Objectives To resolve the challenge of straw decomposition in cold and arid regions, microbial agents with both low - temperature tolerance and high efficiency are commonly used to promote the efficient decomposition of straw. We investigated the mechanism of microbial agents M44 on accelerating straw decomposition under different straw returning methods.

Methods A split-plot design was adopted in a field experiment, the main plots included deep plowing with straw return (DPR), subsoiling with straw mixing return (SSR), and no-tillage with straw mulching return(NTR), and the subplots was with or without application of microbial agent M44 (CK, and M44). At the same time, 20 g of straw, 1 kg of soil from the treated plots, and 0.04 g of M44 microbial agent were thoroughly mixed, placed into nylon mesh bags, and buried in the corresponding plots, with a total incubation period of 320 days. Periodically, samples were collected to determine indices such as straw degradation rate, nutrient release rate, soil extracellular enzyme activity, and soil nutrient content.

Results Compared with the CKs, the M44 treatments significantly increased the average straw degradation rate, lignocellulose degradation rate, and nutrient release rate by 2.33−9.81, 3.10−14.81, and 3.55−18.59 percentage points, respectively. And the enhancement of straw degradation efficiency during 0−185 days were as high as 28.40−71.83%. Compared with the CKs, the M44 treatments increased the activities of urease, sucrase, alkaline phosphatase, and catalase by averagedly 0.39, 0.33, 0.28, and 0.35 mg/(g·d), respectively. they also enhanced the activities of β-1,4-xylosidase, β-1,4-glucosidase, and N-acetylglucosaminidase by an average of 11.79, 18.28, and 3.12 nmol/(g·h) throughout the whole incubation period, respectively (P<0.05). The straw decomposition and soil fertility improvement effects of DPR+M44 and SSR+M44 were superior to those of NRT+M44, as evidenced by a higher straw degradation rate (10.66−25.60 and 8.69−26.08 percentage points) and release rates of straw carbon, nitrogen, and phosphorus (3.55−18.59, 6.35−7.58, and 6.99−11.36 percentage points) (P<0.05). Structural equation modeling analysis revealed that urease, alkaline phosphatase, and leucine aminopeptidase directly promoted straw degradation. Further path analysis indicated that soil extracellular enzyme activities indirectly mediated straw nutrient release by directly accelerating straw degradation. Additionally, straw nutrient release was directly driven by urease and β-1,4-xylosidase, while nutrient release from straw enhanced soil nutrient levels.

Conclusion Under various straw return methods, the application of microbial agent M44 can accelerate straw degradation, shorten the degradation process, enhance the nutrient release rate from straw, increase soil enzyme activities and nutrient contents, with even more pronounced effects observed under deep plowing with straw return and subsoiling with straw mixing return methods.