Oxygen depletion and phosphorus release following flooding of a cultivated wetland area in Denmark

Geografisk Tidsskrift, Danish Journal of Geography 108(2):17–25, 2008

The rewetting of altered wetlands is becoming increasingly widespread. When flooding cultivated soils, the oxygen (O₂) availability is reduced, subsequently, ferric hydroxides can dissolve and associated inorganic phosphorus (P) will be mobilized. This study shows the temporal and spatial dynamics of O₂ depletion following flooding using planar optodes and the subsequent release of Fe and P in two depth intervals in an experimental column set-up. The column was kept flooded for 48 days and thereafter partly drained and flooded again. Results document that large amounts of P (0.2 t P ha-¹) have accumulated in the present plough-layer (Ap) during the last 22 years, which represent roughly 15% of the present inorganic P stock in the Ap. As a result of flooding, fully anoxic conditions were observed within 3 days (within 10 h in Ap) and concentrations of dissolved Fe and P in the soil solution increased simultaneously after 7 days of flooding. Thus, P reaction kinetics was markedly delayed as compared to O₂ availability. P concentrations in soil water afterflooding (up to 0.15 mg L^?1) accounted for only 0.034% of the inorganic P stock in the Ap which is a significantly smaller fraction of the potential P-release as compared to previous investigations.

This is considered a result of only a minor fraction of the total inorganic P being directly associated with ferric hydroxides and thereby sensitive to short-term anoxic conditions as well as differences in the methodology used in this study (soil/water ratio). Finally, reactions releasing Fe and P were noted to be partly reversible upon draining.