Comparison of Soil Greenhouse Gas Fluxes during the Spring Freeze–Thaw Period and the Growing Season in a Temperate Broadleaved Korean Pine Forest, Changbai Mountains, China

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 (F_temperature = 185.54, P < 0.001; F_moisture = 117.47, P < 0.001) and CH₄ fluxes (F_temperature = 123.68, P < 0.001; F_moisture = 14.55, P < 0.001), and their interaction also had significant impacts on CO₂ (F_CO2 = 3.16, P < 0.05) and CH₄ fluxes (F_CH4 = 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.