Estimating lateral nitrogen transfer through the global river network using a land surface model

Abstract. Lateral nitrogen (N) transport from land to oceans through rivers is an important component of the global N cycle. We developed a new model of this system, called ORCHIDEE-NLAT, which simulates the routing of water in rivers, and the pertaining transport of dissolved inorganic N (DIN), dissolved organic N (DON) and particulate organic N (PON) as well as the accompanying biogeochemical processes of decomposition for DON and PON, and denitrification for DIN during the transit from land to oceans through the river network. Evaluation against global observation-based datasets reveal that the model captures both the magnitude and seasonal variations of riverine water discharges and total nitrogen (TN) flows well. The ORCHIDEE-NLAT model was then applied to reconstruct the historical evolution of global TN flows from land to rivers, as well as the denitrification of DIN within the river network. Due to anthropogenic activities (e.g. mineral fertilisers and manure application, sewage water injection in rivers and land use change) and indirect climate and CO₂ effects, the TN exports are modelled to increase from 27.1 Tg N yr^-1 over 1901–1910 to 40.8 Tg N yr^-1 over 2001–2014, with DIN (80 %) contributing most of this increase. The annual mean TN flow and DIN denitrification rates show substantial spatial heterogeneities. The seasonal amplitude of TN flow is of similar magnitude as the large-scale spatial variability. Compared to previously published global aquatic N transfer models (IMAGE-GNM, FrAMES-N, MBM, DLEM and Global NEWS2), our model produces similar global and continental-scale TN exports to the ocean, but shows distinct patterns at finer scale spatial scales (e.g. basin scale). ORCHIDEE-NLAT could also be coupled with other land surface models such as those used in the Nitrogen Model Intercomparison Project (NMIP). Our model provides a full simulation of N transport and reactivity from soils to oceans at an unprecedented spatio-temporal resolution (daily fluxes at 0.5° globally).