Effects of a Diatom–Bacillus megatherium Biocrust on Nutrient Limitation and Ryegrass Growth in Fluvo-Aquic Soil Along the Yellow River

Biological soil crusts (biocrusts) promote plant growth by regulating soil nutrient dynamics and enhancing soil structure through the microorganisms they host. However, their impact on microbial nutrient limitation, a critical factor in nutrient cycling, remains underexplored. This study hypothesized that different types of biocrusts modulate soil nutrient limitations, influencing plant growth. A pot experiment was conducted to evaluate the effects of four treatments—control, diatom, Bacillus megatherium, and diatom–B. megatherium biocrusts—on soil structure, nutrient availability, microbial nutrient limitation, and ryegrass growth after 40 days of cultivation. The results indicated that the B. megatherium treatment exacerbated microbial C and N limitations and reduced available phosphorus (by 41.80%) and ryegrass biomass (by 29.19%) compared to the control. The diatom-B. megatherium treatment alleviated nutrient limitations but increased nutrient competition between soil microbes and plants, impairing plant performance. In contrast, the diatom treatment enhanced soil structure, alleviated microbial nutrient limitations, and significantly improved total capillary porosity (by 10%), available phosphorus (by 22.91%), saturated water content (by 21.81%), and ryegrass biomass (by 76.05%) while reducing soil bulk density (by 9.63%). These findings provide practical insights and a theoretical foundation for utilizing biocrusts to improve fluvo-aquic soil quality and promote sustainable plant growth.