Environ Sci Pollut Res DOI 10.1007/s11356-014-3760-4

REVIEW ARTICLE

Rice management interventions to mitigate greenhouse gas emissions: a review Saddam Hussain & Shaobing Peng & Shah Fahad & Abdul Khaliq & Jianliang Huang & Kehui Cui & Lixiao Nie

Received: 18 September 2014 / Accepted: 20 October 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Global warming is one of the gravest threats to crop production and environmental sustainability. Rice, the staple food of more than half of the world’s population, is the most prominent cause of greenhouse gas (GHG) emissions in agriculture and gives way to global warming. The increasing demand for rice in the future has deployed tremendous concerns to reduce GHG emissions for minimizing the negative environmental impacts of rice cultivation. In this review, we presented a contemporary synthesis of existing data on how crop management practices influence emissions of GHGs in rice fields. We realized that modifications in traditional crop management regimes possess a huge potential to overcome GHG emissions. We examined and evaluated the different possible options and found that modifying tillage permutations and irrigation patterns, managing organic and fertilizer inputs, selecting suitable cultivar, and cropping regime can mitigate GHG emissions. Previously, many authors have discussed the feasibility principle and the influence of these practices on a single gas or, in particular, in the whole agricultural sector. Nonetheless, changes in management practices may influence more than one gas at the same time by different mechanisms or sometimes their effects may be antagonistic. Therefore, in the present attempt, we estimated the overall global warming potential of each approach to consider the

Responsible editor: Philippe Garrigues S. Hussain : S. Peng : S. Fahad : J. Huang : K. Cui : L. Nie (*) National Key Laboratory of Crop Genetic Improvement, MOA Key Laboratory of Crop Ecophysiology and Farming System in the Middle Reaches of the Yangtze River, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan 430070, Hubei, China e-mail: [email protected] A. Khaliq Department of Agronomy, University of Agriculture, Faisalabad 38040, Punjab, Pakistan

magnitude of its effects on all gases and provided a comprehensive assessment of suitable crop management practices for reducing GHG emissions in rice culture. Keywords Carbon dioxide . Crop management . Global warming potential . Greenhouse gases . Methane . Nitrous oxide . Rice

Introduction Global warming is one of the most prominent challenges in the present era. Global warming is caused by the increased concentration of greenhouse gases (GHGs) in the atmosphere and leads to a phenomenon widely known as “greenhouse effect.” Increased food demands necessitate amid global efforts to increase crop production that ensure food security and also protect environment and natural resources through reduced GHG emissions (Burney et al. 2010; Tilman et al. 2011). The global mean annual temperature is considerably increasing due to enhanced greenhouse effect. The projected rise in temperature by the end of the twenty-first century is estimated at 1.1 to 6.4 °C (IPCC 2007). The rising of the Earth’s temperature is governed by various exotic components including water vapor, ozone, methane (CH4), carbon dioxide (CO2), nitrous oxide (N2O), and chlorofluorocarbons, which absorb heat leading to a concomitant increase in atmospheric temperatures with consequences of disasters like floods, hurricanes, and drought. Increasing GHG emissions is also evident from the literature (Smith et al. 2007; IPCC 2007), and Verge et al. (2007) predicted that GHG emissions may increase by 35–60 % at the end of 2030 (Fig. 1). Globally, CO2, CH4, and N2O contribute 60, 15, and 5 % to the anthropogenic GHG effect, respectively (Rodhe 1990). Of the total anthropogenic emissions, CH4 and N2O are the major contributors to GHGs owing their origin to the agricultural sector (Fig. 2a).

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Both of these gases have 298 and 25 times more global warming potential (GWP) as compared to CO2 (IPCC 2007). Rice is arguably one of the most important cereal crops feeding more than half of world’s population. Globally, rice fields cover around 153 million hectares comprising approximately 11 % of the world’s arable lands (FAOSTAT 2011). The world demand for rice is predicted to increase by about 24 % in the next 20 years (Van Nguyen and Ferrero 2006). Moreover, rice fields are major sources of CH4 (Fig. 2b) and N2O and can also be source or sink of CO2. According to IPPC (2007), rice fields contribute about 30 and 11 % of global agricultural CH4 and N2O emissions, respectively. The GWP of rice crop is 467 and 169 % higher than wheat and maize (Fig. 2c; Linquist et al. 2012a). In rice fields, CO2 is emitted mainly from microbial decay (Janzen 2004) and burning of plant litter and soil organic matter (SOM) (Smith 2004). Production of CH4 is derived by decomposition of organic materials in anoxic flooded rice cultures by the processes of production, oxidation, and transport (Le Mer and Roger 2001). N2O is produced by the microbial transformation of nitrogen (N) in soils and manures and is often boosted where available N exceeds plant demands, especially under wet conditions (Smith and Conen 2004; Oenema et al. 2005). The burgeoning population and increasing rice demand in the future have induced tremendous concerns on the stimulation of GHG emissions (van Beek et al. 2010; Zhang et al. 2011). Therefore, developing technologies and practices to overcome GHG emissions is inevitable for sustainable and productive rice-based systems. In rice fields, the emission of

Fig. 1 Global trend of GHG emissions by region. S south, NC north and central, SW south-west (source: Verge et al. 2007)

GHGs mainly depends on crop management practices, but changes in management regime also offer possibilities for mitigation. Often a practice may affect more than one gas, by different mechanisms, sometimes antagonistically, so that the net benefits depend on the integrated effects of that practice on all gases (Schils et al. 2005). Moreover, the temporal pattern of influence may vary among different practices or among gases for a given practice, leading to temporary or indefinite reductions in GHG emissions (Six et al. 2004). In this paper, we analyzed and evaluated the different crop management options on the basis of existing data and estimated the overall GWP of each, to consider the magnitude of its effects on all gases. We discussed the crop management practices in relation to GHG emissions without compromising crop productivity.

