Phosphorous dynamics in periodically flooded and drained riparian soils
PublisherUniversity of Reading
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The aim of the research was to evaluate the processes that determine P stability in riparian floodplain soils, with emphasis on chemical transformations in flood-drained calcareous soil. Flooding of soil columns showed an increase in P solubility (from 0.01 to 1.2 μg ml-1) largely due to the reductive dissolution of Fe minerals (solution Fe increased from < 0.05 to > 50 μg ml-1) and increased solubility of Ca-P minerals (solution Ca increased from < 100 to > 1000 μg ml-1). The periodic drainage of the flooded soil exported the equivalent of approximately O.l5 kg ha-1 (over six flood-drain cycles). Exported P correlated with soil solution P immediately before drainage (r2 = 0.99; P < 0.001). After drainage, there was a reversal of the reactions described above, with solution Fe, Ca and P returning to preflooding levels. Over the course of the 6 flood-drain cycles inorganic P became more stable, with Ca-P and Fe-P pools increasing (from 9 and 11 % respectively both to 14 % of TP) at the expense of labile pools. Similar patterns of P release and retention were observed during batch incubation studies of aerobic - anaerobic cycles using moist soil samples. Solution P and Fe were strongly correlated in both non-calcareous (r2 = 0.96) and calcareous (r2 = 0.73) soils. The role of Ca-P mineral solubility was disguised by the dissolution of calcium carbonate. Drying of the soil can lead to P release on re-wetting (up to eight-fold). This is most likely due to a release of P from ruptured microbial cells and transformations of Fe mineral surfaces. Monitoring suggested that all of these processes were active in the field, as the water table advanced and receded, though more slowly than in the laboratory. Over a four month flood-drain cycle losses of dissolved P from the floodplain were estimated at 0.034 kg ha-1. It is unlikely that this is significant relative to upstream inputs of P to the river from point sources and other types of agricultural land in the River Thames catchment.