Global agriculture in 21st century faces the tremendous challenge of providing sufficient and healthy food for growing population under increasing water scarcity, while minimizing environmental consequences. In agriculture sector, rice is a major food grain crop in the world and it will be necessary to produce about 60% more rice than currently produced to meet food demands by 2025. In Bangladesh, rice is the prime food grain crop cultivated in 77% (28.49 million hectares) of the total cropped area that covers 5th largest position of the world in respect to rice cultivation area (BBS, 2012). But rice cultivation especially in dry winter (Rabi) season requires huge amount of irrigation water, which is a great challenge due to energy crisis. From the time immemorial, rice is grown in low land areas under continuous flooding in Asian tropics and subtropics that requires between 700 and 1500 mm water per cropping season depending on soil texture (Bhuiyan, 1992) that leads to high surface runoff, seepage and percolation account for between 50% - 80% of total water input. Water availability in Bangladesh is around 90 billion cubic meters (BCM) during dry season against the demand of 147 BCM, a shortage of nearly 40%, resulting in drought situation in large parts of the country (Quassem, 2010). Boro rice in Bangladesh is fully irrigated and the Aman rice is partly irrigated. An estimated 3000 to 5000 litres of water is required to produce one kg rice (BRRI, 2016). So, Bangladesh is going to face severe water crisis within next couple of decades. In future, it will be necessary to produce more food with less water or increase the efficiency of water management. Again, flooded rice cultivation is a major anthropogenic source of methane (CH4) which is one of the important greenhouse gases having 25 times more global warming potential than carbon dioxide (Nieder & Benbi, 2008). These flooded soils characterized by low oxygen and high organic substrates offering an ideal environment for methanogen bacteria generate anaerobic condition that favors CH4 production as an end product from organic matter degradation (Conrad, 2002). Methane from soil is derived from C mineralization, moisture, pH, soil Eh, etc. and emitted to the atmosphere by molecular diffusion, ebullition or plant mediated transport like aerenchyma (Zheng et al., 2007). The CH4 concentration in the atmosphere has reached 1.77 ppm by volume in 2005, which is more than double that of its preindustrial value (Solomon et al., 2007). Therefore, it is necessary to solve water scarcity and minimizing CH4 emission from rice paddy without aggravating rice production.
Two main types of water efficient irrigation practiced in Bangladesh are: 1) Alternate Wetting and Drying (AWD) and 2) System of Rice Intensification (SRI). AWDI developed by International Rice Research Institute, is a water saving and methane mitigation technology where rice fields are not kept continuously submerged but are allowed to dry intermittently during growing stages. But a severe AWD regime in where photosynthesis is severely inhibited and plants cannot rehydrate overnight could markedly decrease grain yield, although it reduces methane emission from paddy fields (Win et al., 2015). Again, SRI is an efficient, resource-saving and productive way to practice the use of younger rice seedlings planted singly at wider spacing, together with intermittent irrigation and organic fertilization adoption. In our country, it is difficult to follow SRI because of some limitations like organic fertilization and more laborious that sum up more cost (Uphoff & Kassam, 2009). On the contrary, alternate wetting and drying irrigation for rice has become popular in Bangladesh than SRI due to economic and environmental feasibility (Price et al., 2013). Although, few studies in Bangladesh showed the effect of AWDI on rice yield, they do not give us a clear idea to sketch out the effect of AWDI on rice yield and CH4 emission during rice cultivation. Therefore, this research experiment was conducted to evaluate different water saving irrigation management practices in paddy field for mitigating methane emissions and sustaining rice productivity.
