Cumulative Effects of Different Application Time Scales of Biogas Slurry on Agricultural Soils Based on Meta-Analysis

Returning biogas slurry to fields is an important approach for the resource utilization of organic waste. It enhances soil fertility and crop growth, but its long-term effects on soil ecosystems need evaluation. A meta-analysis approach was employed to comprehensively assess the influence of biogas slurry application from multiple perspectives (soil enzyme activities, heavy-metal accumulation, microbial communities, and fundamental nutrients) across different time scales: short-term (3 years). The results demonstrate that the ecological effects of biogas slurry application exhibited a pronounced time-dependency. Specifically, short-term application (less than 1 year) significantly increased soil urease (23.0%) and sucrase activity (22.1%), along with organic matter and available nutrients. However, it also resulted in the rapid accumulation of heavy metals, including Hg and As. Under medium-term application, sucrase activity further surged (108.1%). Meanwhile, phosphatase activity and total potassium content decreased, and heavy-metal accumulation showed declining trends. Following long-term application, microbial richness substantially improved, but certain heavy metals (Cu, Zn and Cd) continued to accumulate. In summary, this study reveals distinct temporal-scale mechanisms by which field biogas slurry application regulates soil functions.