Microbiological mechanisms of lignin- and humus-derived small molecule addition promoting straw conversion into soil organic matter in a sodic soil

Straw return is the main practice used to increase soil organic matter (SOM) in agricultural ecosystems. To increase the efficiency of straw conversion to SOM, a large number of microbial inoculants have been developed. However, their effects are poor because of the complex water and temperature conditions, especially under sodic conditions. Small molecules can rapidly shift soil microbial communities and improve their ability to transform exogenous organic matter into SOM, providing a new direction for promoting high-efficiency straw conversion into SOM. In this study, we conducted a 13C-straw degradation experiment using lignin- (LSM) and humus-derived small molecules (HSM) as activators, investigating their effects on the microbial communities and formation of new mineral-associated (13C-MAOM) and particulate organic matter (13C-POM) in both sodic and non-sodic soils. 13C-straw was mainly converted into MAOM, accounting for 73.97%–92.67% of the newly formed SOM. Biopolymer-derived small molecules decreased the exchangeable sodium percentage (ESP). In addition, biopolymer-derived small molecules increased13C-MAOM and 13C-POM through shifting the microbial communities, strengthening microbial cross-trophic interactions, enhancing enzyme activities, and increasing microbial residues in both soils. HSM had greater impacts on 13C-MAOM formation than LSM. 13C-MAOM and 13C-POM formation negatively correlated with ESP, and positively correlated with microbial cross-trophic interactions and enzyme activities in both soils. The results suggest that biopolymer-derived small molecules promote 13C-MAOM and 13C-POM formation associated with trophic interactions between protist predators and primary decomposers. Our study provides scientific support for future attempts to stimulate microbial trophic interactions for boosting SOM accumulation under stress conditions.