River nutrient fluxes are a key driver of coastal eutrophication, an excessive buildup of aquatic nutrients. Soil erosion, the primary path that moves solid-form nutrients from land to rivers, has close links to climate and land use change. However, researchers lack a deep understanding of the relationship between erosion-related nutrient fluxes and climate and land use change at continental and global scales. A new model-based study highlights the role extreme rains have played in driving the substantial increase of erosional nitrogen and phosphorus fluxes to the northern Gulf of Mexico in recent decades.
Earth system models are important tools for understanding global biogeochemical cycles in the context of climate and land use change. Although river nutrient fluxes driven by soil erosion play a key role in coastal biogeochemistry, they are rarely included in Earth system models. By representing erosional nutrient fluxes in the Energy Exascale Earth System Model (E3SM), researchers found that the increased extreme rains in recent decades drive the observed increase of solid-form nutrients reaching the northern Gulf of Mexico. With extreme rains projected to increase with warming, these erosional nutrient fluxes will likely continue to rise and complicate efforts to reduce regional eutrophication.
Soil erosion delivers enormous amounts of macro-nutrients—including nitrogen and phosphorus—from land to rivers, potentially sustaining bioavailable nutrient levels in inland and coastal waters for decades. Although previous field studies showed the sensitivity of erosional nutrient fluxes to climate and land use change, the relationship is not well understood at continental and global scales. This motivated scientists to represent erosional nutrient fluxes in E3SM and apply the model to the conterminous United States. Model simulations indicate that increased extreme rains in the Mississippi river basin cause erosional nitrogen and phosphorus fluxes to the drainage basins of the northern Gulf of Mexico increased by over 50% from 1991–2019, with land use change and improved agricultural management practices playing only minor roles. Importantly, simulations also show that solid-form phosphorus is the dominant phosphorus constituent in rivers, with over half coming from soil phosphorus pools that could become bioavailable within decades. As warming conditions are projected to lead to increased extreme rains, the regional erosional nutrient fluxes will likely continue to rise and complicate future efforts towards reducing eutrophication in inland and coastal waters.
Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov
The U.S. Department of Energy Office of Science, Biological and Environmental Research program supported this study as part of the Earth System Modeling program area through the E3SM project.
Published: June 8, 2021
Z. Tan, et al., “Increased extreme rains intensify erosional nitrogen and phosphorus fluxes to the northern Gulf of Mexico in recent decades”, Environmental Research Letters, 16, 054080, (2021). [DOI: 10.1088/1748-9326/abf006]