Effects of Tillage Years on Soil Organic Carbon Fractions in the Poyang Lake Reclaimed Paddy Fields

Wetland conversion to paddy field is a typical land use change in the middle and lower reaches of the Yangtze River. Poyang Lake is the largest fresh water lake in China. There were total 1 466.9 km² Poyang Lake wetlands that were converted to croplands, and most of them were used as paddy cultivation. Soil samples were collected from Poyang Lake reclaimed paddy fields with different tillage years. Subsequently, soil total organic carbon (SOC), heavy fraction organic carbon (HFOC), light fraction organic carbon (LFOC) and microbial biomass carbon (MBC) were measured. The objective was to clarify the effect of tillage years on soil organic carbon fractions in the Poyang Lake reclaimed paddy fields. Results indicate that the top 30 cm soil HFOC and LFOC ranged from 5.89 to 24.01 and 0.47 to 4.14 mg·g^-1, respectively, whereas MBC varied from 12.43 to 850.53 mg·kg^-1. All three organic carbon fractions were significantly positively correlated with soil total organic carbon. HFOC, LFOC and MBC were significantly affected by tillage years. The contents of all three organic carbon fractions increased with tillage years. However, the ratios of various organic carbon fractions to soil total organic carbon differed among the three components and varied with soil depth. After 50 years tillage, HFOC in the reclaimed paddy fields was relatively stable, however, the ratio of HFOC to SOC declined in comparison to natural wetlands. Hence, to some extent, reclamation declined the stability of soil carbon pool.