Linking dissolved organic carbon cycling to organic carbon fluxes in rice paddies under different water management practices

Although paddy soils are generally characterized by relatively high dissolved organic carbon (DOC) concentrations and fluxes, little is yet known on how water management influences the cycling of this important organic C pool. This work aims at providing insights into the link between DOC cycling during rice cropping and organic C input to the subsoils and export with surface waters, as well as methane (CH4) emissions in a temperate paddy soil as a function of different water management practices. DOC quantity, quality and fluxes, as well as CH4 emissions were evaluated at field-scale over two cropping seasons for three water management systems including continuous flooding, dry seeding with delayed flooding, and intermittent irrigation. DOC cycling in the different water management systems were strongly linked to the reducing soil conditions resulting from field flooding. In contrast to dry seeding or intermittent irrigation, adoption of continuous flooding not only favoured the accumulation of DOC in the topsoil (> 10-20 mg C l(-1)), but also enhanced C inputs to the subsoil (33-51 g C m(-2)), and exports with surface waters (18-44 g C m(-2)). Moreover, changes in DOC quality in paddy soils were linked to a positive feedback on the abiotic release of soil-derived DOC, and substrate availability for CH4 production. Water management practices in rice paddies strongly affect the temporal trends in DOC quantity and quality over the cropping season, with important implications on organic C fluxes.