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A laboratory incubation test was conducted to study the organic C mineralization in gray forest soils after land use conversion and the temperature sensitivity of the mineralization. It was shown that after the conversion from wildwood land to farmland, the organic C and total N contents in 0-10 cm and 10-20 cm soil layers decreased by 68.5% and 76.8%, and 40.5% and 44.4%, and the average mineralization rate and cumulative mineralization of soil organic C in farmland were 24.4%-43.2% and 9.20%-13.7% of those in wildwood land, respectively. At lower temperature (< 25 degrees C), there was no significant difference in the temperature sensitivity of soil organic C mineralization between the two land use types; while at higher temperature ( > 25 degrees C), this temperature sensitivity in farmland soil was higher in 0-10 cm layer but significantly lower in 10-20 cm layer, compared with that in wildwood land soil.

With incubation tests in laboratory, the mineralization of organic carbon in various size aggregates of paddy soil was investigated under aerobic and submerged conditions. The results showed that the organic carbon mineralization in various size aggregates decreased quickly at the beginning of the incubation, but remained stable during the late period of incubation. The mineralization rate varied significantly with the size of the aggregates. Through the incubation time, the organic carbon in 1-2 mm aggregates had the highest mineralization rate, while that in < 0.053 mm aggregates had the lowest one. Statistic analyses indicated that the mineralization rate of organic carbon in various size aggregates was significantly and linearly correlated with the contents of organic carbon and microbial biomass carbon in the aggregates. 0.25-1 mm aggregates had the highest contribution to the cumulative mineralization of soil organic carbon, accounting for 41.77% under aerobic condition and 34.11% under submerged condition, while < 0.053 mm and 1-2 mm aggregates had the lowest contribution under aerobic and submerged conditions, accounting for 7.8% and 6.6%, respectively.