Effect of Soil Amendments on Methane Emission and Rice Productivity nearby the Dingaputa Haor area of Netrokona District, Bangladesh

An experiment was conducted nearby the Dingaputa haor area of Netrokona District during boro season. The aim of the study was to find out the most suitable soil amendment for reducing CH4 emission and maximizing the yield attributes of BRRI dhan58. In this experiment six treatments, such as T1: 100% recommended dose of urea (220 kgha), T2: 50% Urea+Vermicompost (4 tha-1), T3: 50% Urea+Azolla incorporated (4 tha-1), T4: 25% Urea+Azolla incorporated (4tha-1)+ Vermicompost (4 tha-1), T5: 25% Urea+Azolla incorporated (6 tha-1)+Vermicompost (2 tha1), T6: 25% Urea+Azolla incorporated (6 tha-1)+Azolla dual cropping (1tha-1) with Cyanobacterial mixture were used. At 14 DAT, CH4 flux was very low and no significant differences were observed among the treatments. At 70 DAT, CH4 emissions peaked in all treatments where highest peak was recorded (30.39 mgm–2h–1) in treatment T6 and the lowest was recorded (16.38 mgm–2h–1) in T2. The highest grain yield (6.50 th–1) was found in the treatment T4 while lowest grain yield (5.37 th–1) was found in the treatment T3. After rice harvest the soil properties such as soil pH, total nitrogen, organic carbon, phosphorus, potassium and sulphur was found 6.94, 0.16%, 1.58%, 14.52 ppm, 0.10 meq 100g–1 and 9.65 ppm respectively. Considering all the above parameters it may be concluded that, the application of 25-50% of the recommended Urea along with Azolla incorporated (4tha-1) and Vermicompost (4tha-1) amendment could be suitable for maximizing of rice yield and reducing CH4 emission.


Introduction
Bangladesh is an agricultural country and rice is the main food crop. Rice has been growing over 25 million hectares of land under irrigated and rain fed conditions, which cover about 84% of total cropped area in Bangladesh. The pressure on Bangladesh land resources to produce more rice will aggravate in the coming years due to increasing population and demand for food. Rice demand would increase by 25% to keep pace with population growth [1]. Rice is produced at least twice in the same crop field in Bangladesh. High fertilizer responsiveness is an essential criterion for a high yielding rice varieties and nitrogen is one of the major nutrient elements for crop production that can contribute a lot for higher yield of rice [2]. In case of Rice Fallow-Rice cropping pattern, one rice crop is fully irrigated (Boro rice) and another is mostly rainfed (T. Aman rice). The flooded rice paddy has been identified as one of the most important sources of anthropogenic CH 4 emission. The CH 4 is an important greenhouse gas (with a 21-fold higher global warming potential than CO 2 over a 100-year time horizon, [3], which has been reported to account for 95% of total CO 2 equivalent emissions from paddy fields [4]. The differences in plant growth duration among rice cultivars affected the total seasonal CH 4 emission from flooded soil. Combination of various factors such as the supply of organic matter, inherent characteristic, depth of water level, size of the root space and oxidation rate in the rhizosphere have also been identified to affect the CH 4 flux from various rice cultivars. To date no systematic study on organic fertilizers have been conducted to determine an optimum organic fertilizer management •02• The area of each plot was 10m 2 (4m × 2.5m). There was 100 cm drain surrounding of each unit of the plot. The total area of the experimental plot was 18 plots x 10 m 2 = 180 m 2 ( Figure 1).

Fertilizer application
Standard recommended doses of fertilizers were used in the experimental plots. At the time of final land preparation nitrogenous fertilizer in the form of urea (prilled) was applied as basal dose at the rate of 220 kg ha -1 and amounts of urea was applied in 2 equal splits at 30 and 60 days after transplanting. Organic fertilizers were applied after making sub-plots at the time of final land preparation according to the design (Tables 1-3).

