Controls on dissolved organic carbon composition and export from rice-dominated systems

Abstract

Rice field outflow can contain high con- centrations of dissolved organic carbon (DOC), which plays a crucial role in drinking water quality and aquatic ecosystem processes. This study examined the relationship between potential determining factors (i.e. rice area, outflow, drainwater reuse, soil proper- ties, and time, measured as the day in the growing season) and the concentration and composition of DOC exported from 11 rice-dominated subwatersheds. Samples were collected from subwatershed inflow and outflow every 1-2 weeks from May through Septem- ber 2008 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 DOC composition. Concentrations of DOC across all subwatersheds and sampling dates ranged from 1.56 to 14.43 mg L -1 (mean = 4.32 mg L -1 ). Linear mixed effects (LME) analysis indicated that DOC concentra- tion decreased over time, and that THMFP, and DOC and THM flux, decreased over time, but increased with outflow. LME analysis of the SUVA254 and S param- eters indicated that the fraction of aromatic DOC moieties increased with time, outflow, and reuse. Additionally, apparent peaks in DOC concentrations, THMFP, and SUVA254 coincided with the onsets of flooding and draining. Lastly, subwatersheds with outflow less than approximately 4,700 m 3 ha -1 behaved as sinks of DOC. Our findings suggest that water management factors such as outflow, reuse, and discrete irrigation events, all of which vary over the course of the growing season, were the dominant determinants of DOC concentration and composition.