Phosphorus Limitation Governs N:P Stoichiometry in Semi-Arid Shrublands: Evidence From Organ, Plant and Community Scales.

Understanding plant community adaptation in semi-arid ecosystems is crucial for predicting their resilience under ongoing climate change. We conducted a multi-scale investigation of nitrogen (N) and phosphorus (P) stoichiometry in 28 shrub ecosystems in northwestern China, a fragile region facing increasing aridity. Our analyses spanned plant organs, plant individuals, and the entire community, encompassing shrubs, herbs, litter, and soils. The community baseline stoichiometry averaged 12.18 g/kg N, 0.84 g/kg P, and an N:P ratio of 16.64. Phosphorus limitation was evident across all biological scales: foliar N:P ratios depended more on P than on N availability, and soil P stoichiometry correlated with multi-scale N:P patterns. Vegetation displayed coordinated adaptive nutrient-use strategies, with photosynthetic tissues having 49%-253% higher N or P concentrations than non-photosynthetic organs, and dominant shrubs exhibiting 37% higher foliar P than accompanying species. Mean annual precipitation (MAP) regulated plant community N:P stoichiometry through dual pathways: a direct effect on soil N:P ratios and an indirect effect mediated by vegetation restructuring. Our findings demonstrated that precipitation was the overarching environmental factor that shaped and modulated phosphorus limitation in these ecosystems, with soil P availability acting as the proximate limiting factor. This multi-scale perspective elucidated how plant communities adapt stoichiometrically to environmental constraints, offering a mechanistic basis for incorporating P limitation into the restoration of vulnerable dryland ecosystems.