Phosphorus leaching from anaerobic peat columns under steady-state flow

Rewetting of organic lowland soils may result in a large and prolonged phosphorus (P) load to the aquatic environment as legacy iron (Fe) is reductively dissolved and the associated P released. We hypothesize that the remaining P sorption capacity in anaerobic soils determines P mobilization. To study P mobilization and transport under steady-state flow, a convective discharge experiment was conducted in the laboratory at 10 °C. Sixty undisturbed soil columns were taken from two different soil depths (5-25 cm, 25-50 cm) in six Danish lowlands characterized by wide variability in P, as well as Fe and aluminum oxide contents. The column experiment used oxygen-free deionized water flowing at a rate of 1 mm per hour over a period of 28 days. The cumulated effluent was analyzed for different P and Fe forms, dissolved organic carbon (OC), ammonium, nitrate, and other solutes on day 5, 14 and 28 during the experiment. Active flow volume and non-equilibrium flow conditions were determined with the help of a tritium tracer. Upon completing the leaching experiments, the soil columns were dismantled for determination of relevant soil properties.

Across the six sampling sites, OC content varied strongly with subsoil OC% (1 – 49%) consistently exceeding topsoil OC% (1 – 44%). Molybdate reactive P (MRP) leaching generally followed site-specific OC patterns, indicating a strong link between C availability and P mobilization. Sites with higher OC showed elevated MRP leaching rates, and higher MRP release, especially in topsoils. In subsoils, MRP leaching was lower and less variable across sites. Columns remained strongly anaerobic during the experiment. Iron-poor sites showed higher MRP leaching. Release rates of MRP declined sharply with increasing molar ratios of bicarbonate-dithionite or oxalate-extractable Fe and P (Fe:P) indicating strong sorption control by Fe oxides. This effect was much more pronounced in topsoils. Saturated hydraulic conductivity also varied substantially among and within sites, ranging from 0.04 to 156 cm d⁻¹. Hydrological conditions further influenced P mobilization: higher flow rates and short residence times caused limited reductive Fe(III) dissolution and MRP release, whereas prolonged residence under low-flow conditions enhanced Fe(II) and MRP release. Additionally, P release to the aqueous phase remained low when the soil’s residual sorption capacity (RSC) exceeded 100 mmol kg⁻¹. In general, we observed lower P release rates compared to those typically reported for batch experiments with similar soils. We expect that our findings will support improved modeling of P export from rewetted organic lowland soils.