Plant litter crust enhances nitrogen accumulation by regulating microbial diversity and urease activity in semi-arid sandy soils

Plant litter plays a crucial role in regulating soil nitrogen (N) cycling in dryland ecosystems. However, the mechanisms by which plant litter crust drives microbial community composition to influence N levels in sandy soils remain unclear. In this study, we examined the effects of litter crust on sandy soil microhabitat characteristics (temperature, moisture, and porosity), microbial diversity and composition, urease activity, and soil N variables (total-N, nitrate-N, and ammonium-N) across different stages of litter crust development (including early-, mid-, and post-term) in a field setting. We found that litter crust promotes bacterial and fungal alpha diversity in sandy soils, leading to a shift in bacterial community composition from oligotrophs (i.e., Actinobacteria) to copiotrophs (i.e., Proteobacteria and Bacteroidetes), and a shift in fungal community composition dominance from Ascomycota to Basidiomycota. Litter crust enhances the complexity and stability of bacterial and fungal co-occurrence networks in sandy soils, especially in early- and mid-term stages of crust development. Additionally, litter crust increases nitrification (aerobic ammonia oxidation) and decreases denitrification (nitrate reduction) in these soils. Notably, the increase in soil moisture and urease activity, along with the decrease in soil temperature due to litter crust, effectively promotes N accumulation in sandy soils. Our results demonstrate that N levels during the litter crust period are mainly influenced by a combination of bacterial beta diversity and fungal alpha diversity at the 0–5 cm depth, and bacterial alpha diversity along with soil properties (i.e., soil variables and urease activity) at the 5–10 cm depth. Overall, our results reveal that litter crust contributes to N accumulation in sandy soils by regulating bacterial and fungal community composition and diversity, as well as by buffering soil temperature and soil moisture, and enhancing urease activity. These findings provide new insights into the critical role of soil microbes in maintaining N functions during litter crust development in semi-arid sandy ecosystems.