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 (C_LL), 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 C_LL was converted to very labile C (C_VL) and labile C (C_L). 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.

土壤有机碳 (SOC) 及其组分被广泛认为是监测土壤碳 (C) 库和覆盖措施引起的动态的有价值的指标。因此，在黄土高原建立了不覆盖（CK）、碎石覆盖（GM）和地膜覆盖（FM）三种处理的10年试验。结果表明，与CK（8.59 g kg ^-1）和FM（8.09 g kg ^-1）相比，转基因处理（7.88 g kg ^-1）表层土壤（0-20 cm）的SOC显着降低) 没有显着变化。与表层土壤CK相比，FM处理下水溶性C（WSC）平均降低9.36%，GM和FM处理下热水可提取C（HWC）平均分别增加12.36%和24.56%。表层土壤中高锰酸钾（PP）法测定的不稳定有机碳（LOC）在FM处理下没有显着降低，但P-P法测定的顽固有机碳（ROC）平均显着降低了9.88%。与 CK 相比，在 GM 和 FM 处理下，通过改良的 Walkley-Black (M-W-B) 方法测定的不稳定 C (C _LL ) 分别显着降低了 11.21% 和 17.54%，这表明 C _LL被转化为非常不稳定的 C (C _VL) 和不稳定的 C ( _CL )。该结果表明MWB方法在解释C周转方面具有优越性。结合以往研究的C管理指数（CMI）结果，GM处理下CMI始终较低，FM处理下CMI不稳定。敏感性指数（SI）结果表明，HWC 是最敏感的 SOC 部分，而 M-W-B 法测定的 LOC 和 ROC 比 P-P 法测定的更敏感。此外，通过 M-W-B 方法计算的 CMI（10-35%）比通过 P-P 方法计算的 CMI（9-12%）更敏感。综上所述，长期覆盖下SOC的降低主要来自ROC，M-W-B法优于P-P法。