Controls on Dissolved Organic Carbon Composition and Export from Rice-dominated Systems

Rice covers 161 million hectares of the Earth's surface and is estimated to use one-fourth to one-third of the World's developed freshwater resources. Outflow water from rice fields can contain high concentrations of dissolved organic carbon (DOC), which is a key precursor in the formation of carcinogenic and mutagenic trihalomethanes (THMs). This study examined rice area, outflow, drainwater reuse, soil properties, and time, and related them to the concentration of DOC and the composition of dissolved organic matter (DOM) exported from eleven rice-dominated watersheds located in the Sacramento Valley, California during the growing season (May to September 2008). Water samples were collected from subwatershed inflow and outflow every one to two weeks and analyzed for DOC concentration, trihalomethane formation potential (THMFP), and also specific ultraviolet absorbance (SUVA254) and the spectral slope parameter (S), which are indicators of DOM composition. DOC concentrations ranged from 1.56 to 14.43 mg L-1 (mean = 4.35 mg L-1 ). DOC and THM fluxes decreased over time, but increased with outflow. The SUVA254 and S parameters indicated that the fraction of aromatic DOM moieties increased with time, outflow, and drainwater reuse. Additionally, the export of the highest DOC concentrations and THMFP occurred at the onsets of rice field flooding and draining. Lastly, subwatersheds with outflow less than approximately 4,700 m3ha-1 behaved as sinks of DOC. These results suggest that the management of outflow and drainwater reuse are the most feasible tools for controlling DOC export and DOM composition. They further indicate that very specific rice irrigation events pose the greatest potential impact on downstream water quality. The response of DOC export and DOM composition to outflow, drainwater reuse, and discrete rice irrigation events, with their corresponding impacts on DOM degradation, mobilization, and sorption processes occurring in flooded fields over time, highlights that the management of water in rice growing areas has the potential to significantly impact carbon biogeochemical dynamics in aquatic ecosystems.