Effects of winter versus summer flooding and subsequent desiccation on soil chemistry in a riverine hay meadow

Flooding of riparian meadows along rivers leads to a switch to anaerobic soil respiration, causing iron reduction and a corresponding release of phosphate. In addition, pollution of river water with sulphate may lead to higher phosphate release as a result of sulphide-iron interactions. As global climate change is expected to increase both temperature and the risk of summer flooding, floods may occur at higher temperatures, leading to faster anaerobic processes in soils. In a mesocosm experiment we tested the effects of flooding with or without 1 mmol L-1 SO42- at two temperatures on sods from a riverine hay meadow. In the control treatment, the water level was kept 10 cm below the soil surface. After four weeks at 5 degrees C, the temperature was changed to 20 degrees C, mimicking the effects of summer flooding. After seven more weeks, all sods were allowed to dry out. In the inundated sods, redox potential dropped during flooding, leading to higher concentrations of Mn2+, Fe2+, PO43-, NH4+ and Ca2+ and a higher alkalinity of the soil pore water. Upon desiccation, redox potential increased immediately, leading to the oxidation of Mn2+, NH4+ and Fe2+ and causing immobilisation of PO43- and a temporary drop in pH. Inundation at 20 degrees C resulted in a much faster release of Mn2+, Fe2+, PO43- and Ca2+ and a higher acid consumption compared to flooding at 5 degrees C. Reduction of the added sulphate did not lead to additional mobilisation of phosphate through competition with the produced sulphide for binding to iron, because of the high iron concentration in the soil, which is characteristic of many floodplains. It is concluded that seasonality of flooding determines accumulation rates of potential phytotoxins and the release rate of phosphate, which has important implications for floodplain management. (c) 2008 Elsevier B.V. All rights reserved.