Effects of winter flooding on phosphorus dynamics in rice fields

Controlling phosphorous (P) loads from rice fields is important for the conservation of aquatic ecosystems, in part because P is relatively concentrated at its sources. Recently, winter flooding, by which irrigation water is maintained in rice fields during winter, has attracted much attention as a farming strategy for environmental conservation and biodiversity maintenance. However, the effects of winter flooding on nutrient cycles have received little research attention. We evaluated the effects of winter flooding on P loads in rice fields by performing laboratory experiments with soils from rice fields with/without winter flooding. These incubation experiments showed that total and soluble reactive P concentrations in surface solutions are decreased by winter flooding. This decrease may follow co-precipitation of P with iron, which may be dissolved from winter flooded soil and rapidly precipitates in solution. Periphyton, which may increase during winter flooding, may not contribute to this decrease because puddling resets periphyton quantities on surface soils. P loads from rice fields with winter flooding over 16 days after fertilization could be reduced by an average of 26% compared with those without winter flooding, indicating that winter flooding is a valuable strategy for reducing P loads in spring when high P loads occur.