Water–Nutrient Co‐Limitation Shapes Soil Phosphorus Availability and Microbial Functional Traits in Semi‐Arid Grasslands

ABSTRACT Phosphorus (P) transformation in soil is jointly governed by abiotic conditions and microbial functional genes, particularly in semi‐arid grassland ecosystems where water and nutrient limitations restrict productivity. However, the integrated effects of P, nitrogen (N), and water addition on soil P fractions and microbial P‐cycling genes remain poorly characterized. In this study, we evaluated the responses of inorganic and organic P fractions and the abundance of functional P genes ( phoD, gcd, pqqC, ppx, phnK ) and their interactions under five treatments implemented over 18 years in a semi‐arid grassland in North China. The treatments included: control (CK), P addition (P10), N + P addition (NP), water + P addition (WP10), and water + N + P addition (WNP). Results revealed that NaHCO 3 ‐Pi was significantly increased under WNP (17.71 mg kg −1 ), followed by WP10 (16.10 mg kg −1 ), P10 (15.04 mg kg −1 ), NP (11.40 mg kg −1 ) and CK (5.53 mg kg −1 ), while NaOH‐Pi was highest under WNP (96.85 mg kg −1 ), followed by WP10, P10, NP and CK, indicating enhanced inorganic P availability with water addition. For organic P, NaHCO 3 ‐Po peaked under WNP (46.34 mg kg −1 ), followed by WP10, and P10, whereas NaOH‐Po showed a similar trend, with the highest concentration in WNP (64.52 mg kg −1 ), then WP10 (58.01 mg kg −1 ) and P10 (49.75 mg kg −1 ). In contrast, HCl‐Pi and residual P were greatest under P10, suggesting that P‐only application promotes accumulation in recalcitrant pools. Gene abundance patterns mirrored P availability: both phoD (involved in organic P mineralization) and gcd (responsible for solubilizing inorganic P) were highest under WP10, aligning with increased labile P fractions, while pqqC was downregulated under enriched conditions (WNP < NP < WP10 < P10 < CK). phnK , associated with phosphonate metabolism, peaked in WP10, whereas ppx was maximally expressed in P10. Redundancy analysis (RDA) demonstrated strong alignment of phoD and phnK with labile P forms (NaHCO 3 ‐Pi, NaHCO 3 ‐Po), while pqqC and ppx correlated with total and stable P pools. Mantel correlations confirmed that WP10 and WNP treatments exhibited the strongest positive associations between microbial gene abundance, bioavailable P fractions, microbial biomass phosphorus, carbon, nitrogen (MBP, MBC, MBN), and trace elements (Zn, Fe, Mn). Collectively, our findings show that combined water and nutrient inputs synergistically enhance labile P pools and stimulate functionally diverse microbial pathways, thereby improving phosphorus turnover in semi‐arid grassland soils. This study provides mechanistic insight into the interactions between nutrient management and microbial functional dynamics, offering guidance for sustainable soil fertility practices under dryland conditions.