Global inland-water nitrogen cycling has accelerated in the Anthropocene

Inland waters are an important component of the global nitrogen (N) cycle, functioning not only as land-to-sea transporters but also as active biogeochemical reactors. However, the latter role is not well understood regarding mechanisms, quantities or on a global scale. It remains unclear whether, when, how and why global inland-water biogeochemical N cycling has changed. Here we analyse the dynamic global inland-water N cycling processes in the Anthropocene by quantifying the long-term changes in different N forms, including their inputs to inland waters, transformation pathways, retention within inland waters, and river export to oceans. Using a spatially explicit, mechanistic, coupled hydrology and biogeochemistry model, we show that, during 1900–2010, the increase in total nitrogen (TN) river loading (from 27 to 68 Tg yr−1) resulted in an increase in TN export to oceans (from 20 to 42 Tg yr−1), despite an increase in inland-water retention (from 25% to 39%) primarily due to gaseous loss and burial. Moreover, the relative contributions of ammonium (NH4+), nitrate/nitrite (NOx−) and organic nitrogen (ON) changed because of threefold increases in global inland-water mineralization (transforming ON to NH4+) and N burial in sediments, a fourfold increase in nitrification (transforming NH4+ to NOx−) and a sixfold increase in denitrification (transforming NOx− to mainly N2). This Article presents a comprehensive analysis of the dynamic global inland-water N cycling processes using a coupled model of hydrology, nutrient loading and biogeochemical transformation, showing that N export increased more slowly than loading due to increased inland-water retention via enhanced transformation and burial.