Response of paddy soil organic carbon accumulation to changes in long-term yield-driven carbon inputs in subtropical China

A decrease in C inputs from the return of crop residues to soil has occurred in many regions worldwide in recent years. The effects of this decline in C inputs could provide valuable information for assessing the long-term impact of litter C inputs on soil organic C (SOC) in rice paddy soils. The present study aimed to evaluate the response of rice paddy SOC accumulation to changes in actual C inputs in subtropical China, with emphasis on the response of C accumulation to declining C inputs. For this, we used a long-term field experiment on paddy soil in a rice-rice (Oryza sativa L.) cropping system running from 1990 to 2014. The four treatments were CK (control, no fertilizer), OM (organic matter application), NPK (N, P, and K fertilizer application), and NPKOM (NPK and organic matter application). Organic matter application for the OM and NPKOM treatments included rice straw and green manure that were left in the field after harvest and chopped, along with rice residues with stubbles and roots. In all treatments, C sequestration showed an increasing trend (from 0.207 to 0.880 g kg−1 yr−1) in the early and middle stages of the experiment (1990-2006) followed by a decreasing trend (from −0.429 to −0.064 g kg−1 yr−1) in the late stage (2007-2014). The trends were more pronounced for the OM and NPKOM treatments than for their CK and NPK counterparts. The changes in SOC stocks were consistent with changes in C inputs (p < 0.01). During the late stage, yield and litter inputs from crop residues and green manure decreased, quickly affecting SOC stock in paddy soils. This declining trend in annual rice yields was mainly caused by the decline in first rice yields, accounting for 42.3-91.5% of the decrease in annual C inputs. Insufficient P or N and K supply and unfavorable climatic factors (decreases in sunshine duration and both maximum and minimum temperatures) are possible reasons for the decline in first rice yields and green manure biomass in the late stage. Collectively, the results suggest that C stocks in high-productivity paddy soils respond very sensitively to a decline in C inputs. This raises the risk of loss of C stock in paddy soil if, in the long run, a large return of C to soil with crop residues or by other sources, e.g., green manure, cannot be achieved.