Estimation of methane emissions from irrigated boro rice and assessment of possible mitigation measures

Abstract

Among the GHGs, methane is the leading heat trapper and acts as an effective contributor to global warming mechanism. Rice cultivation under continuously irrigated condition is the prime source of methane emission to the atmosphere. As the fourth largest rice producing country in the world, the present emission status of Bangladesh from irrigated Boro rice and applicability of different mitigation approaches is a contemporary concern. In this study an attempt was made to identify the current rate of emission, study the influence of agro-climatic factors on the emission rate and feasibility of mitigation measures. The study was carried out in three villages of Gazipur Sadar Upazilla of Gazipur district. Assessment of country’s present rate of methane emission from 2000 to 2009 was estimated through the methodology proposed by IPCC (2006) and also by using CH4MOD2.5 model (Huang et al., 2004). Calculation was made for Aman (BR 11) rice variety and Boro (BRRI dhan 28 and BRRI dhan 29) rice varieties seasons. CH4MOD2.5 model was also used to analyze the effects of different agro-climatic factors on the rate of emission and forecast the future (in 2030 and 2050) emissions under SRES A1B emission scenario. Three FGDs were conducted with the farmers along with KII to assess the adaptability of the possible mitigation measures. The results of the study show that by using IPCC methods, country emits 1071 Gg (Gigagram) of methane annually from both seasons. From CH4MOD2.5 model, it was estimated as 464 Gg of which 83% came from Boro and 17% from Aman rice. Cultivar specific emissions for BR 11, BRRI dhan 28 and BRRI dhan 29 were 20.4, 69.92 and 96.73 mg m-2 d-1, respectively. Changing of both temperature and CO2, increased the emission by 13% and 16% for BRRI dhan 28 and BRRI dhan 29, respectively, due to the availability of readily decomposable organic C. Increasing of 10 kg/ha of N2 fertilizer had both effect on the emission rate. Sandy soil emitted more methane than clayey soil because of higher cohesive force in clay soil, which retained more water in the pore space. Higher emission was also estimated from rice straw incorporated fields. Around 27% of higher emission rate was estimated in BRRI dhan 29 due to higher plant height, growth duration and yield compared to BRRI dhan 28. Methane emission can be reduced by 71%, 80%, 87% and 93% by applying 3, 4, 5 and 6 IDWR (Intermittent Drainage Water Regime), respectively instead of CFWR (Continuously Flooded Water Regime). Instead of transplanting, direct seeding with 6 AID (Alternate Irrigation and Drainage) reduces emission by 94%. Shifting of 7 days and 14 days EFTP (Earlier from the Traditional Practice) reduced emission by 6% and 19% respectively from BRRI dhan 28, and 9% and 18% respectively from BRRI dhan 29. In the year 2030 and 2050, country will emit around 653 Gg and 906 Gg of methane from Boro rice (an increase of 39% and 56% respectively under A1B scenario) under CFWR. Incorporation of 3 and 5 additional drainages instead of CFWR, emission rate will be reduced around 64% and 84% for both the year 2030 and 2050. From the FGD, it was found that changing of rice cultivars is only applicable when framers are given a variety with higher yield, lower crop duration, moderate plant height and lesser tiller number. By using pellet urea and LCC, farmers used 18% less N2 fertilizer. AWD is already practiced but uncertainty of electric supply, non-availability of irrigation water and low conveyance efficiency are the major difficulties to its adoption. Time and money consuming weed management is an obstacle to direct seeding. Due to fewer livestock, lesser amount of FM is already applied in the rice fields. Shifting of transplanting date is only applicable for high lands. As the study was carried out in one hydrological zone with A1B scenario, further studies in other zones and scenarios are needed for a better understanding of the emission rates and adoptability of the mitigation options.