Cover crops offset recalcitrant soil organic carbon losses under plastic-film mulching by altering microbial functional genes

While cover crop residue (CR) incorporation offsets soil organic C (SOC) losses caused by plastic-film mulching (PFM), the microbial modulators and mechanisms of SOC accumulation remain poorly understood. Using functional gene microarray, soil enzyme activities, and soil density fractionation approaches, we investigated the mechanism, by incorporating CR (13.0 and 9 Mg ha⁻¹ on a dry weight basis) under PFM in organic maize cropping systems, spanning 2 consecutive years. Compared to no-mulching, PFM without CR amendment significantly decreased SOC stock by 23%, corresponding to an increase in CO₂ efflux by 74%, a decrease in light fraction C (LFOC) and heavy fraction C (HFOC) by 13 and 11%, respectively, and an increase in relative abundances of labile as well as recalcitrant C-degrading genes and related soil enzyme activities. However, PFM with CR amendment reduced the SOC stock loss to 11%, corresponding to 36% increase in CO₂ efflux, 22% decrease in LFOC but 12% increase in HFOC, and an increase in relative abundances of labile C-degrading genes and related soil enzyme activities but a decrease in relative abundances of recalcitrant C-degrading genes and related soil enzyme activities. Our results, based on microbial functional genes, suggest that reduced degradation of recalcitrant C was responsible for increased mineral-associated C and thus SOC accumulation under PFM in organically cultivated maize.