The effect of long-term soil surface mulching on SOC fractions and the carbon management index in a semiarid agroecosystem

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Soil organic carbon (SOC) and its fractions are widely considered to be valuable indicators for monitoring soil carbon (C) pools and dynamics induced by mulching practices. Thus, a 10-year experiment with three treatments, including no mulching (CK), gravel mulching (GM) and film mulching (FM), was established on the Loess Plateau. The results showed that SOC under GM treatment (7.88 g kg−1) in surface soil (0–20 cm) decreased significantly compared with that of CK (8.59 g kg−1), and that of FM (8.09 g kg−1) did not change significantly. Compared with CK in surface soil, water soluble C (WSC) under FM treatment decreased by 9.36% on average, and hot-water extractable C (HWC) under GM and FM treatments increased by 12.36% and 24.56% on average, respectively. Labile organic C (LOC) determined by the potassium permanganate (P-P) method did not decrease significantly under FM treatment in surface soil, but recalcitrant organic C (ROC), determined by the P–P method, significantly decreased by 9.88% on average. Compared with CK, less labile C (CLL), determined by the modified Walkley-Black (M–W–B) method under GM and FM treatments, significantly decreased by 11.21% and 17.54% on average, respectively, which suggested that CLL was converted to very labile C (CVL) and labile C (CL). This result indicated that the M-W-B method was superior in explaining C turnover. Combined with the C management index (CMI) results of previous studies, CMI was consistently lower under GM treatment, and it was unstable under FM treatment. The sensitivity index (SI) results indicated that HWC was the most sensitive SOC fraction, and LOC and ROC determined by the M–W–B method were more sensitive than those determined by the P–P method. Moreover, CMI calculated by the M–W–B method (10–35%) was more sensitive than that calculated by the P–P method (9–12%). In conclusion, the decrease in SOC under long-term mulching is mainly from ROC, and the M–W–B method is superior to the P–P method.