[Distribution characteristic of soil organic carbon pool in the plow layer of sloping farmland with different cultivation years in black soil region].

Tillage and erosion simultaneously cause soil movement from the top-slope position to the foot-slope position in a sloping farmland, with consequences on variation of soil organic carbon (SOC) content as well as its stability mechanism. To identify the accumulation-loss characteristics of organic carbon pool (OCP) of black soil under long-term cultivation, the distribution characteristics of different OCPs (free unprotected, physical protection, chemical protection, biochemical protection) of sloping farmland with different years of cultivation were quantified by Stewart physical-chemical fractionation method. The results showed that long-term cultivation and associated erosion resulted in significant accumulation of SOC in the foot-slope deposition area. The contents of four OCPs in the foot deposition area were significantly higher than those in top-slope eroded areas. SOC of sloping farmland was dominated by chemically protected carbon and biochemically protected carbon (＞90%). Furthermore, the top-slope erosion areas and foot-slope deposition areas mainly accumulated chemically protected organic carbon (84.6%) and biochemically protected organic carbon (51.4%), respectively. With the increases of cultivation years, the accumulation rate of four OCPs increased with the stability degree of SOC, that is, biochemical protection carbon pool (48%) ＞ chemical protection carbon pool (42.2%) ＞ physical protection carbon pool (6.4%) ＞ free unprotected carbon pool (3.4%). The proportion and accumulation rate of the free unprotected organic carbon pool were the smallest, but they were most sensitive to external interference. Thus, more attention should be paid to free unprotected organic carbon during the management of black soil.