Abstract: Phosphorus (P) is vital for plant responses to climate change, influencing primary productivity. However, the seasonal variations of foliar phosphorus fractions (FPFs) in deep-rooted desert species are not well understood, especially in hyper-arid ecosystems with P-impoverished soils. Therefore, we investigated seasonal dynamics of Alhagi sparsifolia seedlings under four P-supply gradients (0, 1, 3, and 5 g P m^-2 y^-1) in a two-year open-air pot experiment. We quantified concentrations and proportions of key functional phosphorus forms (FPFs)—metabolic P, structural P, nucleic acid P, and residual P—alongside leaf traits and soil properties across spring, summer, and autumn. Findings showed notable seasonal variations: soil available P concentration decreased by 48.8% in summer compared to spring and autumn, while leaf total P concentration increased by 1.9%. Autumn exhibited higher specific leaf area, leaf P content per unit area, and mass compared to spring and summer. Additionally, metabolic, structural, and residual P concentrations rose significantly with increasing soil P supply, while nucleic acid P decreased. FPF allocation varied seasonally, with summer showing increases in metabolic P (1.6% concentration, 1.9% allocation), nucleic acid P (5.0%, 4.2%), and structural P (4.0%, 2.9%) compared to spring. Similar comparisons with autumn revealed greater summer increases of 3.3% (0.4%), 7.7% (2.7%), and 5.5% (2.5%) respectively. FPF dynamics correlated more strongly with leaf traits than soil properties in spring and summer, but autumn weakened these associations. This study highlights the seasonal allocation patterns and their role in the adaptation of desert plants to poor-P conditions.

Key words: desert ecosystem, seasonality, phosphorus limitation, leaf traits, soil phosphorus availability, Taklimakan Desert