Flooded Rice Agroecosystems and Water Quality in Northern California.

Flooded rice cropping systems in the Sacramento Valley have been identified as a possible source of water quality degradation and contamination in northern California surface waterways.  With increased demands for water resources, maintaining high water quality standards is of the utmost importance to the state of California and its residents.  The objectives of this research were to: (1) measure losses and seasonal concentrations of dissolved organic carbon (DOC), total suspended solids (TSS), total dissolved solids (TDS), dissolved inorganic nitrogen (N), dissolved phosphorus (P), and potassium (K) in rice field drainage waters and (2) assess how these losses and concentrations may affect surface water quality.  A two-year study was conducted to measure water, nutrient, and suspended and dissolved solid fluxes from straw burned and straw incorporated rice fields.  The major constituent of interest related to rice production is DOC, as DOC can react with chlorine during drinking water disinfection to form harmful by-products.  Our results suggest that rice fields, especially during winter flooding periods, may be a significant contributor of DOC to the Sacramento River.  The same is true for TSS losses, which can decrease water clarity and limit survivorship of freshwater organisms.  The TDS concentrations in drainflow from rice fields were generally below water quality standards.  Dissolved N, P, and K losses were small compared to fertilizer inputs, although winter losses of K sometimes exceeded 30 kg ha^-1.  Dissolved N and P concentrations in drainflow were low and were not expected to have human health or ecosystem impacts.  These data suggest that if water quality standards to regulate DOC are enacted, changes in water and straw management in rice systems will need to be considered.