Fallow season straw and water management effects on methane emissions in California rice

In response to legislative mandate to reduce postharvest straw burning and environmental concerns to restore wetland habitat for Pacific flyway waterfowl, California rice growers are incorporating straw into soil and flooding rice fields in winter. These changes were hypothesized to alter soil carbon cycling pathways across the region. The principal objective of this study was to determine how various winter fallowed straw and water management changes would affect year‐round methane emissions. Main plots were winter flood and nonflood, and subplots had straw treatments: burned, soil incorporated, or rolled (partially soil incorporated). Results showed the principal factor controlling methane emissions was the interaction of flooding and straw amendments. The presence of either water or straw alone led to low emissions. Winter emissions accounted for 50% of annual totals in straw‐amended treatments despite lower temperatures and the presence of plants in summer. Summer emissions were significantly influenced by winter straw amendments but not by winter flood. Postdrain peaks after winter drain accounted for 10–13% of annual emissions in treatments with amended straw. Although rolled and incorporated treatments had similar straw inputs, methane fluxes from rolled treatments were higher than from incorporated treatments. Measurements of methane should be conducted year‐round to capture fallow and postdrain fluxes and improve global emission estimates. Regional emission estimates showed that 2.6 times more methane was emitted after flooding plus incorporation was implemented than before the legislative mandate was enacted.