Global pattern and drivers of soil soluble organic nitrogen

Soil soluble organic nitrogen (SON) is a key hub in terrestrial nitrogen cycle, which can be converted to inorganic nitrogen by mineralization and formed from inorganic nitrogen by microbial metabolism. Meantime, SON is also one of the significant sources of nitrogen pollution contributing to eutrophication in water bodies within watersheds due to its high solubility and mobility. Despite its undeniable importance, the global distribution and drivers of SON are still a huge debate due to the scarcity of extensive data. To address this gap, a comprehensive global dataset of SON from 384 published studies was established, which encompassed 5782 subsoil samples (0-30 cm). Machine learning methods were employed to quantify the contributions of environmental factors to SON and subsequently predictions at the global scale. Overall, global SON concentrations varied significantly under various climate, ecosystems, vegetation types, and soil properties and ranged from 0.04 to 1034 (mean 41.36) mg kg-1. The 82% variation of SON concentrations was explained by the 11 selected variables, among which altitude, bulk density, mean annual precipitation, and parent material were the primary factors. Additionally, the generated map of global SON concentrations revealed a latitudinal pattern in which SON concentrations gradually increased from low to high latitudes. The global SON stocks were estimated to be 2.4 Pg. The database is not only of great significance for understanding the nitrogen cycle in terrestrial ecosystems but also provides valuable data support for the improvement of biogeochemical models.