Impact of Nitrogen Fertilizers on Methane Emissions from Flooded Rice

Methane is second most potent greenhouse gas emitted under anaerobic condition in rice soils. Effects of different nitrogen fertilizer application on methane emissions in flooded paddy field were studied. The experiment was laid out in a randomized complete block design with three treatments and three replications. The treatments were control (0 kg N ha-1), urea (120 kg N ha-1) and ammonium sulfate (120 kg N ha-1). In all treatments P (60 kg P2O5 ha -1) along with K (40 kg K2O ha -1) were also applied as basal dose. The cumulative seasonal methane flux was highest in urea 36.3 (kg ha-1) followed by control 35.2 (kg ha-1) and ammonium sulfate 28.5 (kg ha-1). Ammonium sulfate application reduced total seasonal emission by 19.5% as compared to control while it reduced CH4 emissions by 21.6% as compared to urea application. On the basis of this study we can conclude that application of ammonium sulfate is an effective tool for mitigating methane emissions from rice soils.

Methane (CH4) atmospheric concentration has significantly rises due anthropogenic activity. Graedel and McRae (1980) presented first evidence that atmospheric concentration of CH4 is increasing. In agriculture submerged paddy soils are the major source of CH4 emission to atmosphere. Under continues standing water soil redox potential (Eh) drops sharply within few days and leads to process methanogenesis in soil . In methanogensis, soil archaea methanogens degraded organic matter and produce CH4 . CH4 emission from rice soil is a net balance of production by methanogens in reducing environment after oxidation by methanotrophs in oxidizing environment and it is influenced by several factors such as water conditions, Eh, soil temperature, pH, fertilizer managements, and organic matter , Hussain et al., 2015. Rice (Oryza sativa L.) is second most consumed cereal in world after corn and out of total rice 90% is cultivated in Asia under irrigated conditions (Pramanik and Kim, 2016). CH4 is second most potent greenhouse gas after carbon dioxide and it is 25 times greenhouses gas as compared to carbon dioxide (Bhatia et al. 2011). According to IPCC (2014) CH4 contributes 16% of total emissions at global level and out of total rice field alone contribute 10% of total CH4 emission at global level (Global Methane Initiative, 2011).  reported that by the end of twenty first century global mean temperature may rise up to 1.5 ˚C due to raise in global greenhouses gases atmospheric concentrations. Global warming is major concerned of 21 st century for scientific and policy maker. As the world population was increasing so under such scenario CH4 mitigation from rice field needed without having any negative impact on rice production. Rice production depends on type and amount of nitrogen (N) based fertilizer applied for cultivation. N based fertilizer amendments may be used for CH4 emissions mitigation from the rice soil. Impact of different N based fertilizer on CH4 emission is less elevated so it is needed. The objective of field experiment is to evaluate the impact of nitrogen fertilizer on CH4 emissions from rice soil under continuous flooded condition. Institute, New Delhi, India, during kharif cropping season of year 2015 (Fig. 1.) The climatic condition of the region was sub-tropical, semi arid that was characterized by dry winter and maximum rainfall occurs during from June to September of year Fig. 2.The soil in experimental site was sandy loam in texture and pritransplanting physicochemical of experimental site soil are mentioned in Table 1.

Experimental Design and Treatments Details
The experiments consist of three treatments with three replicate each which are arranged in RBD. Composition and dose of various treatments were mentioned in Table 2. Pusa Basmati-1509 variety of rice (Oryza sativa L.) was adopted for conducting the experiment. Two to three rice seedlings (23 days age) were transplanted at 15 x 20 cm spacing. Continuous flooding condition at 8 ± 4 cm water level was maintained by groundwater irrigation for entire cropping period. The field was naturally allowed to dry three weeks before harvesting of crop. No chemical interventions (pesticide and herbicide) were applied to avoid their additional effects. Weeding in done manual when were required.

Methane Sampling Collection and Analysis
Gas samples were collected at 7 days regular interval throughout the cropping years using manual closed chamber technique (Hutchinson and Mosier 1981). Gas samples were collected between 9 am to 11 am and samples were withdrawn from top of the chamber using 20 ml air-tight syringes at 0, 1/2 and 1 hrs. Concentration of CH 4 gas in the collected gas samples were measured by using gas chromatography equipped with column and a flame ionization detector according to Hutchinson and Mosier (1981).

RESULTS AND DISCUSSION
Methane emission among all treatments was low during first three weeks and significantly increased with plant growth and lower soil Eh. The highest flux peak was observed between 35-42 days after transplanting (DAT) and second peak occur between 56-70 DAT (Fig 1). Two higher CH 4 peaks may be due to degradation of soil organic matter by methanogens bacteria under anaerobic conditions and similar flux were also reported by Suryavanshi et al. (2013) in rice soil. The cumulative seasonal CH 4 flux was 35.2 kg ha -1 under the control treatment. The maximum cumulative CH 4 flux was recorded in urea (36.3 kg ha -1 ) treatment followed by control (35.2kg ha -1 ) and ammonium sulfate (28.3 kg ha -1 ). As compare to control, urea fertilizer application enhances CH 4 emissions by 2.72% and ammonium sulfate amendments reduce CH 4 emissions by 19.5% as wwww compared to control (Fig. 2.). Ammonium sulfate application mitigates total seasonal CH 4 emissions by 21.6% over to urea Fig. 2. The higher CH 4 emission under nitrogen applied plots over no nitrogen amendments has been reported (Xia et al. 2014). Urea application enhances the ammonium ions concentration in soil and due to structural symmetry between CH 4 and ammonium ion ) methanotrophs bind with  Control N (0 kg N ha -1 ), P (0 kg P 2 O 5 ha -1 ), K (0 kg K 2 O ha -1 )
Ammonium Sulfate N (0 kg N ha -1 ), P (60 kg P 2 O 5 ha -1 ), K (40 kg K 2 O ha -1 ), AS (120 kg N ha -1 ) P and K were applied basally, while N (Ammonium sulfate) applied in three splits in 50% (basal) and, 25% (tillering) and 25% (panicle initiation) of total dose. wwww ammonium ions instead of CH 4 therefore results in less CH 4 oxidation by methanotrophs in soil which final result in higher CH 4 emission from soil (Schimel, 2000). Minami (1995) observed about more than 15% reduction in average CH 4 flux from rice soil by soil incorporated with ammonium sulfate at 200 kg N ha -1 rate as compared to 200 kg N ha -1 urea incorporation. Similar finding were also observed by Ali et al. (2012) and they reported 16% and 21% reduction in total seasonal CH 4 flux by ammonium sulfate over urea in upland and lowland rice soil in Bangladesh respectively. On ammonium sulfate ions soil concentration of active sulfate ions increases (Ali et al. 2012) which result in higher population of sulfate reducing bacteria in soil. Sulfate reducing bacteria compete with methanogens bacteria for organic matter as they both feed on similar substrate (Hussain et al. 2015) therefore on application of ammonium sulfate suppressed methanogens activity in soil which result in CH 4 flux reduction form rice soil.

CONCLUSION
In is field study we evaluate the impact different nitrogen based fertilizer impact on methane emission from paddy soil. A total cumulative methane emission was highest in urea applied plots and lowest in ammonium sulfate plots. Ammonium sulfate application reduces 19.5% and 21.6% as compare to urea and control respectively. Therefore, based on this field study data it could be suggested that application of ammonium sulfate significantly reduce methane from rice soils.