects of FTCs on CO2 and CH4 fluxes. Since temperature and moisture are considered as potential factors affecting FTCs, two freezing temperatures and three moisture levels were included in the incubation conditions. Our results showed that FTCs significantly affected CO2 and CH4 fluxes, but their response patterns to FTCs were distinct. The FTCs promoted CO2 emission during the soil thawing phase, but reduced CH4 uptake during the freezing phase, resulting at times in the transition of the soil from a sink to a source of CH4. Both freezing temperature and soil moisture had significant impacts on CO2 (Ftemperature = 185.54, P < 0.001; Fmoisture = 117.47, P < 0.001) and CH4 fluxes (Ftemperature = 123.68, P < 0.001; Fmoisture = 14.55, P < 0.001), and their interaction also had significant impacts on CO2 (FCO2 = 3.16, P < 0.05) and CH4 fluxes (FCH4 = 15.19, P < 0.001) during FTCs. The effect of microclimate composed by freezing temperature and soil moisture on CO2 and CH4 fluxes is a direct and dominant pathway (path coefficient ≥ 0.50). Substrate quality, and microbial properties also influenced CO2 fluxes during FTCs with the effect of the substrate being greater than that of microbial properties; substrate was important but the role of microorganisms was insignificant for CH4 fluxes. Our study revealed that the different responses of CO2 and CH4 to FTCs in soil cores from an alpine forest ecosystem could improve the understanding of soil carbon release during FTCs and support accurate assessment of the carbon balance.