2. Materials and Methods
2.1. Experimental Field Preparation and Rice Cultivation
The experiment was carried out at the research field of Bangladesh Agricultural University in 2017. The soil of experimental site was well drained, loamy texture having a soil pH (6.43) with moderate organic matter (1.78), 0.17% total nitrogen, 13.9 ppm available phosphorous and 16.3 ppm available potassium. The experimental field had three side-by-side blocks for three replications and each plot (20 m2) was laid down in a randomized complete block design where block to block and plot to plot distances were 1.5 m and 1m to prevent interactions. The field was tilled with a tractor and subsequently ploughed and cross ploughed followed by laddering and cleaning. These plots were surrounded by 25 cm wide and 20 cm high levees separated by 1 m transition zones. The applied fertilizer doses were Urea 220 kg∙ha−1, TSP 115 kg∙ha−1, MoP 100 kg∙ha−1 and Gypsum 60 kg∙ha−1. Among them, nitrogenous fertilizer (urea) was applied thrice as basal dose after transplantation, whereas others were applied only once before transplanting. Rice seedlings (21 days old BINA Dhan 10) were transplanted into experimental plots at three seedlings hill−1 with 25 cm × 20 cm spacing between the rows and hills, respectively.
2.2. Placement of Perforated Pipes According to Treatments
In this study, five irrigation treatments were followed: T1 = Saturated, T2 = Continuous flooding (5 cm standing water), T3 = AWDI, 5 cm irrigation applied when water level in the pipe fell 10 cm below ground level, T4 = AWDI, 5 cm irrigation applied when water level in the pipe fell 15 cm below ground level and T5 = AWDI, 5 cm irrigation applied when water level in the pipe fell 20 cm below ground level. Perforated PVC pipes were installed in the experimental plots 10 days after transplanting (DAT) according to treatments for measuring soil water depletion to follow AWDI techniques. The diameter of pipe was 8 cm and the lengths were 25 cm, 30 cm and 35 cm, respectively. After irrigation had been applied, water entered through perforations and water levels inside the pipe were at same level as that of outside. With the progress of time, water got depleted in AWDI plots, but a close observation revealed that there was water inside the pipe and again 5 cm irrigation was done when the depleting water table inside the pipe reached a certain level from ground soil.
2.3. Determination of Effective Rainfall, Crop Water Requirement and Water Savings
Effective rainfall was estimated by USDA Soil Conservation Method (Smith, 1992).
1) (when, P total < 250 min.)
2) P effective = 125 + 0.1 × P total (when, P total > 250 win)
where, P effective = effective rainfall (mm) and P total = total rainfall (mm).
Total rainfall data of cropping period of that year was collected from the department of Irrigation and Water Management, Bangladesh Agricultural University. Again, water requirements for rice cultivation was computed by adding applied irrigation water, effective rainfall during growing season and water for land preparation (Rashid, 1997). In the study, water saving percentage was calculated as follows:
Water loss was calculated based on water amount supplied in each plot.
2.4. Gas Sampling, Analysis and Estimation of CH4 Emission
A closed-chamber method (Ali et al., 2008) was used to estimate CH4 emission during rice cultivation. Gas samples were collected by 50 ml gas-tight syringes at 0, 15 and 30 minutes after chamber placement over flooded plots at different rice growth stages to get average CH4 emissions. The dimension of closed chamber was 62 cm × 62 cm × 112 cm. Samples were analyzed to determine CH4 concentration by gas chromatograph (Shimadzu, GC 2014, Japan) with a Flame Ionization Detector. The temperatures of column, injector and detector were adjusted at 100˚C, 200˚C and 200˚C, respectively. A closed-chamber equation (Rolston, 1986) was used to estimate methane fluxes for every treatment.
where, F (Flux) = CH4 emission rate (mg CH4 m−2∙hr−1), ρ = gas density (0.714 mg∙cm−3), V = volume of chamber (A × h; m3), A = surface area of chamber (length × width; m2), h = height of the chamber (m), Δc/Δt = rate of increase of CH4gas concentration (mg∙m−3∙hr−1), T (absolute temperature) = 273 + mean temperature (˚C).
