Mitigating seawater flooding induced greenhouse gas emissions and nutrient leaching using soil amendments

Saltwater intrusion and flooding into coastal agricultural lands are growing concerns due to their detrimental impact on surface and groundwater, soil health, agricultural productivity, and ecosystem services. This study aimed to evaluate the efficacy of soil amendments (biochar, compost, gypsum, and hydrogel) in mitigating the impacts of seawater flooding on nutrient release and greenhouse gas emissions. Column experiments were conducted on two soil types: Biscayne soil, high in Ca and total Kjeldahl nitrogen (TKN) content, and Krome soil, low in Ca and TKN. Soil amendments were applied at two different rates (1.5 % and 3 % w/w), with a control for comparison. Porewater samples were collected from three depths after one and seven days of flooding to measure concentrations of nutrients (Ca²⁺, Mg²⁺, K⁺, NH₄⁺‒N, NO₃⁻‒N, ortho-phosphorus (P), total phosphorus (TP), Na⁺, and electrical conductivity). Greenhouse gas samples were collected using PVC chambers and analyzed for carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). One day after flooding, all amendments (except hydrogel) at both rates effectively decreased CO₂ flux in Biscayne soil. Similarly, most amendments, except 1.5 % hydrogel in Biscayne and 1.5 % biochar and 3 % compost in Krome soil, decreased N₂O flux. The application of 3 % compost induced a 100 % decline in N₂O flux in both soils, while the highest CO₂ flux reduction (78.1 %) was observed from a 3 % biochar application rate in Biscayne soil. Biochar decreased NH₄⁺‒N, P, and TP concentrations in Krome soil, while 3 % compost decreased NO₃⁻‒N and P concentrations in Biscayne soil. The highest reductions in Na⁺ and Mg²⁺ concentrations were observed in Biscayne soil with 3 % hydrogel, while Ca²⁺ and K⁺ reductions were greatest in Krome soil with 1.5 % biochar and 3 % compost. These findings highlight the potential of soil amendments to mitigate greenhouse gas emissions and nutrient release from flooded soils, emphasizing the importance of soil characteristics and element-specific selection of amendments.