Greenhouse gas emissions and soil redox potential in irrigated lowland rice as affected by elevated temperatures

Irrigated lowland rice fields are considered as one of the major source of anthropogenic global methane (CH sub 4). While, the source strength of CH sub 4 from rice field can only be estimated with considerable uncertainty, this level of uncertainty could be compounded by increasing global temperatures. In this study, experiments were conducted from February 2010 to December 2011 at the International Rice Research Institute using variety of approaches to control temperature of canopy, water, and air, respectively. These experiments encompassed field conditions, greenhouse and walk-in growth chambers (WGC). The study determined the effect of elevated temperatures, in combination with rice straw incorporation, on: a) CH sub 4 and N sub O emissions and dynamics of CO sub 2 fluxes from the soil, b) soil redox potential (Eh), and c)biomass partitioning and yield of rice in lowland condition. Methane emission rates increased from vegetable stage to reproductive stage, and peaked from flowering to maturity period. On the other hand, N sub 2 O emission was apparent at initial stage of soil flooding and after harvesting of rice coinciding with progressive drying of the soil. Fluxes of CO sub 2 widely fluctuated and did not show any trend within the rice growing season. In the field experiment using infrared heaters, an increase in canopy temperature by 2-3 deg C above ambient did not affect CH sub 4 emission. In the greenhouse, as water temperature increased up to 34-35 deg C, CH sub 4 emission rates increased, but decreased emission beyond 35 deg C. The experiment in WGC demonstrated that CH sub 4 emission rates increase throughout the rice growing period under elevated air temperatures (29-35 deg C). The diurnal variations in CH sub 4 emission rates were positively correlated with air and soil (5 cm depth) temperatures. Highest emission rates occurred from 1100H-1400H. On the other hand, soil Eh, N sub 2 O emission and CO sub 2 fluxes were not affected by elevated temperatures. Elevated temperatures in the canopy (2-3 deg C) and water (2-4 deg C) did not affect the rice yield. However, high air temperature (35 deg C) under WGC experiment significantly altered the biomass partitioning, i.e., increased straw but decreased grain biomass due to increased spikelet sterility. Rice straw incorporation led to increases CH sub 4 emissions, but decreased N sub 2 O emissions alongside with the rapid reduction in soil Eh. Soil Eh was negatively correlated with CH sub 4 emission, while positively correlated with N sub 2 O emission. The combined rice straw and elevated temperature affected CH sub 4 emission rates and cumulative emission, such as far greater emissions were observed under high temperatures with rice straw in WGC experiment. Both increasing global temperatures and rice straw incorporation could alter the greenhouse gas emissions and biomass partitioning of rice leading to reduction in grain yield.