Different responses of CO and CH 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 CO and CH fluxes. However, the effects of FTCs on CO and CH 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 CO and CH 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 CO and CH fluxes, but their response patterns to FTCs were distinct. The FTCs promoted CO emission during the soil thawing phase, but reduced CH uptake during the freezing phase, resulting at times in the transition of the soil from a sink to a source of CH. Both freezing temperature and soil moisture had significant impacts on CO (F = 185.54, P < 0.001; F = 117.47, P < 0.001) and CH fluxes (F = 123.68, P < 0.001; F = 14.55, P < 0.001), and their interaction also had significant impacts on CO (F = 3.16, P < 0.05) and CH fluxes (F = 15.19, P < 0.001) during FTCs. The effect of microclimate composed by freezing temperature and soil moisture on CO and CH fluxes is a direct and dominant pathway (path coefficient ≥ 0.50). Substrate quality, and microbial properties also influenced CO 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 CH fluxes. Our study revealed that the different responses of CO and CH 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.