Dynamics of soil labile organic carbon fractions and C-cycle enzyme activities under straw mulch in Chengdu Plain

• Straw mulch had positive/no obvious effect on LOCF in various wheat growth stages.
• WSOC and MBC were significantly sensitive indicators for straw mulch.
• Cellulase and/or β-glucosidase were positive impact factors on LOCF.
• Polyphenol oxidase was a positive/negative factor on LOCF at various soil depths.
• Peroxidase was a negative impact factor on LOCF.

Soil labile organic carbon fractions (LOCF) are valuable indicators of changes in soil quality and soil total organic carbon (TOC), and they are easily influenced by changes in soil management practice. To explore the sensitivity of LOCF and their enzymatic conversion under straw mulch for scientific management of crop straw use on the Chengdu plain, a straw mulch field experiment was conducted to observe the LOCF changes and explore their relationships with C-cycle enzymes during wheat growth. The treatments included no fertilizer or straw (Control), only mineral fertilizer (MF), 10% rice straw nitrogen (N) plus 90% fertilizer N (RS1), 20% rice straw N plus 80% fertilizer N (RS2), 30% rice straw N plus 70% fertilizer N (RS3), 100% rice straw (RS). The water-soluble organic carbon (WSOC) and microbial biomass carbon (MBC) under straw treatments in the 0–20 cm soil layer were significantly higher than those of Control and MF treatments from the seedling stage, and showed significant differences among straw treatments from the tillering stage. Correspondingly, the dissolved organic carbon (DOC) and permanganate oxidized carbon (POXC) showed no obvious differences among straw treatments in most growth stages. Therefore, WSOC and MBC were the most sensitive indicators for assessing soil organic carbon (SOC) change under straw mulch. Moreover, stepwise multiple linear regression revealed that cellulase and β-glucosidase were significantly positive impact factors for LOCF in the two soil layers, while polyphenol oxidase had both positive and negative impacts on WSOC at different soil layers, and peroxidase was a negative factor for LOCF.