Responses of soil bacterial composition and network complexity to O₃ stress and straw return in a soybean cropland

Soil microbial community and function play a key role in root nutrient acquisition and growth. Yet, the combined effects of O₃ stress and straw return on soil microbial composition and network complexity remain unclear. Here, a pot experiment was conducted in open-top chambers to monitor the responses of soil microbial diversity, composition, and network complexity at branching and podding stages of soybean (Glycine max; a species highly sensitive to O₃ stress) to O₃ stress (45 ± 5 ppb; 80 ± 10 ppb; and 110 ± 10 ppb) and straw treatment (no straw return and straw return). Under O₃ stress at 110 ± 10 ppb, SOC and microbial biomass C and N significantly increased at the branching stage and decreased at the podding stage. Under O₃ stress, the Chao1 and PD indices, along with the number of observed species, were significantly increased by straw return. The bacterial network complexity was negatively affected by O₃ stress (80 ± 10 ppb) and positively affected by straw return. Microbial diversity (Chao1, Shannon, and PD indices) showed greater total standard effects than the factors of soil stoichiometry (C/N, C/P, and N/P ratios). Straw return had the lowest total standard effect on microbial network community and complexity than the other factors. Collectively, the slight alleviating effect of straw return on the reduced microbial topology induced by O₃ stress was mainly associated with bacterial functions rather than soil physicochemical properties.