Fate of carbon in California rice systems

Legislated reduction of burning post-harvest rice straw residues combined with winter flooding of rice fields have altered straw management practices in California. To understand the effects of these changes on straw decomposition and carbon cycling, fluxes of CH4 and CO2, carbon budgets, factors controlling fluxes at different times of the year, and straw decay rates were quantified. Two experiments with split plot designs were implemented in two commercial rice fields. Winter flood and non-flood treatments were main plots with four straw treatment sub-plots (Burn, Incorporate, Roll, and Bale) in this three year study. A microcosm experiment additionally included four rice soils as another sub-plot. Results showed that winter flooding and autumn straw amendments increased winter CH4 emissions such that they accounted for 50% of annual totals. Straw inputs increased total carbon (CO2-C+ CH4-C) emissions, and increased atmospheric radiative effects compared to the conventional bum treatment. Winter flooding also enhanced winter straw decay probably by increasing leaching. Other factors such as, temperature, presence of plants, soil moisture, and soil type were important in particular treatments at certain times of the year. Warmer temperatures were likely responsible for greater summer carbon emissions than those in winter despite smaller straw inputs in the spring than autumn. Straw decay rates were enhanced by higher summer temperatures and soil incorporation. Summer aboveground rice biomass was a good predictor of CH4 emissions and up to 20 times as much CH4 was emitted from planted than unvegetated areas in the summer. Soil water content was the sole predictor of total carbon flux during post-drain winter and summer periods. Low soil pH depressed straw decay and sand content increased CH4 emissions. In conclusion, winter flooding increases straw decay which can facilitate spring seedbed preparations. Straw amendments increase atmospheric radiative effects but have the potential to improve long term soil nutrient status. The systems-based approach used in this study measured important parameters throughout the year contributing to greater understanding of the carbon cycling dynamics in water seeded rice. It provides the basis for farmers to evaluate management choices necessitated by the reduction in straw burning.