Cultivation Age Drives Soil Organic Nitrogen Transformation in Vineyards via Reshaping chiA-Harboring Microbial Communities and Hy-Drolase Activities

Long-term grape cultivation alters soil acid–base properties and nitrogen transformation processes, yet the mechanisms underlying the changes in soil physical and chemical properties as well as nitrogen components remain unclear. In this study, four grape vineyards with cultivation ages of 0.5 (newly established, Y0.5), 4 (Y4), 16 (Y16), and 22 years (Y22) in Penglai, Yantai, China, were selected as research subjects. Soil nitrogen pools, key nitrogen hydrolase activities, and chiA gene community assembly, as well as their internal regulatory relationships were investigated. The results revealed that cultivation age significantly reshaped soil nitrogen speciation: Y16 vineyards exhibited the highest concentrations of total nitrogen, mineral nitrogen, microbial biomass nitrogen, and labile hydrolyzable ammonium nitrogen (HAN), representing a critical inflection point of soil nitrogen supply capacity. With prolonged cultivation to 22 years, soil nitrogen pools shifted toward recalcitrant stable organic nitrogen (stable-SON), accompanied by significant accumulation of amino sugar nitrogen (ASN). Activities of N-acetyl-β-D-glucosaminidase (NAG), urease, amidase and leucine aminopeptidase were markedly suppressed in Y16 soils. Both Chao1 richness and Shannon diversity indices of chiA gene communities reached the minimum values at 16 years, and PCoA confirmed distinct community separation of Y16 from other age groups. Soil pH, ammonium nitrogen and nitrate nitrogen were the dominant environmental filters governing chiA community composition. Actinomycetota and Pseudomonadota dominated the chiA-harboring microbiota, with acid-tolerant Streptomyces significantly enriched in Y16, while keystone genera Nonomuraea and Pseudoxanthomonas mediated the conversion between labile and stable organic nitrogen pools. Redundancy analysis demonstrated that chiA community composition (25.5% explanation), chiA α-diversity (19.3% explanation), and NAG activity (14.6% explanation) were the primary drivers of soil organic nitrogen fraction variation. Results support a four-level cascading framework wherein stand-age-induced soil acidification and altered N inputs drive shifts in chiA-harboring microbial diversity and community structure, which in turn modulate nitrogen hydrolase activities and ultimately govern the reciprocal transformation between labile and stable organic nitrogen pools, resulting in stage-specific nitrogen supply capacity. This study clarifies the microbial gene–enzyme–nitrogen pool coupling mechanism in chronosequence vineyards and provides theoretical guidance for targeted nitrogen management of aged vineyards.