Abstract: To understand the effects of applying a microbial promoting-decomposing agent at different times on physicochemical properties and fungal communities in black soil under no-till straw mulching, we conducted an experiment in the black soil area of Hulunbuir City, Inner Mongolia. There were five treatments: conventional plowing with straw returning (CK1), no-till with straw mulching without microbial promoting-decomposing agent (CK2), no-till straw mulching with microbial promoting-decomposing agent applied at planting (NT1), no-till straw mulching with microbial promoting-decomposing agent applied 15 days after planting (NT2), no-till straw mulching with microbial promoting-decomposing agent applied 30 days after planting (NT3). The results showed that compared with CK1, CK2 significantly increased soil bulk density and reduced soil pH in the 0-40 cm layer, and increased the contents of alkaline hydrolyzable nitrogen (AN), available phosphorus (AP), and soil organic matter (SOM) in the 0-10 cm layer. Compared with CK2, delayed application of the microbial promoting-decomposing agent increased soil nutrient content in the 0-10 cm layer, with the NT3 showing the most significant effect. Specifically, NT3 increased soil pH, AN, AP, available potassium (AK), dissolved organic carbon (DOC) and SOM by 5.17%, 17.81%, 12.18%, 43.31%, 27.29%, and 4.15%, respectively, and reduced soil bulk density by 4.96%. In the 10-20 cm layer, NT3 significantly increased AP, AK, and DOC contents by 38.07%, 24.51%, and 22.34%. Treatments with the microbial promoting-decomposing agent increased fungal αdiversity in the 0-10 cm layer and elevated the relative abundance of Humicola and Penicillium in the 0-40 cm layer, while reducing Fusarium abundance in the 0-20 cm layer. Among them, NT3 had the lowest Fusarium abundance and the highest enrichment of Mortierella. The relative abundances of Humicola and Penicillium were significantly positively correlated with soil nutrient content in the 0-10 cm layer. In conclusion, applying the microbial promoting-decomposing agent at 30 days after notill sowing with straw mulching achieved the optimal results, providing critical guidance for its application in black soil regions.

Key words: no-till straw mulching, microbial promoting-decomposing agent, soil nutrient, fungal community structure