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.1. 土壤有机氮（Soil Organic Nitrogen, SON）转化过程对全球养分循环与生态系统生产力至关重要，但目前对于其对气候变化的敏感性在不同土地利用类型间的差异仍未得到充分阐明。2. 本研究针对中国亚热带地区沿显著气候梯度分布的成对森林与农田土壤，探究了总蛋白质解聚作用（Gross Protein Depolymerization, GPD）、微生物生长、总氮矿化作用（Gross N Mineralization, GNM）以及微生物氮利用效率（Microbial N Use Efficiency, NUE），旨在明确这些SON转化过程的气候敏感性差异，并阐明其独特的生物地球化学调控机制。3. 尽管总氮矿化速率相当，但森林土壤的总蛋白质解聚速率、微生物氮生长速率分别较农田土壤高出82%与132%，微生物氮利用效率也提升了26%。尤为关键的是，森林土壤中的SON转化过程对气候响应极强：年平均气温与降水量升高通常会促进总蛋白质解聚、微生物生长与总氮矿化，但会降低微生物氮利用效率。与之相反，农田土壤的SON转化过程响应显著减弱，仅在气温升高时观测到微生物生长与氮利用效率的小幅提升。机制分析揭示了二者截然不同的调控路径：在森林土壤中，气候通过矿物-酶相互作用（例如铁/铝氧化物调控蛋白酶活性）与资源化学计量比（例如溶解性有机碳与有效磷的比值）间接影响SON转化过程；总蛋白质解聚与微生物生长、总氮矿化紧密耦合，是速率限制步骤。而在农田土壤中，温度的影响是间接的，通过碱基阳离子与铁/铝氧化物比值、资源有效性（游离氨基酸、碳氮比）以及微生物功能基因丰度级联介导，且SON转化过程整体呈现解耦状态。