Saltwater intrusion alters nitrogen and phosphorus transformations in coastal agroecosystems

Abstract

As sea levels rise, coastal regions are becoming more vulnerable to saltwater intrusion (SWI). In coastal agricultural areas, SWI is causing changes in biogeochemical cycling in soil and waterways. These changes are leading to the release of excess nitrogen (N) and phosphorus (P) from farm fields, which in turn can cause impaired water quality downstream. I explored the effects of saltwater intrusion on N and P concentrations of surface water and soil porewater on Maryland’s Eastern Shore in the Chesapeake Bay Watershed on three spatial and temporal scales: 1) a three-year field study through farmland and various surrounding habitats; 2) a one-month laboratory soil incubation study; and 3) a regional study of tidal tributaries (sub-watersheds) along Maryland’s Eastern Shore where I utilized 35 years of observational data on nutrient concentrations and salinity from the Chesapeake Bay Water Quality Monitoring Program. The results of the field and incubation studies suggest that SWI can cause a large release of N and P from the soils of coastal landscapes to downstream water bodies such as tidal creeks and marshes. However, the results of the regional study suggest that the relative magnitude of SWI-driven contributions of N and P to waterways as compared to other sources and drivers of N and P differ depending on the spatial and temporal scale considered. Defining mechanisms through which SWI spurs nutrient release from soils of agricultural fields and surrounding habitats as well as the magnitude of these processes is critical for quantifying N and P export in coastal watersheds. The results of these three studies can potentially be used to inform water quality models for individual tidal tributaries, which would allow for more targeted approaches to nutrient load reductions in sub-watersheds of the Chesapeake Bay and other watersheds globally.