Total methane flux for the entire cropping period was computed by the formula (Singh et al., 1999):
Total CH4 flux = ,
where, Ri = rate of methane flux (g∙m−2∙d−1) in the ith sampling interval, and n = number of sampling intervals.
2.5. Estimation GWP of CH4
To estimate the GWP, CO2 is typically taken as the reference gas, and an increase or reduction in emission of CH4 is converted into “CO2-equivalents” by means of their GWPs. In this study, we used the IPCC factors to calculate the combined GWP for 100 years (GWP = 25 × CH4, kg CO2-equivalents ha−1) from CH4 under various agricultural irrigation practices. In addition, the greenhouse gas intensity (GHGI) was calculated by dividing GWP by grain yield for rice (Mosier et al., 2006).
2.6. Explorations of Rice Plant Growth and Yield
Rice plant growth parameters like plant height, root length were investigated during different growing period. Again, 4 - 5 cm long root samples collected at reproductive stage were conserved in proper tagged glass tubes (with FA solutions) for free hand sectioning to show anatomical view whether aerenchyma developed or not under a scanning electron microscope. Yield components such as panicle number per hill and grain numbers per panicle were determined at harvesting stage. When 95% grains became yellow, harvesting was done. After harvesting, crops were separately collected, bundled and tagged for threshing. Finally, the grains were sun dried up to14% moisture content, then weighed as kg∙ha−1.
2.7. Estimation of Water Productivity Index
Water productivity index is the ratio of crop yield (kg/ha) per unit water (m3/ha) supplied (Jaafar et al., 2000) and calculated as follows:
2.8. Investigation of Soil Properties
Soil redox potential (Eh) were measured during rice cultivation at certain time intervals by glass electrode Eh meter. At harvesting stage, soil bulk density (BD) was analyzed using cores (volume 100 cm3, inner diameter 5 cm), filled with fresh moisture soils. The collected core samples were oven dried at 105˚C for 24 h and then measured the weight of dried core samples. Soil porosity was calculated using BD and particle density (PD, 2.89 Mg∙m−3) according to the equation:
At harvesting stage, chemical properties of the collected soil samples were analyzed for organic carbon by wet oxidation method (Walkley & Black, 1965), organic matter content by multiplying the percent organic carbon with Van Bemmlen factor of 1.73, total nitrogen by Micro-kjeldhal method (Page et al., 1982), available phosphorus by Olsen method (Olsen et al., 1954), exchangeable potassium by Flame photometer (Brown & Lilleland, 1946).
2.9. Statistical Analysis
The compiled and tabulated data of rice growth, yield, soil properties and CH4 emission were statistically analyzed by Analysis of Variance (ANOVA) to examine whether treatment effects were significant or not. The mean differences among treatments were compared by Duncan’s Multiple Range Test. The computer software MSTAT-C was used for statistical analysis.
3.1. Effect of Irrigation Managements on CH4 Emission during Rice Growing Season
The different irrigation managements significantly influenced on CH4 emission from paddy field at all stages of growing season (Figure 1). Although, CH4 emission rate showed increasing trend from the very beginning of growing stage, suddenly fluctuated at active tillering stage (42 DAT), then gradually increased onwards and sharply increased from panicle initiation stage (56 DAT), peaked at flowering stage (84 DAT) and finally dropped at grain maturation stage (91 - 105 DAT) for all irrigation systems. In study, the highest CH4 peak was detected in continuous flooded plots and the lowest emission was in AWDI plots. Among AWDI plots, T3 (10 cm) showed comparatively higher CH4 flux than T4 (15 cm) and T5 (20 cm). Different considerable factors influencing CH4 emission are given below:
A negative correlation was observed between soil Eh and CH4 emission rate. It was found that soil redox potential significantly decreased by advancement of growth stage and AWDI treatments showed less soil Eh reduction than other
Figure 1. Changes in CH4 emission rate and soil redox status (Eh) under different irrigation treatments.