Analytical techniques of gas sample collection
Gas samples were collected by using the closed-chamber method [13] during the rice cultivation. The dimensions of close chamber were 62 × 62 × 112 cm 3 . Three chambers were installed in each experimental plot. Gas sample was collected at different growth stages to get the CH 4 emissions during the cropping season. Gas sample was collected in 50 ml gas-tight syringes at 0, 10-and 20-minutes intervals after chamber placement over the rice planted plot. The samples were analyzed for CH 4 by using gas chromatograph equipped with an FID (flame ionization detector). The analysis column used a stainless-steel column packed with Porapak NQ (Q 80-100 mess). The concentration difference between 0, 10 and 20 min give the total emission occurred when gas chamber practice to maintain a high yield of rice grain while reducing CH 4 emissions to a minimum [5]. The CH 4 is produced in soils by the microbial breakdown of organic compounds in strictly anaerobic conditions at redox potential less than -150 mV [6]. There are two major sources of methane emissions, one is natural source and another is anthropogenic source. More than 50% of the global annual CH 4 emission is of anthropogenic origin [3]. It is reported that irrigated rice accounts for more than 75% of global rice production and these rice fields are one of the major sources of CH 4 gas [7]. Since irrigated rice remains continuously flooded most of the time during growing season, this creates the ideal condition for CH 4 emission. Recent estimates of CH 4 emission from rice fields show that its rate varies within the range of 39 and 112 Tg CH 4 year -1 which is equivalent to 6 to 18% of total global CH 4 flux [7]. A statistical analysis of the CH 4 emission fluxes from rice fields in Asia showed that the average CH 4 flux during the growing season is significantly affected by water management, organic matter application, soil organic carbon content, soil pH, and climate [8]. It is also influenced by soil type, weather, tillage management, residues, fertilizers, and rice cultivar. Therefore, manipulation of this factor can help reduce CH 4 emission. Thus, several studies were conducted to mitigate CH 4 emission in rice fields through soil and water management [9]. Cyanobacteria play an important role in maintenance and buildup of soil fertility, consequently increasing rice growth and yield as a natural bio-fertilizer [10]. The agricultural importance of cyanobacteria in rice cultivation is directly related with their ability to fix nitrogen and other positive effects for plants and soil [11,12]. The beneficial effect of cyanobacteria in decreasing the headspace concentration of methane (CH 4 ) by increasing dissolved oxygen concentration which enhance the methane oxidation at source is also reported [10]. Blue Green Algae (BGA) reduce methane (CH 4 ) flux without reducing rice yields that can be used as a practical mitigation option for minimizing the global warming potential of rice ecosystem. Considering such thing this study was undertaken to find out the effects of soil amendments on CH 4 emission during rice cultivation; to determine the soil properties after rice harvest and to determine the growth and yield of rice under different soil amendments.

Materials and Methods
The experiment was carried out during boro season (December 2015 to April 2016). The study was undertaken nearby the Dingaputa haor area located between the latitudes of 24°52′ N to 24

Experimental Design
The experimental design was laid out in a Randomized Complete Block Design (RCBD) with 3 replications. The experimental field was divided into 3 blocks with 6 treatments. Thus, the total numbers of unit plots were 18.

Before Transplantation
After Transplantation    •03• was closed. The temperature of column, injector and detector were adjusted at 60°C, 120°C, and 220°C, respectively. Methane emission from the paddy field was calculated from the increase in CH 4 concentrations per unit surface area of the chamber for a specific time interval. A closed-chamber equation [14] was used to estimate methane fluxes during rice cultivation.

Estimation of methane emission
Methane emission from the paddy field was calculated from the increase in CH 4 concentrations per unit surface area of the chamber for a specific time interval. A closedchamber equation [14] was used to estimate methane fluxes during rice cultivation.

Statistical analysis
The findings were analyzed by partitioning the total variance with the help of computer by using MSTAT program. The treatment means were compared using Duncan's New Multiple Range Test (DMRT) as outlined by [15].

Result and Discussion
A field experiment was carried out to find out the results of the study regarding the effect of different soil amendments on total CH 4 emission and rice productivity.

