Carbonate Weathering Enhances Nitrogen Assimilatory Uptake in River Networks

Abstract Geological processes such as carbonate and silicate weathering can alter the riverine dissolved inorganic carbon (DIC) concentration and subsequently modify the carbonate chemistry. However, their effects on aquatic nitrogen cycles are usually overlooked. Here we present field measurements of 84 rivers with more than 210 sites across Pearl River Basin (PRB), China in both dry and wet seasons, and the results show that dissolved organic nitrogen (DON) follows the variation of dissolved inorganic nitrogen (DIN) and solar radiation in the carbonated-dominated region during the wet season, while not the case in the non-carbonate-dominated region. We conduct in-situ incubations and show that organic nitrogen production is strengthened in the high DIC region in the wet season with higher temperatures and solar radiation. DON zonation in the dry season is not obvious due to low temperatures that limit nitrogen assimilation. We identify this fertilization effect of carbonate weathering on organic nitrogen formation and find it is not unique in PRB but a global phenomenon. The abundant DIC induced by carbonate weathering eliminates the DIC limitation that can exist in silicate rock regions and boosts the nutrient uptake potential by 16% to 31% for global river networks under excess nutrient availability.