The Effect of Biochar on Soil Nutrients and Microorganisms During Reclamation of Karst Sloping Farmland That Has Been Abandoned for 20 Years

Abandoned slopes often encounter problems such as compacted soil and lack of nutrients. Biochar, as a promising soil amendment agent, can effectively enhance soil fertility. Moreover, evaluating the nutrient and microbial characteristics during the improvement process is of great significance for revealing its mechanism of action in improving abandoned land. This study analyzed the characteristics of soil nutrients, microbial community structure, and co-occurrence network after reclamation under different application rates (0%, 1%, 2.5%, 4%, 5.5%, hereinafter referred to as CK, T1, T2, T3, T4) of corn straw biochar. The results showed that biochar significantly increased soil organic carbon (by 60.74–164.82%), total nitrogen (11.31–27.73%), and total phosphorus (13.32–56.03%) content, and the effect was best at a rate of 4% (T3). With the increase in biochar application rate, soil bulk density generally showed a downward trend, and pH generally showed an upward trend. Significant levels ( p < 0.05) were reached from T2 to T4. There was a strong linear correlation between biochar application rate and soil organic matter, total nitrogen, and pH in the fitted model, with R 2 values reaching 0.753, 0.601, and 0.706, respectively. Microbial community analysis showed that biochar application changed the bacterial community structure. With the increase in soil depth, the Shannon index and Chao index of each treatment generally increased, indicating that soil depth is one of the key factors regulating the community structure. Biochar application promoted the proliferation of beneficial bacterial groups such as Pseudomonadota and Acidobacteriota, by increasing the number of co-occurrence network nodes and edges enhancing the complexity and stability of the microbial network.