treatments which might be due to having capability of AWDI to improve the soil Eh of rice paddy soils. In study, CH4 emission was increased significantly with the development of soil reductive condition (Figure 1). When water supply was stopped before harvesting (after 84 DAT), the oxidation process was more for available oxygen that increased Eh values. The maximum and sharply decreased soil Eh value after tillering stage (42 DAT) from continuous flooded and saturated plots could be related to the development of anaerobic conditions and rapid decomposition of soil organic matter under waterlogged conditions. The intense reductive condition, e.g. soil Eh value −120 to −220 was noticed during reproductive stage which caused significant amount of CH4 emissions. AWDI significantly improved the soil redox potential status which ultimately decreased CH4 emissions during the entire rice growing period.
The highest CH4 emission rate was found in continuous flooded plots where aerenchyma tissue was well developed, probably due to waterlogging conditions (Figure 1). The most interesting phenomenon in this study is aerenchyma development in rice root during reproductive stage that was significantly influenced by different irrigation managements. The deficiency of oxygen mainly triggers root aerenchyma development which facilitates gas diffusion between roots and aerial environment. In study, AWDI plots showed less remarkable aerenchyma for more availability of oxygen as like as CH4 emission influenced by different irrigation managements (Figure 2). Again, the formation and more intensity of aerenchyma development in continuous flooded and saturated plots might have accelerated the gas exchange rate between the rhizospheric soil environment and the atmosphere.
The decomposition rate of soil organic matter was more under irrigated field most probably due to the presence of available water. In study, the decomposition rate was more in continuous flooded plots that showed the highest CH4 emission among all treatments. AWDI plots showed more soil organic carbon due to less decomposition that might be for less water requirements (Figure 3). After heading stage, CH4 flux was decreased constantly due to lack of CH4 producing conditions caused by the end of irrigation and plant senescence, when labile organic C are no longer released due to the lack of water for decomposition.
Figure 2. Development of aerenchyma tissue in rice root cortex at reproductive stage under different irrigation treatments. (a) Saturated conditions; (b) Continuous flooded; (c) AWDI, 10 cm; (d) AWDI, 15 cm; (e) AWDI, 20 cm.
Figure 3. Changes in soil organic carbon with plant growth stages under different irrigation water management practices.
3.2. Effect of Irrigation Managements on Rice Grain Yield and Water Productivity
During first 2 weeks after transplantation, 5 cm standing water was maintained in all plots to avoid weed infestation. Water required for crop establishment was 62.6 cm and for land preparation was 45 cm. Then, plots were irrigated according to treatments. Effective rainfall was 21.7 cm. In study, the maximum 13 irrigations required for continuous flooded plots followed by saturated plots (10 irrigations) in where AWDI treatments T3 (10 cm), T4 (15 cm) and T5 (20 cm) received 8, 7 and 6 irrigations, respectively. The required water volume were 19,430 m3∙ha−1, 17,930 m3∙ha−1, 16,930 m3∙ha−1, 16,430 m3∙ha−1 and 15,930 m3∙ha−1 for continuous flooded, saturated plot, AWDI-10 cm, AWDI-15 cm, AWDI-20 cm, respectively. Finally, it was found that AWDI-20 cm could save highest amount (18%) of irrigation water followed by AWDI-15 cm (16%) and AWDI-10 cm (13%) in where saturated plot could save only 8% irrigation water compared to continuous flooded plot.
The grain yield of rice was significantly increased by AWDIs. In present study, AWDI-10 cm gave the highest grain yield (6250 kg∙ha−1) followed by AWDI-15 cm (5810 kg∙ha−1) but the lowest yield (4283 kg∙ha−1) was found in AWDI-20 cm due to more water stress. Again, continuous flooded plots gave 5783 kg∙ha−1 grain yield most probably for more waterlogged conditions throughout the growing season. Table 1 also shows the highest water productivity index (0.369 kg∙m−3) was obtained in AWDI-10 cm followed by AWDI-15 cm (0.354 kg∙m−3), saturated plots (0.299 kg∙m−3), continuous flooded plots (0.298 kg∙m−3) and AWDI-20 cm (0.269 kg∙m−3).
