Effects of Biological Soil Crust Development on Extractable Nutrient Fractions in Adjacent Surface Soils of the Gurbantunggut Desert

Biological soil crusts (BSCs) are important components of dryland ecosystems, yet nutrient patterns in adjacent uncovered soils remain unclear. We surveyed 70 sites across the Gurbantunggut Desert and collected 209 composite samples from the adjacent 0–5 cm surface-soil layer, categorized into three groups: uncrusted bare sand (n = 32), soil adjacent to algal–lichen crusts (n = 48), and soil adjacent to moss crusts (n = 129), with bare sand serving as the uncrusted reference category. The results showed that: (1) total nitrogen differed among the three BSC-associated soil categories (p = 0.041), whereas soil organic carbon, total phosphorus, and total potassium did not (all p > 0.05); (2) NO3−-N, NH4+-N, extractable inorganic N, NaHCO3-extractable phosphorus, and NH4OAc-extractable potassium differed significantly among categories (all p < 0.001) and were generally highest in soil adjacent to moss crusts; and (3) random forest models explained approximately 38–65% of nutrient variation and identified EC, total nitrogen, and the site-level BSC metric as the leading predictors. The final piecewise structural equation model explained 35.3–76% of nutrient variation. These findings indicate marked regional heterogeneity in extractable nutrient fractions associated with different BSC types.