Divergent assembly mechanisms mediate plant stoichiometric responses to soil P constraints during ecosystem restoration

Stoichiometric coupling within plant-soil-microbial systems underpins ecosystem development. However, the assembly mechanisms governing these interactions during restoration in resource-limited environments remain poorly understood. We investigated carbon (C), nitrogen (N), and phosphorus (P) stoichiometry across plant, soil, and microbial pools along a restoration chronosequence in a semi-arid sandy ecosystem. Our results revealed an intensifying relative P constraint driven by disproportionate resource accumulation: biologically mediated N accrual outpaced geochemical P supply. Alongside shifts in ecoenzymatic stoichiometry, microbial biomass P declined in the fixed dune and sparse forest-grassland stages despite rising soil total P, a decline negatively associated with soil water content. Regarding plant responses to this belowground constraint, while species identity explained the largest share of leaf stoichiometric trait variation (42.8%–63.3%), community-level assembly followed divergent pathways. Variance partitioning revealed that species turnover dominated evolutionarily constrained structural traits (leaf C, 71.7%) and environmentally responsive N investment (leaf N, 45.8%). Conversely, for P-related traits, negative covariance between species turnover and intraspecific trait variation (ITV) contributed −38.1% for leaf P and −23.7% for leaf C:P, counterbalancing turnover-driven changes in these traits. Leaf N:P was co-regulated by turnover (15.26%) and ITV (15.32%). These strategies allowed the plant community to mitigate belowground P constraints on metabolic stoichiometry. Our findings suggest that plant communities respond to geochemical constraints through trait-specific assembly pathways: structural investment is filtered by species replacement, whereas P-related traits reflect counterbalancing turnover–ITV contributions. Thus, the capacity of plant communities to buffer against biogeochemical constraints may depend on both species composition and ITV-driven functional flexibility.