Land use overrides climatic controls on soil organic nitrogen transformations

1. Soil organic nitrogen (SON) transformation is critical for global nutrient cycling and ecosystem productivity, yet how its sensitivity to climate change differs across diverse land use types remains poorly resolved.2. Here, we investigated gross protein depolymerization (GPD), microbial growth, gross N mineralization (GNM), and microbial N use efficiency (NUE) in paired forest and cropland soils along a significant climatic gradient in subtropical China. We aimed to determine the differential climate sensitivity of these SON transformations and elucidate the distinct biogeochemical controls.3. Forest soils exhibited 82% higher GPD and 132% higher microbial N growth rates, alongside 26% higher NUE than cropland soils, despite comparable GMN rates. Critically, SON transformations in forests were highly sensitive to climate with elevated mean annual temperature and precipitation generally enhancing GPD, microbial growth, and GMN, but reducing NUE. In contrast, cropland SON transformations showed markedly dampened responses, with only minor increases in microbial growth and NUE observed with higher temperature. Mechanistic analyses elucidated distinct regulation pathways. In forests, climate indirectly influenced SON transformations via mineral-enzyme interactions (e.g., iron/aluminum oxides modulating protease activity) and resource stoichiometry (e.g., dissolved organic carbon: available phosphorus ratios). GPD was tightly coupled to microbial growth and GMN, acting as a rate-limiting step. In croplands, temperature effects were indirect, mediated through a cascade involving base cation to iron/aluminum oxides ratios, resource availability (free amino acid, carbon to N ratios), and microbial functional gene abundances, with SON transformation processes largely decoupled.