Substantial effort has focused on understanding spatial variation in dissolved inorganic nitrogen (DIN) export to the coastal zone and specific basins have been studied in some depth. Much less is known, however, about seasonal patterns and controls of coastal DIN delivery across large spatial scales. Understanding sub-annual patterns and controls of catchment DIN export is critical to efforts to predict and mitigate impacts of coastal N delivery such as algal blooms and hypoxic areas, which are often seasonal phenomena. In addition, seasonality of DIN export may be more sensitive to climate change than annual average DIN export due to anticipated shifts in the timing of water transport and agricultural practices. Here we describe, evaluate and apply the first global, spatially explicit model capable of predicting seasonal DIN export by rivers to coastal regions as well as the sources and sinks for that DIN (NEWS2-DIN-S). For model development and calibration, we compiled a dataset of observed seasonal DIN export and key catchment characteristics for 76 major world rivers. To drive NEWS2-DIN-S, we developed seasonal, spatially explicit N input budgets for >6,000 large river basins.
Results/Conclusions
We found that observed catchment DIN transport efficiency was positively related to runoff and negatively related to temperature, due likely to flushing effects and increased retention by plants and soils, respectively. Agricultural input was the largest source of N during growing seasons in North America, Europe, Asia, and Australia. Atmospheric deposition was most important in northern latitudes during winter. Global model runs of NEWS2-DIN-S found that average DIN yield ranged from 17 kg N km-2 season-1 (July-September) to 34 kg N km-2 season-1 (October-December). In all seasons, DIN export was greatest from catchments in densely populated regions of western Europe, the eastern US, and South Asia. Sensitivity analysis found that changes in runoff had a stronger effect on modeled DIN export than temperature, suggesting that the effects of climate change on DIN export will depend on the interaction of these factors. As such, predicted climate-related changes in temperature and runoff could increase coastal DIN delivery in the United States by 1%, while decreases of 4-6% are possible in India and China.