Phosphate release from seasonally flooded soils: A laboratory microcosm study

Phosphorus derived from agricultural practices has been targeted as a leadi ng cause of water quality degradation in Lake Champlain, Mobilization of P from seasonally flooded agricultural soils is a concern. Using 14 soils fro m a research farm in New York's Champlain Valley, we characterized the avai lable P status, extractable Fe and Al, P sorption capacities, and soluble p hosphate release in flooded laboratory microcosms. Quantities of NH4-acetat e available P ranged from 3 to 100 mg kg(-1) and fluoride-extractable P fro m 10 to 211 mg kg(-1). Flooding soils induced significant release of phosph ate to the porewater over a 60- to 90-d period in 13 of the 14 soils studie d. Porewater phosphate increases ranged from 2.2 to 27.0 times the initial phosphate concentrations. However, floodwater phosphate increases were much lower, with a maximum of 3.6 times the initial concentration. Average pore water phosphate concentrations over the flooding period ranged from 0.046 t o 7.0 mg L-1 and average floodwater P from 0.032 to 3.70 mg L-1. Ammonium-a cetate P and the degree of phosphorus saturation (DPS) were highly correlat ed with the average porewater and floodwater phosphate concentration. Avera ge ratio of porewater to floodwater phosphate concentrations ranged from 1. 0 to 3.3. Five soils that were lower in fluoride-extractable P had increasi ng porewater phosphate accompanied by increasing porewater Fe2+ and decreas ing floodwater phosphate, Results suggest that P solubility and mobility we re a function of both the available P status and redox cycling.