Saltwater intrusion increases phosphorus abundance and alters availability in coastal soils with implications for future sea level rise

Authors: GU, CHUNHAO - University Of Delaware; JOSHI, SUNENDRA - University Of Delaware; Fischel, Matthew; TOMASZEWSKI, ELIZABETH - Us Geological Survey (USGS); DONALD, SPARKS - University Of Delaware

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2024
Publication Date: 4/22/2024
Citation: Gu, C., Joshi, S., Fischel, M.H., Tomaszewski, E., Donald, S. 2024. Saltwater intrusion increases phosphorus abundance and alters availability in coastal soils with implications for future sea level rise. Science of the Total Environment. 931: Article e172624. https://doi.org/10.1016/j.scitotenv.2024.172624.
DOI: https://doi.org/10.1016/j.scitotenv.2024.172624

Interpretive Summary: Climate change causes global sea levels to rise and floods coastal regions with salt water. The salty flood waters can change the chemistry of soils due to their high pH and dissolved ions and release nutrients like phosphorus into the environment. This study determined how sea level rise will impact phosphorus release and retention in sandy coastal soils using a series of sites in Rehoboth Bay, Delaware. We found the flooding and saltwater increased the phosphorus content and altered the chemical form. These results help inform decisions for nutrient management in areas impacted by future sea level rise. The findings also inform scientists and policy makers of risks associated with phosphorus release and sea level rise in coastal areas with the goal of reducing harm to human and environmental health.

Technical Abstract: Climate-driven sea level rise (SLR) promotes saltwater intrusion into coastal soils across the globe at an increasing rate, which can impact phosphorus (P) dynamics and affect adjacent water quality. However, P molecular speciation and availability, which largely determines its bioavailability and transformation in coastal soils, remains poorly understood. We evaluated SLR impacts on soil P transformation using X-ray absorption near edge spectroscopy (XANES) and the modified Hedley sequential fractionation from a salinity gradient at the Rehoboth Inland Bay in Delaware. The salinity gradient consists of five sampling sites along a transect representing different degrees of SLR-induced saltwater intrusion. With increasing distance from the Bay, both soil salinity (29.3 to 0.07 mmhos cm-1) and P concentrations (683.1 – 304.0 mg kg-1) decreased. Sequential fractionation showed that occluded P (i.e., P in the residue after HCl extraction) was dominant (86.9 – 89.5% of total P). With increasing salinity, labile P increased initially, reached a plateau, and then decreased. Qualitative analysis of bulk XANES spectra showed that soil P was likely dominated by Al-P. Hence, with increasing saltwater intrusion, soil P became increasingly accumulated in an occluded pool mainly as Al-P. Qualitative analysis of µ-XANES spectra revealed that Ca-P existed in hotspots of the highest salinity soil while Fe-P or Al-P occurred in hotspots of the low salinity soil, consistent with the greater HCl-P (presumably Ca-P) in the former soil. Overall, results demonstrated that SLR markedly influenced P cycling in coastal soils.