Methane pool and flux dynamics in a rice field following straw incorporation

Concerns for air quality have led to legislation restricting rice straw burning in some parts of the world. Consequently, growers must dispose of large amounts of residual rice straw by incorporation into the soil, which may have large effects on CH4 emissions from those fields. Our objective was to characterize how this recent change in management has affected overall CH4 emissions in a California rice field and establish relationships between organic matter availability, CH4 pool sizes and CH4 fluxes. Closed chamber measurements were used to monitor diurnal and post drain fluxes, to describe the seasonal pattern of CH4 emissions and estimate total CH4 fluxes on a large on-farm field trial during the 1997 growing season. Soil redox, temperature and plant growth and yield were also monitored. To establish relationships between CH4 pool sizes and fluxes, soil interstitial CH4 concentrations were monitored in the field and available organic matter in the spring was estimated with a laboratory incubation. Redox values in the soil were found to be 50 mV lower in plots in which straw had been incorporated (−275 mV) than those in which it had been burned (−225 mV). No significant treatment differences were seen in total soil organic matter contents in the spring. However, available organic matter was 1.5 times higher in straw incorporated than straw burned plots. Methane emissions peaked between 22.00 and 23.00 h on two different diurnal sampling dates. Methane emission after draining was about 10% of the flooded period total. A 5-fold increase in total CH4 emissions over the rice growing season was observed in plots in which rice straw had been incorporated each fall for 4 yr. Total cumulative CH4 flux, 1 May–1 October 1997, was 8.87 g C m−2 in incorporated, winter flooded plots; 9.52 g C m−2 in incorporated, non-winter flooded plots; 1.63 g C m−2 in burned, winter flooded plots; and 2.25 g C m−2 in burned, non-winter flooded plots. Soil CH4 concentrations at 10–15 cm depth was strongly associated with emissions to the atmosphere (r=0.89). A model developed by Nouchi et al. (1994) [Nouchi, I., Hosono, T., Aodi, K., Minami, K., 1994. Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water, rice biomass and temperature and itʹs modeling. Plant and Soil 161, 195-208.] which could predict the CH4 flux based on soil CH4 concentrations and temperature was fit to our data. The model was very successful at predicting flux rates and cumulative fluxes because conductance (CH4 flux divided by CH4 concentration in soil water) was highly correlated with soil temperature (r=0.88) throughout the period of high CH4 emissions. Organic matter availability and CH4 pool and flux dynamics were altered by straw incorporation practices as evidenced by increased conductance at the same interstitial CH4 concentration and increased emissions per unit available organic matter in rice straw incorporated plots.