Composting versus stacking of selenate-supplemented spent mushroom substrate: Effects on soil selenium...

Selenate-supplemented spent mushroom substrate (Se(VI)-SMS) requires stacking or composting pretreatment for safe reuse as a selenium (Se)-enriched organic fertilizer, yet how these pretreatments influence the release dynamics of Se from the resulting fertilizer in soil remains unclear. This study elucidated the mechanisms governing Se transformation in soil amended with differently pretreated Se(VI)-SMS fertilizer by linking the changes in soil Se fractions, dissolved organic matter (DOM) characteristics, and bacterial communities. Compared with the unamended control soil (Se mobility factor, MF = 23.2%), the composted fertilizer amendment reduced the soil Se MF by 12.1%-13.2%, whereas the stacked fertilizer amendment increased it by 16.7%-17.9%. The application of both fertilizers enhanced Se sequestration into the organic matter-bound (OM-Se) and humic acid-bound (HA-Se) fractions, with the composted fertilizer treatment showing greater increases (OM-Se: +15.4%-21.6%; HA-Se: +30.5%-40.1%). Soil DOM analysis revealed that the composted fertilizer promoted DOM aromatization and humification, while the stacked fertilizer primarily increased dissolved organic carbon content. Furthermore, the stacked fertilizer induced stress responses in the soil bacterial community, whereas the composted fertilizer shifted the community's functional profile from stress response to metabolic activity. Co-occurrence network analysis indicated that the composted fertilizer strengthened interactions between stable DOM components and bacterial taxa (e.g., Pseudomonadota) linked to Se transformation. These findings demonstrate that composting pretreatment promotes soil Se stabilization via DOM-microbial interactions, providing a sustainable approach for recycling Se(VI)-SMS into a Se-enriched organic fertilizer that balances Se bioavailability with ecological safety.