Nitrification Inhibitors : A Perspective tool to Mitigate Greenhouse Gas Emission from Rice Soils

Rice fields are significant contributors of greenhouse gases mainly methane and nitrous oxide to the atmosphere. Increasing concentrations of these greenhouse gases play significant role in changing atmospheric chemistry such as mean air temperature, rainfall pattern, drought, and flood frequency. Mitigation of greenhouse gases for achieving sustainable agriculture without affecting economical production is one the biggest challenge of twenty first century at national and global scale. On the basis of published scientific studies, we hereby assess the use of nitrification inhibitors for greenhouse gas mitigation from rice soil. Biologically oxidation of ammonium to nitrate is termed as nitrification and materials which suppress this process are known as nitrification inhibitors. Soil amendment by addition of certain nitrification inhibitors such as neem oil coated urea, nimin-coated urea; dicyandiamide, encapsulated calcium carbide, and hydroquinone reduce cumulative methane and nitrous oxide emission from rice. Firstly, these inhibitors reduce nitrous oxide emissions both directly by nitrification (by reducing NH4 + to NO3 -) as well as indirectly by de-nitrification (by reducing NO3 availability in soil). Secondly, methane emission from rice soil can be reduced by enhancing methane oxidation and suppressing methane production and further by reducing the aerenchymal transportation through rice plant. Application of some of the nitrification inhibitors such as calcium carbide and encapsulated calcium carbide reduce methane production by releasing acetylene gas which helps in reducing the population of methanogenic microbes in the soil. Application of nitrification inhibitors also helps to maintain soil redox potential at higher level subsequently reducing cumulative methane emission from soil. Plant derived organic nitrification inhibitors (neem oil, neem cake, karanja seed extract) are eco-friendly and possess substantial greenhouse gas mitigation potential from rice. In the current scenario of global warming and environmental pollution, application of organic plant derived nitrification inhibitors is much needed for sustainable agriculture.

Global climate change is one of the biggest challenges of the twenty first century.Enhance greenhouses effect lead to rise in mean global air temperature and it is projected that mean temperature may increases from 1.5 to 4.5 º C by the end of 21 st century (IPPC 2013).Rise in atmospheric greenhouses gases (GHGs) such carbon di-oxide, methane (CH 4 ), chlorofluorocarbon and nitrous oxide (N 2 O) concentration in atmosphere due to anthropogenic activities leads to global warming (IPCC 2007).According to IPCC (2014) carbon dioxide (Fossil fuel and industrial processes), Carbon di-oxide (Forestry and other land use), CH 4 , N 2 O and fluorinated gases contributes 65 %, 11 % , 16%, 6% and 2 % respectively at global level in 2010 (Figure 1).Methane and nitrous oxide are two major GHGs emitted from rice (Oryza sativa L.) agro-ecosystem.At global level rice cultivation alone contribute 10 % of total CH 4 emission (GMI 2011) while the global warming potential of N 2 O is 298 times higher (Rees

CONCluSION
The increasing trend of human population in Indian and at global level creates treamdmous pressure on agricultural system for feeding.This demand leads to evolution of modern agriculture and rice is important stable food for majority of population in world.Rice production is major source for greenhouse gases emission which leads to global warming.In this study we synthesized the published data to provide suitable rice management for greenhouses mitigation by fertilizer management (Nitrification inhibitors).We found that nitrification inhibitors interventions in rice can be one effective tool to anticipate global warming.For instances, inorganic nitrification inhibitors such DCD, HQ and thiosulphate application have methane and nitrous oxide mitigation potential from rice cultivation.Recently the mitigation potential of few organic nitrification inhibitors such as karanjin, nimim and neem oil coated urea was also explored.Fertilizer management practices like nitrification inhibitors application sound environmental friendly and help in achieving sustainable agricultural goal.
et al. 2013) than carbon di-oxide, so mitigation of both CH 4 and N 2 O is needed to combat global warming.There are mainly four different types of rice ecosystem namely upland, rainfed, irrigated and deer rice ecosystem (Adhya et al. 2014).Deep rice and irrigated rice ecosystem are main sources of CH 4 emissions to atmosphere will upland and dry period in between continuous flooded are main sources of N 2 O emissions to atmosphere from rice soil.In deep and continuous irrigated rice anaerobic conditions lead to sharp decline in soil redox potential (Ali et al. 2015; Hussain et al. 2015; Dubey 2005) which results in CH 4 production.In flooded rice methanogens bacteria consume soil organic carbon and emit CH 4 (Nazaries et al. 2013; Penning and Conrad 2007).Methane produce by methanogens in rice soil, generally emits to atmosphere by three (diffusion, ebullition and aerenchymal transportation) possible mechanisms (Green 2013; Tokida et al. 2013; Das and Baruah 2008; IPCC 1996; Neue 1993).In rice soil N 2 O is produce by both biological (nitrification and denitrifcation) and chemical decomposition process (Lan et al. 2014; Baggs 2011; Ussiri and R. Lal 2007; Freney 1997; Bremner 1997).Nitrogen base fertilizer are main sources of N 2 O production in rice soil and about 1.25 % of the total applied nitrogen is converted into atmospheric N 2 O (Bouwman 1994) under aerobic condition in soil but under flooded rice less than 0.

and nitrous oxide mitigation potential of nitrification inhibiters in rice
1 % of applied N fertilizer is emitted as N 2 O (Freney et al 1997).Methane and N 2 O production is rice soil is effect by several factors such as water managements, soil pH, redox potential, temperature, soil matter of the soil, soil microorganisms diversity, transplanting methods, rice cultivar, Crop duration and type of time of fertilizer application (Hussain et al. 2015; Hadi et al. 2010; Conrad 2007; Dubey 2005; Conrad 2002, Le Mer and Roger 2001).The nitrification inhibitor play potent role in mitigating GHGs emissions from different rice argo-ecosystem.In this review study we develop understating of NI in rice soil to mitigate GHGs emissions to combat global warming issues.Biological oxidation of ammonium to nitrate through nitrite by nitrifying bacteria species Nitrosomanas and Nitrobacter respectively is known as Nitrification and material which retard or inhibit nitrification process termed as Nitrification inhibitors (NI) ( Hussian et al. 2015; Saharwat 2004).In current scenario of global warming application of NI for reducing greenhouse gases emissions such as N 2 O and CH 4 from rice soil have good environmental sound as this compound also reduce nitrate water pollution load also.NI reduces N2O emission directly by retarding nitrification in soil and by reducing availability of nitrate for de-nitrification indirectly.

table 1 : Influences of different nitrification inhibiters on GHGs emission in rice soil
Malla et al. (2005)la et al. (2005)reported that HQ, DCD, thiosulpahte along with urea and calcium carbide coated urea mitigate N 2 O emission by3.95, 17.11, 34.21and 28.05 % as compare to control from rice soil (Table 1).Li et al. (2009) observed that HQ and DCD basal application in rice reduce total cumulative N 2 O emission to 3.90 kg ha -1 as compare to control 3.90 kg ha -1 .HQ/DCD application at tillering and panicle intiation stages of rice reduces N 2 O emission by 55.64 and 17.18 % respectively in Chinese soil (Table 1).Pathak et al. (2002) reported that soil amendment by DCD along with urea mitigate N 2 O emission by 21.39 % than soil amendment by urea alone in rice soil.Xu et al. (2002) also observed that HQ, DCD and HQ plus DCD reduces N 2 O emissions to 13.2, 9.14 and 6.51 mg N 2 O-N pot -1 as compare over control (17.25 mg N 2 O-N pot -1 ) respectively.