Elevated hydrolase activity rather than oxidase activity enhances the accumulation of organic matter in black soil under maize stover retention

Abstract

Stover incorporation is a widely practice enhancing soil organic matter (SOM), improve soil quality, and support agricultural sustainability. However, the specific enzymatic mechanisms driving SOM accumulation under different maize stover incorporation modes (SIMs) in black soils remain poorly understood. In this study, we investigated soil humus fractions, the activities of hydrolase and oxidase enzymes, and the abundance of functional genes related to SOM transformation under five-year SIMs, including stover removal (CK), direct incorporation of chopped stover (SD), stover mulching with no-tillage (NT), and biochar application derived from an equivalent amount of stover (BC). The results showed that SOM content in the 0–10 cm soil layer increased under SD, NT, and BC treatments. Both SD and NT significantly elevated the levels of fulvic acid (FA), humic acid (HA), and humus structural complexity (as indicated by the HA/FA ratio and PQ value) in the surface soil. These treatments also significantly enhanced the activities of soil cellobiohydrolase, α-glucosidase and β-1,4-N-acetyl-glucosaminidase. Notably, the SD treatment increased the abundance of GH3-Bacteria, GH3 - Fungi, amyA, chiA, and lig genes in the surface soil, while the NT treatment increased amyA gene abundance. In contrast, the BC treatment reduced GH3-Bacteria gene abundance in the subsoil layer (10–20 cm). In addition, exg gene abundance was positively correlated with BG activity in the surface soil. FA and HA contents were positively correlated with hydrolase activities (BG, BGal, NAG, and CBH; P < 0.05). Overall, stover incorporation enhanced SOM and humus carbon contents by stimulating cellulose-degrading enzyme activities and promoting related functional gene expression, with the SD and NT treatments showing the most pronounced effects in surface soil.