Effects of seasonal changes on the carbon dynamics in mixed coniferous forests

We characterized seasonal changes in the residual rate and mass-loss rate of litter, as well as the carbon release dynamics of litter and soil, in mixed coniferous forests in the Xiaoxinganling region by conducting a controlled freeze–thaw experiment. The carbon release rate and mass-loss rate of litter at two levels of decomposition (undecomposed and semi-decomposed) were measured during four seasons: the unfrozen season, freeze–thaw season, frozen season, and thaw season. The temperature was higher, the mass-loss rate was faster, and the overall mass-loss rate of litter was higher in the unfrozen season than in the other three seasons; litter organic carbon increased and soil organic carbon increased due to the strong carbon sequestration capacity of plants. The temperature fluctuated above and below 0°C during the freeze–thaw season, which results in the physical breakage of the undecomposed litter and increases in the mass-loss rate. This leads to increases in the organic carbon of undecomposed litter and decreases in the soil organic carbon of undecomposed litter; the opposite patterns were observed for changes in the organic carbon of semidecomposed litter and soil organic carbon. There was noticeable mass loss of litter during the frozen season, and the rate of mass loss of litter during the thaw season was the lowest. Litter organic carbon decreased and soil organic carbon increased in both seasons. The organic carbon of undecomposed litter was highest in the thaw season, followed by the freeze–thaw season, frozen season, and unfrozen season. The organic carbon of semi-decomposed litter was highest in the frozen season, followed by the thaw season, freeze–thaw season, unfrozen season to freeze–thaw season, frozen season, thaw season, and unfrozen season. After freeze–thaw treatment, the organic carbon in deadfall soil was highest in the unfrozen season, freeze–thaw season, frozen season, thaw season to unfrozen season, frozen season, thaw season, and freeze–thaw season. The findings of this study provide new insights into the material cycling process under freeze–thawing, as well as information on the effect of seasonal freeze–thaws on the forest carbon cycle. ### Competing Interest Statement The authors have declared no competing interest.