3.3. Effect of Irrigation Managements on Seasonal CH4 Flux, GWP and GHGI
The total GWP of CH4 significantly decreased with AWDI treatments as compared to continuous flooded plots. The total and highest GWP from the continuous
Table 1. Rice grain yield, water productivity, water saving percentage, seasonal CH4 flux, GWPs and GHGI under different irrigation system.
In a column, figures having similar letter(s) or no letters do not differ significantly whereas figures with dissimilar letter(s) differ significantly as per DMRT. ** = Significant at 1% level of probability and * = Significant at 5% level of probability. aGWPof CH4 were calculated by multiplying 25 times on seasonal CH4 fluxes; The IPCC GWP factors (mass basis, kg CO2-equivalent ha−1) for CH4 is 25 in the time horizon of 100 years (Froster et al., 2007); bGHGI (kg CO2-equivalent kg−1 grain yield) was calculated by dividing GWPs of CH4 emissions by rice yield.
irrigated plot was 5.16 Mg CO2 ha−1 followed by saturated plots (3.82 Mg CO2 ha−1). Conversely, under AWDI treatments the lowest GWP from the AWDI-20 cm was 2.54 Mg CO2 ha−1 followed by AWDI-15 cm (3.05 Mg CO2 ha−1) and AWDI-10 cm (3.26 Mg CO2 ha−1). In general, intermittent irrigation system (AWDI) significantly reduced the total GWPs (Table 1), even though continuous flooded and saturated plots showed two-fold higher GWPs. The GHGI also decreased remarkably with AWDI treatments compared to other treatments. As a result, AWDI-20 cm reduced highest percentage (50%) of CH4 emission rate but gave lower yield due to more water stress compared to continuous flooded plots. AWDI-15 cm could reduce 40% CH4 emission with satisfactory yield although AWDI-10 cm reduced only 36% CH4 emission with the highest grain yield production.
3.4. Effect of Irrigation Managements on Nutrient Status Prior to Rice Harvesting Stage
In study, Table 2 shows that the intermittent AWDI system increased soil porosity, the content of soil organic matter compared to continuous irrigated control plots in where total N content, available P and K content showed non-significant variation under different irrigation treatments. AWDI did not affect the soil nutrient status in addition, increased organic carbon as like as soil organic matter status. Soil organic matter significantly varied from 2.10% to 1.90% in where the highest % OM was found in AWDI-15 cm and the lowest was in continuous flooded plots.
Table 2. Chemical properties of soil prior to rice harvesting stage.
** = Significant at 1% level of probability; * = Significant at 5% level of probability and NS = Not significant.
3.5. Correlations Correlation of CH4 Emissions with Other Parameters
There were negative correlations between total seasonal CH4 flux with grain yield, water productivity index, soil Eh, organic matter, total N, available P and K content except no. of irrigation, total volume of water supplied for rice grain production, soil porosity and soil pH under continuous irrigated treatment (Table 3). On the other hand, CH4 flux showed a strong positive correlation with no. of irrigations, total volume of water supplied for paddy field, grain yield, water productivity index, the availability of soil organic matter, soil pH, total N, available P and K content, while there were only negative correlation with soil Eh and soil porosity under AWDI systems.
In study, Alternate Wetting and Drying Irrigation (AWDI) significantly decreased the seasonal CH4 emissions during rice cultivation. The lower CH4 emission from AWDI plots was due to increased aeration, stabilization of soil organic carbon, improved soil redox potential status. Among AWDI treatments, T3 (10 cm) reduced 36% CH4 flux followed by T4-15 cm (40%) and T5-20 cm (50%). This result revealed that AWDI treatments are the most effective options for reducing CH4 emission during Boro rice cultivation, which was consistent with the findings of Jain et al. (2013). This result might be also due to highly promoting soil reduced condition that was influenced by different irrigation treatments (Supparattanapan et al., 2009). Ali et al. (2013) also reported that intermittent irrigations significantly reduced total seasonal CH4emissions by 27% compared to conventional (124 kg CH4/ha) irrigated rice paddy field.
