Dissolved organic carbon losses from rice agroecosystems under various straw and water management practices

Managed wetland ecosystems (including rice agroecosystems) dominate the landscape of California’s Sacramento Valley.  Historically, rice straw was burned, but since the mid-1990s, most rice straw is incorporated and fields are flooded over winter to promote decomposition.  While this practice has improved air quality, some water quality concerns have arisen related to the potential increase in dissolved organic carbon (DOC).  Greater quantities of DOC exported into surface waters can affect in-stream biogeochemical processes, and in turn, the quality of drinking water.  The objectives of this study were to (1) measure seasonal concentrations and fluxes of DOC from rice agroecosystems and (2) assess how varying straw and water management practices affect DOC losses.  At four locations, straw-burned and straw-incorporated fields were monitored between 2005 and 2007.  Water samples were collected from field inlets and outlets during winter flooding and growing season flooding.  Weirs and pressure sensors were used to estimate drainflow.  During the first month of winter flooding, DOC concentrations among all straw-incorporated fields ranged between 35 and 77 mg L^-1.  Burned fields were not typically flooded over winter, but DOC concentrations in runoff ranged between 6 and 14 mg L^-1.  At the onset of drainage during the growing season, DOC concentrations from straw-incorporated fields were larger (2 sites) or similar (2 sites) compared to straw-burned fields.  No practical differences in DOC concentration were observed later in the growing season.  Results indicate that changes in rice straw management have increased annual DOC flux and seasonal DOC concentrations from these managed wetlands.