Divergent nutrient limitations and stoichiometric patterns between cyanobacterial crusts and underlying soil along an aridity gradient in northwest China

Drylands play a crucial role in global biogeochemical cycles, yet how increasing aridity affects elemental stoichiometry in biocrusts and soils remains unclear. This study investigated carbon (C), nitrogen (N), and phosphorus (P) stoichiometry in cyanobacterial crusts and underlying soil along a precipitation gradient in northwestern China. Cyanobacterial crusts exhibited significantly higher average contents of C (1608.81 mmol kg−1), N (97.10 mmol kg−1), and higher molar C:P (114.5) and N:P (7.1) ratios, but lower C:N (16.7) than the underlying soil (C:P = 74.5; N:P = 2.8; C:N = 29.0). Scaling analysis revealed isometric C-N coupling in cyanobacterial crusts (slope = 1.03), but allometric scaling in soil (C > N = P). Our results confirmed that cyanobacterial crusts were primarily P-limited, as evidenced by their significantly higher C:P and N:P combined with low P content, whereas underlying soil was co-limited by N and P, indicated by its elevated C:N ratio, low N and P contents, and low N:P ratio. Aridity explained over 70% of variance in crusts C, N, P, C:P, and N:P, whereas soil stoichiometry was primarily driven by crusts P content (explaining >45% variance). These findings highlight the distinct stoichiometric roles of cyanobacterial crusts in drylands and underscore their importance in stabilizing surface substrates and modulating soil nutrient balance under increasing aridity.