The increased Eh values might have suppressed the methanogens’ activities that were responsible for CH4 emission from rice field, while stimulated the methanotrophs activities, and eventually decreased methane emission rate during rice cultivation (Ali, 2008). On the other hand, the highest CH4 emission rate
Table 3. Correlation of CH4 emissions with selected plant parameters and soil properties.
was found in continuous flooded plots due to more conductivity of CH4 gas through aerenchyma channel, more decomposable organic carbon and highly soil reduced status under waterlogged condition. Tarlera et al. (2016) also observed more CH4 emission rate in the treatment of continuous flooding than AWDI treatment. In the present study, although saturated plots showed low CH4 emission than continuous irrigated control plots but always gave high rate of CH4 emission than AWDI treatments.
The seasonal CH4 emissions gave more or less similar pattern in both cultivation systems, i.e. CH4 flux was higher during reproductive stage of rice growth in all treatments due to the development of root aerenchyma (Kludge et al., 1993), and decomposition of soil organic materials (Dubey, 2005). The sudden and sharp fall in CH4 emission rates at grain maturation stage in all might be due to the aging of rice plant, the end of irrigation and plant senescence, when labile organic C are no longer released by organic matter decomposition due to lack of available water that was also supported by Cai et al. (1997). Finally, AWDI treatments significantly could mitigate the rate of CH4 emission upto 50% (20 cm), 40% (15 cm) and 36% (10 cm) from that of continuous flooding in a cropping season which was supported by Win et al. (2015) and Itoh et al. (2011).
In our experiment, AWDI-20 cm saved upto 18% irrigation water with the lowest CH4 emission rate compared to continuous flooded treatment but gave lower yield due to more water stress. But AWDI—10 cm and 15 cm saved upto 13% and 16% irrigation water, respectively with minimum rate of CH4 emission compared to continuous flooded treatment as supported by Chapagain and Yamaji (2010). Yang et al. (2016) also reported that moderate AWDI—in which photosynthesis was not severely inhibited and plants could rehydrate overnight, during soil drying period or when water table was maintained at 10 to 15 cm below soil surface, could increase not only WUE but also grain yield.
AWDI treatments stimulated the more rice yield, which were negatively interlinked with total seasonal CH4 flux in where more root depth was found most probably for the more soil porosity that supported more tillers and ultimately more yield of rice. This might be due to more depth of rice root support more nutrient uptake from different soil nutrient layer. Rice grain yield was increased by 11%, only in AWDI (10 cm) in where AWDI (15 cm) gave more or less similar grain yield over the control irrigated plots in our experiment. Conversely, rice grain yield was negatively correlated with seasonal CH4 flux, which was supported by Denier van Der Gon et al. (2002). Among the all treatments, AWDI (10 cm) and AWDI (15 cm) are economically feasible and effective for mitigating seasonal CH4 flux and increasing rice productivity over the conventional flooded rice cultivation.
This field experiment confirmed that AWDI is a promising method which provides duel benefits of water saving and optimum rice productivity with low CH4 emission compared to conventional flood irrigation during dry season Boro rice cultivation. Conclusively, suitable alternate wet and dry irrigation water management practices (Irrigate when water level falls below 10 - 15 cm from surface) could be implemented in farmers’ field based on site specific agro-ecological zone for significant water savings, reducing methane emission and sustaining rice productivity. Further field trials in different agro-ecological zones may provide wider adaptability and farmers’ acceptability.
The authors are highly grateful to the National Science and Technology, Ministry of Science and Technology, 2015-2016, Government of the People’s Republic of Bangladesh for financial support after this research.
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