Long‐Term Nitrogen Addition Weakens the Linkage Between Phosphorus Cycling Gene Abundance and Phosphatase Activity Through Soil Acidification in a Larix gmelinii Plantation

ABSTRACT Anthropogenic nitrogen (N) enrichment propels global forest ecosystems toward phosphorus (P) limitation, theoretically triggering the upregulation of microbial P‐acquisition capacity to maintain stoichiometric balance. However, the extent to which N addition regulates this adaptive capacity remains uncertain. Here, leveraging an 8‐year N addition experiment in a larch ( Larix gmelinii ) plantation, we measured soil biochemistry, enzyme activities and microbial functional gene abundances involved in organic P mineralization to examine the microbial mineralization responses. Nitrogen addition caused significant soil acidification, reduction in 16S rRNA gene abundance and activities of enzymes hydrolyzing organic P, but increases in the relative abundance of P‐acquisition functional genes. Relative to the control plots, soil pH significantly reduced from 5.0 to 4.6, microbial biomass carbon and gene copy number of 16S rRNA reduced by 26.6% and 35.5%, acid phosphatase and phytase activities reduced by 32.6% and 51.1%, whereas relative abundance of appA gene increased by 29.2% in the N addition plots. Redundancy analysis identified that variation in soil pH explained 57.5% of N‐induced changes in organic P mineralization. Crucially, these findings decouple the assumed positive correlation between genetic potential and functional realization, revealing a stress‐induced functional mismatch that challenges the applicability of stoichiometric theory under severe acidification stress. Furthermore, it highlights a pitfall of relying solely on metagenomic metrics for diagnosing soil health. Consequently, promoting soil acid‐buffering capacity is essential to prevent functional decline of soil microorganisms in forest ecosystems in response to atmospheric N deposition.