Effect of Different Chloride Concentrations on Nutrient Release in Wetland Soils: A Phytometer Assessment in the Botshol Wetlands,The Netherlands

Turbid water, high phosphorus (P)-loading and disappearing Chara communities forced local water authorities to carry out restoration measures in the lakes and marshes of the Botshol The Netherlands. The reduction of the external-P input could be reached by chemical treatment of the brackish suppletion water and by separating the area from nearby agricultural areas. A side effect of these measures was an increase of chloride from 400 mg l⁻¹ to 1000 mg l⁻¹ in the surface water of Botshol. Internal biogeochemical processes were investigated with phytometers and direct measurements of soil nutrient availability in greenhouse experiments. The increased chloride levels were assumed to increase soil pore water P. The first experiment showed higher P in the peat-soils treated with the highest Cl-concentration and an increased leaching of PO₄ from the lake-bottom peat-soils. No reaction of the phytometer Epilobium hirsutum was found. In the second experiment the 800 mg l⁻¹ Cl-treatment resulted in significantly higher biomass of Carex acutiformis grown on treated bank soil. N-uptake by the phytometer Carex acutiformis was significantly higher. The available-P and total-P in the bank soil did not show a treatment effect. The two studies showed under similar ‘standardized’ conditions a treatment effect of chloride on the P-availability, resulting in higher PO₄ leaching and increased plant nutrient concentrations and biomass. The field study showed higher available-P concentrations in the shore zone than in remote areas. The high chloride levels after restoration impact internal nutrient availability in the Botshol wetlands, on soil loaded with P in recent and historic times.