Abstract
Energy, water and CO₂ fluxes were continuously measured in a rice paddy field in Mymensingh, Bangladesh using the Eddy Covariance (EC) technique from February 2006 to December 2007 in Central farm of Bangladesh Agricultural University to study the characteristics of those fluxes and their fluctuations in magnitude due to seasonal, annual and interannual variability to key environmental factors. The results show that energy fluxes were governed by the availability of solar radiation and water with dominating latent heat flux (λE). λE was higher in magnitude 5-18 MJm⁻²d⁻¹ during the growing periods as well as the monsoon fallow because of the water abundance, on the other hand H was found to be very small 1-2 MJm⁻²d⁻¹. Slight variation of H & λE in their magnitude were experienced during winter fallow when fields were dry with almost no rainfall. The response of NEP was positive with LAI and growing stages. The NEP increased substantially with advancing growth stages and gradually reduced when the plants reached maturation and senescence occurred. For both Boro and Aman rice seasons the variation in NEP followed the same pattern but varied in magnitude. In 2007, Boro had less NEP just after transplanting than Aman, those were 2 μmolm⁻²s⁻¹ and 4 μmolm⁻²s⁻¹ respectively. Significant difference between Boro and Aman NEP was observed during full growing season which was 26 μmolm⁻²s⁻¹ and 14 μmolm⁻²s⁻¹ respectively. During maturation it was in decreasing trend while Boro had 12 μmolm⁻²s⁻¹ and Aman had 15 μmolm⁻²s⁻¹ NEP. Differences in LAI in two cropping seasons were very important for this kind of variations because NEP is directly proportional to LAI and it was observed that Rs was lower in 2007 than 2006, which might be considered as a factor for variation between these two seasons. It was observed that climatic variability and weather had direct impact on the fluxes measured during Boro and Aman growing seasons. Interannual variability was also found and climate dynamics such as amount of rainfall, solar radiation, wind, air temperature and humidity profoundly influenced it make year 2006 more productive than 2007 but surprisingly monsoon fallow influence total CO₂ sequestration than winter fallow due to standing water at site and abundance of rice stubbles with weedy plants. Variations in surface condition were evident during crop growth stages in 2006 and 2007, which also influenced the energy and water exchange in addition to weather variability. λE was dominant among the energy fluxes in both seasons with highest value of 600 Wm⁻² in 2006 and 587 Wm⁻² in 2007 those were more or less similar and could be attributed to the paddy rice system and abundance of water. H was more or less analogous ranging within 150 Wm⁻², considerably less due to abundance of energy partitioning into λE; G was also identical ranging within 100 Wm⁻². Throughout the cropping span, differences in energy and water fluxes were attributed to the availability of sunlight, abundance of water, and difference in sunshine hours among winter and summer. Irrigation during winter Boro and rainfall during summer Aman had significant effect on the flux partitioning of MYM site. Under these circumstances, there are a lot of other aspects that needs to be comprehensively studied like carbon balance of paddy fields and exchange of other greenhouse gases like methane, nitrous oxide and their seasonal variability due to surface condition. Moreover, net carbon gain and net carbon removal could be another vital aspect to determine the actual contribution of rice paddy ecosystems in the perpetual global warming phenomenon. It is thus imperative that extensive field studies are required to be carried out for another 3-5 years to comprehend the flux dynamics which in turn will facilitate the understanding about the factors which influence and control various fluxes resulting either in contribution or mitigation of additional carbon in the atmosphere.