Global pattern and drivers of soil soluble organic nitrogen

Soil soluble organic nitrogen (SON) is a central component of the terrestrial nitrogen cycle, acting as both a precursor to inorganic nitrogen via mineralization and a product of microbial assimilation of inorganic forms. Due to its high solubility and mobility, SON also represents a critical pathway for nitrogen loss and is a key contributor to nitrogen pollution and eutrophication in downstream aquatic ecosystems. Despite its ecological significance, the global distribution and environmental drivers of SON remain poorly understood, largely due to a lack of spatially explicit data. To address this gap, we compiled a comprehensive global SON dataset comprising 5,782 topsoil samples (0-30 cm depth) from 379 published studies, including detailed information on geographic location, climate, and soil properties. Overall, the global SON concentrations ranged from 0.04 to 1034 mg kg^-1, with an average concentration of 41.36 mg kg^-1. The random forest model could explain 82% of the variations in SON concentrations. Elevation, parent material, and mean annual precipitation emerged as the most influential predictors. Machine learning was used to quantify environmental contributions to SON and predict its global stock, estimated at 2.4 Pg N. The resulting global SON map revealed a distinct latitudinal gradient, with SON concentrations increasing toward higher latitudes. In addition, meta-analysis showed that except for reduced precipitation, biochar, and no-tillage, other global changes increased global SON to varying degrees (-4.45% – 70.71%). This study provides the first global map of SON distribution and identifies key biophysical and environmental controls of SON dynamics.