Management strategies for reducing phosphorus levels in saltwater-intruded agricultural fields

As sea levels continue to rise and high tide flooding events increase in frequency, researchers and farmers alike are looking for solutions to adapt to and mitigate the effects of saltwater intrusion (SWI). Some landowners on the Lower Eastern Shore of Maryland respond to SWI by taking land out of agriculture. For example, they (1) attempt to remediate salt-damaged soils (e.g., planting switchgrass, Panicum virgatum), (2) restore native marsh grasses (e.g., planting saltmarsh hay, Spartina patens), or (3) abandon fields altogether (e.g., allow for natural recruitment). This work examines the ability of each of these land management practices to reduce phosphorus (P) levels in soils and porewater, with the overall goal to benefit both the farming community and water quality in the Chesapeake Bay. We show that remediation and restoration practices are efficient at taking up soil P and reducing porewater P concentrations through biomass P uptake. After three years of growth, we observed an increase in P uptake in biomass of Panicum virgatum (remediation species; 11–30 kg ha⁻¹) and Spartina patens (restoration species; 4–18 kg ha⁻¹) and a decline in available soil P pools (M3P; 30–50 % kg M3P ha⁻¹). At all farms, under all three management strategies, the P fertility index value (FIV) in the topsoil was 33–50 % lower than baseline conditions, likely reducing the potential release of P to nearby waterways. Results from this work will help inform state-level coastal management policies and determine optimal strategies for climate resilience.