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

Concerns for air quality have led to legislation restricting rice straw bur ning in some parts of the world. Consequently, growers must dispose of larg e 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 o rganic matter availability, CH4 pool sizes and CH4 fluxes. Closed chamber m easurements were used to monitor diurnal and post drain fluxes, to describe the seasonal pattern of CH4 emissions and estimate total CH4 fluxes on a l arge on-farm field trial during the 1997 growing season. Soil redox, temper ature and plant growth and yield were also monitored. To establish relation ships between CH4 pool sizes and fluxes, soil interstitial CH4 concentratio ns were monitored in the field and available organic matter in the spring w as estimated with a laboratory incubation. Redox values in the soil were fo und to be 50 mV lower in plots in which straw had been incorporated (-275 m V) than those in which it had been burned (-225 mV). No significant treatme nt differences were seen in total soil organic matter contents in the sprin g. However, available organic matter was 1.5 times higher in straw incorpor ated than straw burned plots. Methane emissions peaked between 22.00 and 23 .00 h on two different diurnal sampling dates. Methane emission after drain ing was about 10% of the flooded period total. A 5-fold increase in total C H4 emissions over the rice growing season was observed in plots in which ri ce straw had been incorporated each fall for 4 yr. Total cumulative CH4 flu x, 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-wi nter 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. S easonal variation in methane flux from rice paddies associated with methane concentration in soil water, rice biomass and temperature and it's modelin g. 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 cond uctance (CH4 flux divided by CH4 concentration in soil water) was highly co rrelated with soil temperature (r=0.88) throughout the period of high CH4 e missions. Organic matter availability and CH4 pool and flux dynamics were a ltered by straw incorporation practices as evidenced by increased conductan ce at the same interstitial CH4 concentration and increased emissions per u nit available organic matter in rice straw incorporated plots. (C) 1999 Els evier Science Ltd. All rights reserved.