Phosphorus accumulation and stability in sediments of surface-flow constructed wetlands

Surface-flow constructed wetlands (CWs) are systems that replicate the nutrient reduction processes of natural wetlands and are considered suitable measures to reduce the nutrient losses from agricultural drainage discharge to surface waters. The present study aimed to investigate the biogeochemical stability of phosphorus (P) in three CWs. Intact soil cores were collected from each CW zone and included the soil and the sediment layer (average 28 mm thickness) formed by deposition of the incoming sediments with the water flow. Soil and sediment layer were analysed geochemically, including bulk density, pH, and total contents of carbon, nitrogen, P as well as different extracts of iron (Fe), aluminium and P. In situ measurements of soil redox potential, dissolved oxygen concentration and temperature were carried out in the CWs during 16 months. Grab water samples were collected across the CWs for determination of P species, total Fe, sulphate and pH. Soil core analysis revealed that the major P storage in the CWs was attributed to retention of Fe-bound P in the sediment layer. The Fe and total P (TP) retention differed significantly between CWs (11–63 g Fe m−2 yr−1 and 1–9 g TP m−2 yr−1). Amorphous Fe oxides constituted the major P sorbent fraction in the soil and sediments of all CWs, and the higher P sorption capacity in the sediment layer demonstrated the continuous supply of P sorbents to this layer. The stability of Fe-bound P in the anaerobic sediments seemed to be controlled by the high molar ratios of bicarbonate-dithionite extractable Fe (FeBD) to associated P, which varied from 21 to 49, and the presence of an aerobic sediment-water interface.