High Spatial and Fast Changes of Iron Redox State and Phosphorus Solubility in a Seasonally Flooded Temperate Wetland Soil

Abstract

Wetland soils may act as sinks for phosphorus from agricultural soils but P sorption is sensitive to soil redox conditions. When soils become flooded and anoxic, FeIII is reduced and P sorption decreases. To get improved knowledge about the relationship between redox state and P solubility under field conditions, a grid of 4 × 5 sampling points laid out in an Fe-rich meadow soil bordering a Danish river were monitored during 11 weeks in the spring comprising in total five sampling dates. The redox state was quantified as the degree of Fe reduction (DRFe), i.e. percentage of soluble plus sorbed FeII to total pedogenic, non-silicate Fe. Average DRFe was highest (76 %) at beginning of April and decreased to 36 % within 9 weeks. The temporal and spatial variation of DRFe and the soil solution concentrations of Fe and P were high. Thus, average P in solution (Psol) ranged from 24 to 4 μM, closely correlated with average DRFe (R2 = 0.83). Soils farthest away from the river drained first and hence exhibited the biggest change in DRFe and Psol over time. The study demonstrates the great spatial and temporal variability of redox state and P solubility in such wetlands. Some likely consequences of redox oscillations are indicated.