Effect of soil amendments on CH 4 emission
CH 4 emission rate was significantly affected by different soil amendments (Figure 2). CH 4 emission was recorded 0.88 to 2.48 mg m -2 h -1 at 14 DAT where no significant differences were observed among the treatments. Similarly, at 28 DAT or active tillering stage, CH 4 emission ranged from 6.12 to 12.94 mg m -2 h -1 where treatment T 6 showed the highest (12.94 mg m -2 h -1 ) and treatment T 2 showed the lowest (6.12 mg m -2 h -1 ) CH 4 emission. At 49 DAT or panicle initiation stage, treatment T 6 and T 2 further showed the highest and lowest CH 4 emission (17.64 mg m -2 h -1 and 9.90 mg m -2 h -1 respectively). Similar trend was observed at 70 DAT. At 70 DAT, CH 4 emissions peaked in all treatments where highest peak was recorded (30.39 mg m -2 h -1 ) found in the treatment T 6 and the lowest (16.38 mg m -2 h -1 ) was recorded in T 2 . Finally, at 91 DAT or ripening stage treatment T 6 and T 2 further showed the highest and lowest CH 4 emission at 18.27 •04• mg m -2 h -1 and 11.73 mg m -2 h -1 respectively. The highest percentage of CH 4 emission (30.39%) occurred when the rice plots were treated with 25% Urea+6 ton azolla incorporated ha -1 +1 ton azolla dual cropping ha -1 with cyanobacterial mixture(T 6 ) and the lowest percentage of emission was obtained (0.88%) in those plots which were treated with 50% Urea + 4 ton vermicompost ha -1 (T 2 ). Therefore, this result revealed that the azolla with cyanobacterial mixture application as organic amendment was responsible for increasing CH 4 emission in rice field as compared to urea or vermicompost. In the study on an average, CH 4 emission rate during rice cultivation followed the sequences: T 6 > T 3 > T 1 > T 5 > T 4 > T 2. [4] studied that the application of azolla incorporated and other organic fertilizers increases CH 4 emission than that from inorganic fertilizers application. CH 4 emission increased with increasing amounts of rice straw and a peak in CH 4 emission at the end of the reproductive stage was observed in all fields receiving rice straw. Total CH 4 emissions ranged from 4.04 to 40.8 g CH 4 m -2 per growing season and emissions were 2-10-fold greater than from fields with no rice straw [4]. Methane emission was higher during flowering to maturity stages which dropped during later stages before harvesting. Ali MA, et al (2012) [16] Reported that, 25% recommended urea + Azolla Cyanobacteria (1 tha -1 ) decreased CH 4 emission by 11% in low land rice field and rice grain yield was increased by 8%.

Effect of soil amendments on soil properties
Soil redox potential (Eh): The effect of soil amendments on soil redox potential is presented in Figure 3. where (T 2 )50% Urea + Vermicompost (4 t ha -1 ) showed the less reduction of Eh (-42.87 mV) and treatment T 4 (25% Urea + Azolla incorporated (4 t ha -1 ) + Vermicompost (4 t ha -1 ) showed the highest reduction of Eh (-92.40 mV). At 28 DAT or active tillering stage, less reduction of Eh was observed from the 50% Urea + Vermicompost (4 t ha -1 ) in treatment T 2 and more reduction of Eh (-171.33 mV) was found in treatment T 4 while lowest reduction of soil redox potential of Eh (-98.23 mV) in the treatment T 2 . At 49 DAT or panicle stage, highest and lowest Soil redox potential (Eh) was found 186.33 mV and -227.00 mV, respectively in the treatments of T 2 : (50% Urea+4 ton Vermicompost ha -1 )and T 6 : (25% Urea + Azolla incorporated (6 t ha -1 )+ Azolla dual cropping (1 t ha -1 ) with Cyanobacterial Mixture). At 70 DAT or flowering stage, treatment T 4 showed the higher capability to reduce the soil redox potential (-238.33 mV) which was not significantly differed (196.00 mV) from T 5 (25% Urea + Azolla incorporated (6 t ha -1 ) + Vermicompost (2 t ha -1 ). Finally, at 91 DAT or ripening stage, the above significant variation range of Eh revealed that, the lowest (-131.00 mV) and highest (-174.00 mV) reduction of Eh were recorded from the T 6 and (196.00 mV) by T 4 . From the above result, it is found that the application of Azolla cyanobacteria as soil amendments significantly reduced the soil redox potential (Eh). Similar trends of changes in soil Eh was reported by [13]. In this study soil redox potential value (Eh) showed follow the sequence: T 2 > T 4 > T 5 > T 1 > T 3 > T 6 .

