Reducing drainage water phosphorus concentration with rice cultivation under different water management regimes

Phosphorus is a critical water quality indicator of farm drainage systems. In South Florida, rice (Oryza sativa L.) cultivation has the potential to reduce phosphorus concentration in drainage water through crop uptake under varying flooding systems. A two-year study was conducted to assess drainage water quality under four different flooding systems conventionally used for rice production - 15 cm continuous flood; 5 cm continuous flood; 15 cm with midseason drawdown; and 5 cm with midseason drawdown. In the first growing season, 15 cm continuous flood treatment had the highest reduction of total phosphorus and particulate phosphorus concentrations in drainage water. However, in the second growing season, we did not observe significant differences among water treatments, which was likely due to the release of phosphorus from other sources like soil and rice straw to the water column which masked treatment effects. On average, in both years, all flood treatments reduced total phosphorus concentrations by 42 ± 8% between inflow and outflow water. No phosphorus fertilizer was added and with each harvest, rice grain potentially removed 15.7 ± 3 kg phosphorus ha−1. The results indicate that rice cultivation in fertile soils can successfully reduce phosphorus concentrations in drainage water under well managed water-flow conditions.