Tillage effects on soil carbon fractions in the Sanjiang Plain, Northeast China

Soil organic carbon (SOC) is important for crop production, improving soil quality, and regulating global C cycling. The objective of this study was to estimate C fraction dynamics under the influence of tillage in the Sanjiang Plain of Northeast China. Rate of decline in the free light fraction (FLF) C concentration was much greater than that of intra-aggregate light fraction (ILF) C, heavy fraction (HF) C and SOC during the initial 5 years of cultivation (p < 0.05). Cultivation led to accumulation in HF-C compared to the other fractions. The contribution of different fractions to respiration varied with cultivation. In native marshland, HF-C, FLF-C and ILF-C were responsible for 33%, 63% and 4% of the soil respiration, respectively. In soil cultivated for 1 or 3 years, contributions of combined FLF-C and ILF-C to respiration were higher than HF-C. However, after 5 years of cultivation, contributions of HF-C to respiration (55%) were higher than that of combined FLF-C and ILF-C (45%). In soil cultivated for 35 years, the HF-C was responsible for almost 69% of the respiration. Therefore, C dynamics in the soil are controlled by the behavior of LF-C in the natural soil and short-term (<5 years) cultivated soils. In long-term (>5 years) cultivated soils, C dynamics are controlled by the behavior of HF-C.