Synergistic effects of straw and earthworm addition on microbial diversity and microbial nutrient limitation in a subtropical conservation farming system

Conservation farming systems with straw return and no-tillage are recommended as effective methods for soil quality improvement, and they are often accompanied by increases in soil fauna populations. Recently, attempts have been made to improve the soil fertility in conservation agriculture through enhanced bioturbation by adding earthworms to no-till fields. However, the mechanisms associated with the responses in terms of soil functions, such as microbial diversity and microbial nutrient metabolism, to the synergistic effects of biotic and abiotic factors remain unclear. In this study, plow layer and plow pan soils were collected from subtropic cropland, and a full factorial microcosm experiment was conducted to quantify the effects of straw mulching and adding earthworms on the soil nutrient contents, microbial diversity, and extracellular enzymatic activities. The results showed that the single application of straw significantly increased the soil organic carbon (C), the ratio of C to nitrogen (N), and the ratio of C to phosphorus (P) in the plow layer soil (p < 0.05). The combined addition of straw and earthworms significantly increased the microbial diversity (Observed species, Chao 1 and Shannon), C/N/P-acquiring enzymatic activities, and microbial C limitation (p < 0.05), but decreased the microbial P limitation (p < 0.05) in the plow pan soil. Adding straw or earthworms did not affect the soil nutrient content in the plow pan soil (p > 0.05), microbial diversity, C-acquiring enzymatic activities, and microbial C and P limitations in the plow layer soil (p > 0.05). Structural equation models indicated that straw mulching indirectly affected the soil microbial diversity and microbial nutrient limitation through the buffering effects of aggregate stability and the C/P ratio in the plow layer soil. By contrast, the soil microbial activities were directly influenced by straw and earthworm addition in the plow pan soil. These findings indicate that earthworms enhanced soil microbial metabolism when straw was added by improving the substrate availability, and the soil physicochemical properties determined the asynchronous responses of the soil functions to land management. We suggest that the combined addition of organic matter and earthworms can improve the quality of soil in infertile land.