Natural grass cover enhances microbial network stability via soil metal elements in deep black soil

Microbial network stability is a critical indicator for assessing the resilience of soil ecosystems under long term agricultural disturbance. However, how different land use types affect microbial co-occurrence network stability across deep black soil profiles remains insufficiently understood. Based on a 12-year long term field experiment, this study employed a high-resolution grid sampling approach to systematically evaluate the effects of representative land use systems, including two conservation managed orchard systems with black horticultural fabric covering and natural grass cover, repeatedly plowed maize farmland, and undisturbed woodland, on soil physicochemical properties, microbial community composition, and co-occurrence network stability across the 0–120 cm black soil profile. The results showed that forest soils exhibited a more homogeneous vertical distribution of nutrients and more stable microbial networks. Compared with repeatedly plowed maize farmland, the orchard with natural grass cover had higher surface soil alkali-hydrolyzable nitrogen (AHN) and available potassium contents, which were 3.84- and 12.0-fold those in maize farmland, respectively, and its microbial network stability approached that of undisturbed woodland. In contrast, the microbial network stability of the orchard covered with black horticultural fabric was closer to that of conventionally tilled cropland. Mantel tests demonstrated that soil organic matter, pH, AHN, soil respiration rate, Zr, Sr, Pb, and Ca were most strongly associated with microbial community composition, while land use type significantly modulated the coupling between soil properties and microbial communities. Partial least squares path modeling further indicated that soil metal elements exerted the most direct and significant regulatory effects on microbial network stability. In summary, long-term natural grass cover significantly enhanced microbial network stability in black soils, with soil metal elements identified as key driving factors. These findings provide a theoretical basis for elucidating the mechanisms by which agricultural management regulates soil ecological stability.