Soil pH after harvest of rice:
With the application of soil amendments pH range of post-harvest soil was significantly influenced (Table 4). It was evident that, the higher pH value (6.94) was found in the treatment T 6 (25% Urea + Azolla incorporated (6 t ha -1 ) + Azolla dual cropping (1 t ha -1 ) with Cyanobacterial Mixture) and the lower (6.63) was found in the treatment T 2 : 50% Urea + Vermicompost (4 t ha -1 ). Phy C, et al (2014) [17] Reported that the soil amendments application had significant effect on soil pH.
Total Nitrogen: Total Nitrogen content ranged from 0.13 to 0.16% and varied due to the effect of different soil amendments. Table 5 represented that treatments T 4, T 5 andT 6 was most effective for contributing the TN content in soil as compared to other treatments. Kamara A, et al. (2015) [18] Also reported that the Azolla treated soils improved the chemical properties of soil as well as the N content compared to the control or other treated soil.  •05• Available Phosphorus: The higher content of phosphorus (14.52 ppm) was recorded from the treatment T 4 while T 5 and T 2 treatment produced statistically lower content of P (10.16 ppm and 10.40 ppm respectively) ( Table 6). Kamara A, et al. (2015) [18] Stated that the application of organic fertilizers improved available phosphorus and cation exchange capacity in soils.

DAT
Exchangeable Potassium: The ranges of K content were 0.07 to 0.10 meq 100g -1 while the highest content was found from those soils the treatments T 3 and T 4 respectively while, the lowest content (0.07 meq 100g -1 ) was obtained from T 1 but they were statistically identical due to non-significant variation ( Table 6).

Available Sulphur:
The higher content of sulphur (9.65 ppm) was recorded from the treatment T 4 while T 5 treated soil produced lowest content of sulphur (5.59 ppm). Kimetu Organic Carbon: Organic carbon varied from 1.29 to 1.58% where the lowest amount of organic carbon was found from those soils which were not treated by any organic amendments (only 100% recommended dose of urea) while treatment T 6 showed the higher percentage of OC (1.58%). This result revealed that only Azolla and cyanobacteria as soil amendment can be produced more OC in soil compared to urea fertilizer and similar observation was also found by [19,20] (Tables 5).

Harvest Index (HI %)
With the application of different soil amendments grain harvest index significantly influenced (Table 7). Harvest Index (HI) of different soil amendments influenced in different way at different stages of BRRI dhan58. It was evident that the higher harvest index (44.71%) was found in the treatment T 4 and the lower (41.67%) was found with the use of T 3 . In case of rice production, it was found that organic amendments increased the yield of rice than control treatment.

Conclusions
From the obtained results it was found that the CH 4 emission was significantly varied from 0.88 mg m -2 h -1 to 2.48 mg m -2 h -1 at 14 DAT where no significant differences were observed among the treatments. Similar trend of results was also found at 48 DAT while it was ranges from 6.12 mg m -2 h -1 to 12.94 mg m -2 h -1 in treatment T 6. The highest CH 4 flux was observed in T 6 treatment and where the lowest CH 4 flus was recorded in treatment T 2 . However, methane emission showed the highest peak at 70 DAT in all treatments. The highest amount (30.39 mg m -2 h -1 ) of CH 4 emission flux was observed in treatment T 6 (25% Urea + Azolla incorporated (6 t ha -1 )+ Azolla dual cropping (1 t ha -1 ) with Cyanobacterial mixture) within the 70 DAT while the lowest (16.38 mg m -2 h -1 ) was recorded in the treatment T 2 (50% Urea + Vermicompost (4 t ha -1 ). After rice harvest the value of soil properties such as soil pH, total nitrogen, organic carbon, phosphorus, potassium and sulphur was observed 6.94, 0.16%, 1.58%, 14    •07• and 9.65 ppm, respectively in T 6 , T 4 and T 3 treatments. Considering the CH 4 emission trend during rice cultivation the treatments sequence may be ranked as T 6 > T 3 > T 1 > T 5 > T 4 > T 2 . On the other hand, on the basis of grain yield the treatments may be ranked as T 4 > T 6 > T 2 > T 5 > T 1 > T 3 . Considering all the above parameters it may be concluded that the application of 25-50% of the recommended Urea along with Azolla incorporated (4 tha -1 ) and Vermicompost (4 t ha -1 ) amendment could be suitable for maximizing of rice yield and reducing CH 4 emission. From the knowledge of this experiment, rice fields enriched with different soil amendments are the significant source of plants nutrients. Now rice growers would be able to select the suitable soil amendments for rice cultivation considering the negative effect of CH 4 gas emission from rice field. It would also help the farmer to select easily the different soil amendments which can give more production on the availability to them. As a result, rice production could be increased through utilization of suitable soil amendments while CH 4 gas emission could be controlled from rice field. Considering the above facts of the present study, the following recommendation may be suggested: • • Further study may be needed to ensure the studied performance in another AEZ-9 area for observing the adaptability.
• Different suitable soil amendments may be needed to include for further study to make sure the present findings of the study.