Riverine nitrogen export exerts significant influences on coastal marine ecosystems, where excessive nutrient delivery can lead to eutrophication, hypoxia, and other severe ecological problems. Yet the magnitude and composition of nitrogen fluxes over the past century remain poorly constrained due to sparse observational records and the omission of in-stream transformations in most large-scale assessments. Here we developed a process-based riverine nitrogen module within the framework of the Dynamic Land Ecosystem Model (DLEM) to explicitly simulate dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) fluxes. We further applied this model to assess riverine nitrogen loading and export across southern China during 1901–2020, with DIN export calculated from its inorganic components.
        Riverine nitrogen export across southern China increased from about 1.8 Tg N yr⁻¹ in the 1900s to about 2.6 Tg N yr⁻¹ in the 2010s, an increase of roughly 44%. DIN and DON were the two major forms of nitrogen export in recent decades, together accounting for approximately 80% of the total export, with PON contributing the remaining 20%. Over the same period, nitrogen loading to rivers increased substantially, with the most pronounced rise occurring in the inorganic fraction.
        These changes suggest a marked strengthening of nitrogen transfer from land to river networks and downstream waters across southern China over the past century. The particularly rapid rise in inorganic nitrogen loading points to a shift in the form as well as the magnitude of riverine nitrogen transport. This study provides a century-scale process-based estimate of riverine nitrogen loading and export in southern China and offers a basis for further evaluating nitrogen delivery from terrestrial ecosystems to inland and coastal waters.