Nutrients and sediments in surface runoff water from direct-seeded rice fields: implications for nutrient budgets and water quality.

Nutrient losses from rice fields can have economic and environmental consequences. Little is known about nutrient losses in surface runoff waters from direct-seeded rice systems, which are common in the United States and increasingly more so in Asia. The objectives of this research were to quantify nutrient losses from California rice fields in surface runoff waters and to determine when and under what conditions losses are greatest. Research was conducted in 10 rice fields varying in residue (burned or incorporated) and water management over a 2-yr period. Concentrations of NH-N and NO-N in runoff water across sites, seasons, and management practices averaged <0.1 mg N L. Runoff water PO-P concentration averaged 0.14 mg L and was not affected by season or straw management practices. However, P fluxes were higher in the winter when rice straw was burned (2.59 kg ha) as opposed to incorporated (0.44 kg ha). Average seasonal runoff water K concentrations did not vary with season and straw management, although they were highest at the onset of the winter season. Average total suspended solids (TSS) concentrations did not vary by season but were highest during the winter in the straw-incorporated fields (46 mg L). Rice fields were sinks for K (4.9 kg K ha) during the growing season. Fields were not significant sources of nutrients or TSS during the growing season; however, during the winter fallow they could be sources of NH-N, P, K, and TSS, especially as water fluxes from fields increased.