Unfortunately, irrigation water is a costly input to rice farming, which accounts for 28% of the total cost of rice production. It has been estimated that approximately 22 million hectares of irrigated dry-season rice may suffer from “economic water scarcity” in Asia by 2025. Furthermore, rice farming may become increasingly threatened due to climate change and extreme natural disasters in terms of increasing temperatures and uncertainty of precipitation, drought, floods and salinity .Irrigated rice farming is an integral part of rice production system in Bangladesh,which contributes greatly towards total annual rice production and food security. However, our rice cultivation especially in dry rabi season requires large amount of irrigation water, which is of great environmental concern due to energycris is and methane emission from rice field to the atmosphere, which acts as a potential greenhouse gas with 25 times global warming potential than carbondioxide.Boro rice cultivation is mostly dependent on irrigation water supply and the a man rice cultivation is partly irrigation water dependent.
As groundwater is the main source of irrigation in Boro rice field in northwestern Bangladesh, higher abstraction rate of groundwater may cause negative impacts on groundwater resources in the region.It has already been predicted that Bangladesh is going to face severe water crisis during dry season within the next couple of years. In this regard, alternate wetting and drying of paddy field, developed by International Rice Research Institute, could save a significant volume of irrigation water 15% -30% for rice production, mitigate CH4 emission and sustain rice productivity.The fertility of crop field in Bangladesh has been declining day by day due to continuous cropping and mining of nutrients, and indiscriminate use of chemical fertilizers. In this regard, biochar, mainly the carbon enriched materials with minute amount of plant nutrients obtained from organic matter under high pyrolysis temperature and oxygen limited condition ,could be the best organic manures to rejuvenate degraded soils. Biochar is an anaerobic pyrolysis product derived from organic material, resistant to easy degradation and capable of restoring soil carbon for a longer period of time by reducing greenhouse emission from soil to the atmosphere.
Moreover, the use of biochar will cut down the amount of chemical fertilizers for rice cultivation and GHGs emissions may be suppressed by modifying the paddy ecosystem. It has already reported that combined application of rice husk biochar and FYM with reduced chemical fertilizer under less water inputs was found effective to sustain wheat crop yield in the highly vulnerable dry tropical agro-ecosystem of India.Furthermore, Singh et al. reported that compatible agricultural practices based on specific agroeco system could be effective for climate change adaptations. Therefore, this study was undertaken to determine the suitable combination of biochar and chemical fertilizer for sustaining rice productivity, minimizing yield scaled methane emissions and improving paddy ecosystem through water savings AWDI system. Soil redox status also improved in the biochar amended field plots, probably due to the cumulative effects of free iron oxides in the rice rhizosphere which enhanced electron activity in soil and acted as electron acceptor, thereby enhanced soil porosity . At the end of the experiment maximum increment in soil porosity, soil pH, SOC, SO4-S and free iron oxides were found in biochar amended field plots under conventional and AWDI systems.
In addition, higher concentrations of free iron oxides in soil could be from the total iron content in soil, where biochar amendment might have contributed as an additional source. In this study, the seasonal CH4 emission trends were found significantly high in dry boro season compared to wet a man season, which may be due to the variation in yield potential of the rice cultivars, irrigation water supply and consumption,meteorological and rice rhizosphere environmental variations within the seasons. CH4 flux was higher during reproductive stage of rice plant in all treatments,which may be due to higher availability of labile organic C from the decomposition of soil organic materials , higher diffusion rate ofCH4 gas through rice root and shoot aerenchyma from rice rhizosphere zone being supported by Kludge et al. and Hiya et al. .
The sharp fall inCH4 emission rates at grain maturation stage in all the treatment combinations might be due to the aging of rice plant, leaf senescence, lack of available water and labile organic C as supported by Cai et al. .In this study grain yield was significantly influenced by biochar amend mentsand irrigation practices. The findings confirmed that biochar amendments 15 -20 t/ha with NPKS fertilizers maximized rice yield under AWDI method. In the dry boro season, rice yield was increased by19.6%, 13.6% and 6.5% with biochar amendments at 15 t/ha, 20 t/ha and 30 t/ha respectively, under conventional irrigation; whereas the corresponding yield increments were 19.0%, 14.2% and 6.9% respectively, under AWDI method.