Plant-derived high-C:P amendments enhance legacy phosphorus utilization in phosphorus-enriched greenhouse soil via stoichiometric regulation and microbial functional coupling.

Long-term intensive fertilization has accumulated large legacy phosphorus (P) pools in greenhouse vegetable soils, while substituting chemical fertilizers with livestock manure may exacerbate soil P loading. We evaluated pakchoi growth and legacy P utilization in an acidified, P-enriched greenhouse soil, comparing an unfertilized, unamended control (CK) with equal-carbon inputs of low-C:P, high-P chicken manure (OF) and high-C:P, low-P plant-derived amendments: alkali lignin (AL), straw (ST), and alkali-lignin-modified straw (AST). Compared with the OF, shoot dry weight was 47.0% and 12.4% higher under ST and AST, respectively, while shoot P uptake increased by 80.2% and 76.8%. OF produced an apparent P surplus, whereas ST and AST generated negative apparent P balances, indicating current-season amendment P inputs could not offset shoot P removal and supporting enhanced legacy P utilization without further accumulation of available or labile inorganic P. Plant-derived amendments increased soil C:P and pH, generally enhanced C- and N-acquiring enzyme activities, and reduced ecoenzymatic vector angles, alleviating relative microbial P-acquisition limitation. Metagenomic and random forest analyses showed that enhanced P acquisition was not primarily linked to a single P-cycling process, but more closely associated with resource stoichiometry and soluble N status, alongside coordinated restructuring of microbial C degradation, N transformation, and P-cycling functions. Overall, AL most strongly ameliorated acidity but elicited weak plant responses; ST produced the strongest biomass response, while AST maintained comparably high P acquisition and markedly improved soil pH. These high-C:P amendments provide a low-P alternative to high-P manure inputs, offering a strategy for reduced-P management and legacy P utilization.