Heavy Precipitation Impacts on Nitrogen Loading to the Gulf of Mexico in the 21st Century: Model Projections Under Future Climate Scenarios.

While spatial heterogeneity of riverine nitrogen (N) loading is predominantly driven by the magnitude of basin‐wide anthropogenic N input, the temporal dynamics of N loading are closely related to the amount and timing of precipitation. However, existing studies do not disentangle the contributions of heavy precipitation versus non‐heavy precipitation predicted by future climate scenarios. Here, we explore the potential responses of N loading from the Mississippi Atchafalaya River Basin to precipitation changes using a well‐calibrated hydro‐ecological model and Coupled Model Intercomparison Project Phase 5 climate projections under two representative concentration pathway (RCP) scenarios. With present agricultural production and management practices, N loading could increase up to 30% by the end of the 21st century under future climate scenarios, half of which would be driven by heavy precipitation. Particularly, the RCP8.5 scenario, in which heavy precipitation and drought events become more frequent, would increase N loading disproportionately to projected increases in river discharge. N loading in spring would contribute 41% and 51% of annual N loading increase under the RCP4.5 and RCP8.5 scenarios, respectively, most of which is related to higher N yield due to increases in heavy precipitation. Anthropogenic N inputs would be increasingly susceptible to leaching loss in the Midwest and the Mississippi Alluvial Plain regions. Our results imply that future climate change alone, including more frequent and intense precipitation extremes, would increase N loading and intensify the eutrophication of the Gulf of Mexico over this coming century. More effective nutrient management interventions are needed to reverse this trend. Plain Language Summary: Future climate change is expected to alter nutrient transport from land to rivers, which will have impacts on coastal ecosystems. The impacts of future precipitation changes on nitrogen (N) loading, however, remain unclear. Based upon a well‐tested hydro‐ecological model, this study separates the roles of future heavy precipitation, non‐heavy precipitation, and no‐precipitation days in affecting N leaching loss and predicts the changes in N loading to the Gulf of Mexico. N loading is projected to increase by 30% under two climate scenarios (RCP4.5 and RCP8.5) by the end of the 21st century, half of which is likely driven by heavy precipitation. Future increases in spring heavy precipitation likely result in higher N leaching loss and enhance N loading. Our results indicate that more effective nutrient reduction efforts will be needed to reach the reduction goals of N loading and hypoxia extent in the Gulf of Mexico. Key Points: We examine the responses of nitrogen (N) loading from the Mississippi Atchafalaya River Basin (MARB) to future climate changesN loading from the MARB could increase by 30% under future climate scenarios, and half of the increase would be driven by heavy precipitationAnthropogenic N input would be increasingly susceptible to leaching loss in the Midwest and Mississippi Alluvial Plain under future climate [ABSTRACT FROM AUTHOR]