Characteristics Of Soil Organic Carbon Mineralization Under Two Different Types Of Forest In The Xiaoxing’an Moutains, China

Virgin Korean pine forest and broadleaved secondary forest are two representative forest types in the Xiaoxing’an Mountains. Knowledge of the pool size and turnover of soil organic carbon of the two forests is important to understanding the regional carbon cycle. In this paper, we used laboratory incubation method to determine soil organic carbon mineralization rate and cumulative mineralization of the two forest, and the date from incubation experiment was fitted to a three-pool first-order model which separate soil organic carbon (SOC) into active, slow and resistant pool, we also examined the seasonal dynamics of soil easily mineralizable organic carbon and its correlation with soil environmental factors. The results showed that:Although soil organic carbon mineralization rate and cumulative mineralization (all based on per unit of bulk soil) of broad-leaved secondary forest were higher than that of virgin Korean pine forest, the ratio of mineralized C per unit of SOC (i.e. cumulative mineralization on soil organic carbon base) was lower than virgin Korean pine forest. Soil active and slow carbon pools decreased with soil depth, while their proportion in SOC increased. Soil resistant carbon pool and its proportion in SOC were both greater in broad-leaved secondary forest than in virgin Korean pine forest, suggesting that broad-leaved secondary forest soil organic carbon was relatively more stable.Soil temperature not only increased organic carbon mineralization rate, but also changed the pool size of SOC. Soil active carbon pool and its proportion in SOC increased linearly with incubation temperature, and consequently, slow carbon pool decreased. Virgin Korean pine forest soils exhibited higher increasing rate of active carbon pool along temperature gradient than broad-leaved secondary forest soils, indicating that the organic carbon pool of virgin Korean pine forest soil was relatively more sensitive to temperature change.The MRT of soil active carbon pool was 19-24 d, soil slow carbon pool was 7-42 a. The MRT of soil active carbon pool decreased with soil depth, while the MRT of soil slow carbon pool increased. The response of MRT of active carbon pool to temperature was not obvious, while the MRT of slow carbon pool decreased with temperature. There was no significantly variation of the MRT of soil active carbon pool and slow carbon pool between broadleaved secondary forest and virgin Korean pine forest.SOC mineralization rate and Cm for 0-5 cm soil layer decreased from early spring to late autumn, while for 5-10 cm soil layer the seasonal variation was not statistically significant for both forest types. The C1 in 0-5 cm soil layer and 5-10 cm soil layer varied from 42.92～92.18 and 19.23～32.95 mg·kg-1, respectively, while the Cm in 0-5 cm soil layer and 5-10 cm soil layer varied from 863.92～3957.15 and 434.15～865.79 mg·kg-1, respectively. Both C1 and C0 decreased from early spring to late autumn. The proportion of Co in SOC for two forest types in 0-5 cm soil layer and 5-10 cm soil layer ranged from 0.74% to 2.78% and 1.11% to 1.84%, respectively, and decreased from early spring to late autumn, indicating that SOC tend to more stable as a whole from spring to autumn. The Cm and Co were significantly and positively correlated to in situ soil water content and HWEC (p<0.01), but were not correlated to in situ soil temperature and CWEC (p>0.05). We concluded that soil labile organic carbon, soil physical and chemical properties contributed to the seasonal dynamics of SOC mineralization in the forests.