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-labeled straw degradation experiment using small molecules derived from lignin (LSMs) and humus (HSMs) as activators, investigating their effects on the microbial communities and formation of newly formed mineral-associated (¹³C-MAOM) and particulate (¹³C-POM) organic matter from ¹³C-labeled straw in both sodic and non-sodic soils. The ¹³C-labeled straw was mainly converted into ¹³C-MAOM, accounting for 73.97%–92.67% of the newly formed SOM. Biopolymer-derived small molecules decreased the exchangeable sodium percentage (ESP), but increased contents of ¹³C-MAOM and ¹³C-POM by shifting microbial communities, strengthening microbial cross-trophic interactions, enhancing enzyme activities, and increasing microbial residues in both soils. Addition of HSMs had greater impacts on ¹³C-MAOM formation than LSM addition. The ¹³C-MAOM and ¹³C-POM formation negatively correlated with ESP, but positively correlated with microbial cross-trophic interactions and enzyme activities in both soils. Our results suggest that biopolymer-derived small molecules promote ¹³C-MAOM and ¹³C-POM formation associated with microbial cross-trophic interactions between protistan predators and primary decomposers. Our study provides scientific support for future attempts to stimulate microbial cross-trophic interactions for boosting SOM accumulation under stressed conditions.