Nutrient stoichiometry and limitations shift as cyanobacterial inoculation-induced biocrusts develop

Cyanobacterial inoculation has been recognized as a promising strategy for promoting the development of biological soil crusts (biocrusts) and combating desertification. However, the long-term shifts in enzyme dynamics, nutrient limitations, and stoichiometric balance during induced biocrust development remain poorly understood. In this study, we quantified changes of carbon (C), nitrogen (N) and phosphorus (P) contents, corresponding stoichiometric ratios, and enzyme activities in both biocrusts and subsurface layers soil at long-term restoration sites in the Qubqi Desert, where cyanobacteria were inoculated approximately two decades ago. The induced biocrust communities underwent a clear successional trajectory, from shifting sand to cyanobacteria- and finally to moss-dominated types, accompanied by substantial increases in organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and the major stoichiometric ratios (C: N, C: P, and N: P) in both biocrusts and subsurface soils. Vector analysis of enzyme activities revealed a shift in microbial nutrient demand along with induced-biocrust development, from N limitation to P limitation or co-limitation of C and P, while the subsurface soils remained N limited. Assessments of stoichiometric imbalance between biocrust and subsurface soils indicated a growing vertical nutrient stratification and decoupling with increasingly pronounced C: P and N: P imbalances as induced biocrust development progressed, suggesting increased P demand and unique P-related metabolic processes within developed biocrusts. Our findings highlight that cyanobacterially induced biocrusts not only enrich soil fertility and alter nutrient limitations but also create vertical heterogeneity in soil stoichiometry. All these findings underscore the importance of understanding P cycling within biocrust ecosystems to support the long-term stability and sustainability of biocrust-based desert soil restoration practices.