Sensitivity of soil organic carbon stocks and fractions to soil surface mulching in semiarid farmland

• Gravel mulching significantly decreased soil organic carbon stock in the 20–40 cm layer.
• Plastic film mulching significantly increased soil organic carbon fractions in the 20–40 cm layer.
• Gravel mulching significantly decreased soil organic carbon fractions in the 20–40 cm layer.
• Plastic film mulching could sustainably enhance crop productivity and maintain soil quality.

There is little information regarding the impact of plastic film and gravel mulching on soil organic carbon (SOC) stocks and labile fractions in semiarid areas of northwest China. Based on a 4-year spring maize (Zea mays L.) field experiment at Changwu Agricultural and Ecological Experimental Station, we analyzed the SOC stock, labile SOC pools and the C management index (CMI) in the 0–20 cm and 20–40 cm layers. The three treatments included non-mulched (CK), gravel-mulched (GM) and plastic film-mulched (FM). After the 4-year duration, compared with the initial SOC stock, the SOC stock under GM decreased 0.97 Mg C hm−2 whereas under FM increased 1.14 Mg C hm−2 in the 20–40 cm layer. Across the 0–40 cm layer, the SOC stock under GM significantly decreased, whereas under FM demonstrated some tendency to increase. In contrast to the CK, the FM treatment significantly increased light fraction organic C (LFOC) content (density <1.7 g cm−3) by 17.2 mg kg−1 in the 20–40 cm layer. Although, the FM treatment significantly decreased the CMI in the 0–20 cm layer, compared with the CK, but an increased trend of CMI in the 20–40 cm layer was observed. KMnO4-oxidized organic C and LFOC were more sensitive than other C fractions in comparison of relative changes for labile SOC fractions. In conclusion, plastic film mulching could be the best option for sustainably enhancing crop productivity and maintaining soil quality in the semiarid regions of northwest China.