Phosphorus release from anaerobic peat soils during convective discharge — Effect of soil Fe:P molar ratio and preferential flow

Abstract

Phosphorus (P) accumulation in drained agricultural lowlands causes a risk for P pollution to the aquatic environment following wetland restoration. While extensive knowledge is available on P sorption and desorption from anaerobic soils, very limited information is available on the interacting influence of soil geochemistry and local scale active flow volume. Combining batch incubation experiments and continuous column discharge experiments, we investigated iron (Fe) reduction and P release from 10 anoxic Fe-dominated (oxalate extractable Fe (Feox) from ~5500 to 50,000mgkg−1) lowland peat soils (TOC from 5 to 39%) with a gradient in Fe:P molar ratio (molar ratio between bicarbonate dithionite extractable Fe and P (FeBD:PBD) from 3 to 112) and degree of non-equilibrium (preferential) flow. Short-term batch incubation experiments (21days) indicated that concurrent Fe and P release was controlled by reductive Fe(III) dissolution, and was well predicted from the soil FeBD:PBD molar ratio. Continuous convective column discharge with oxygen-free deionised water at 1mmh−1 for 10 effluent pore volumes resulted in highly variable in situ redox potential (Eh from −200 to 300mV), effluent Fe(II) concentrations (23 to 2000μM) and effluent dissolved reactive phosphorus (DRP) concentrations (<6.5 to 316μM). Effluent P forms changed from dominantly particular P/dissolved organic P (PP/DOP) to dominantly DRP as Eh decreased in all soils. Total phosphorus (TP) release rates during convective discharge (3–66μmolkg−1day−1) were negatively non-linearly correlated with the soil FeBD:PBD molar ratio, which explained 71–73% of the variability, and with FeBD:PBD of 10 as a critical threshold ratio. Fe and P release rates from batch experiments were poorly correlated with convective discharge Fe and P release rates, indicating the overall influence of soil structure. Diffusion was found to be the rate-limiting step for P release after prolonged leaching. Although the soil FeBD:PBD molar ratio turned out as a key explanatory parameter in predicting P release rates following rewetting of these peat soils, the results did indicate the influence of preferential flow in decreasing P release rates. Although, sufficient available P is present for leaching in the initial phase after rewetting P enriched lowland soils, we do expect that soils with pronounced preferential flow will become more rapidly exhausted in available P, and hence limit P release after prolonged discharge.