Mechanism of GHG emissions CH4 production and emission In paddy soils, CH4 is produced as one of the terminal products in the anaerobic food web by the activity of methanogens (CH4-producing bacteria). Anaerobic conditions are the biochemical pathways of CH4 production because anaerobic conditions favor the performance of methanogens resulting in harvesting of organic C and transforming it into CH4 by a process known as methanogenesis (Inubushi et al. 2001; Bloom and Swisher 2010). During flooding in rice fields,

Region Global trend of GHG emissions 2030 2015 2000 1990

SW Pacific

Europe

NC America

S America

Africa

Asia

0

1000

Total GHG emissions (Tg CO2 eq.)

2000

3000

4000

Environ Sci Pollut Res Fig. 2 a GHG emissions from agriculture (Kasterine and Vanzetti 2010), b percentage contribution of various sectors in global methane emission (Global Methane Initiative 2010), c GWP (CO2 eq. ha−1 season−1) and yield-scaled GWP (kg CO2 eq. Mg−1) of three cereal crops based on CH4 and N2O emissions (Linquist et al. 2012a)

(a)

(b)

(c)

the redox potential of soil sharply declines by the sequential biochemical reactions. The organic matter in soil is decomposed during the periods of inundation and acts as the main source of methanogenic substrates. The C that is not utilized by plants is usually released into the soils and is sometimes converted into CH4 by methanogens (Denier van der Gon et al. 2002; Naser et al. 2007). After the production of CH4 from methanogenic processes, its vertical transportation to the atmosphere occurs by three main pathways (Le Mer and Roger 2001). These pathways include diffusion of dissolved CH4 gas (through the water-air and soil-water interfaces), loss through ebullition (the release of gas bubbles regulated by crop management practices or soil fauna), and finally, plant transport into the roots (by diffusion and conversion to gaseous CH4 in the aerenchyma and cortex) and subsequent release of CH4 to the atmosphere through plant micropores. Previous studies revealed that in the temperate rice fields, more than 90 % of the CH4 was emitted through the plants (Schutz et al. 1989; Xie and Li 2002). Nonetheless, under tropical rice, ebullition may also evolve significant amounts of CH4 emission, particularly during the early season where high organic inputs are used (Denier van der Gon and Neue 1994; Wassmann et al. 2000a). N2O production and emission N2O production is mediated by the processes of nitrification and denitrification by soil bacteria during the production of ammonium nitrate. Fertilizer management practices and soil moisture conditions are the main factors determining N2O

emissions from paddy soils. After extended flooding, paddy fields have a unique soil profile which results into the development of oxidizing and reducing layers in the cultivated layer (Xing et al. 2009). Once applied to paddy fields, ammonium-N fertilizers are nitrified in the oxidized layer at the soil-water interface forming nitrate, which moves downwards to the reduced layer where it is denitrified producing N2O. Nonetheless, the denitrification process not only exists in the upper flooded cultivated layer, but also in the underground saturated soil layer (Xing et al. 2002). During the rice growing season, with alternate dry-wet periods and rice-winter upland crop rotation, N2O produced in the underground saturated soil layer could move upwards accompanied with water evaporation and adding to atmospheric N 2O (Xing et al. 2009). Furthermore, Ncontaining gases, which are generated from soil microbial processes, can also be emitted to the atmosphere through rice root-stem tissue, although such gases diffuse slowly and can be detained in the saturated soil layer. Yan et al. (2000) concluded that N2O was released mainly through rice plants in the presence of floodwater while through soil surface in the absence of floodwater. Qin et al. (2010) further reported that under moist soil conditions, easy access N from mineral fertilizers results in higher levels of N2O than the typically more bound organic N. Soil moisture and the availability of C enhance the production of N2O, provided that a suitable nitrate source is available. CO2 production and emission The share of rice in CO2 emission is less than that of CH4 and N2O emission. Residue burning, urea fertilizer, tillage, and

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respiration are the major sources of CO2 emission in rice fields (Janzen 2004; Smith 2004). As flooded soils have very poor conditions for C oxidation (anaerobic), there is likely to be a buildup of soil C and less emission. The CO2 is directly related to SOM stability because decomposition of SOM (associated with soil respiration) results in its emission. Soil C pool is converted into CO2 by the action of soil microbes. CO2 fluxes are considered as important only if they can be linked to soil C change. Rice being a C3 plant is less efficient regarding CO2 assimilation especially due to photorespiration. On the other hand, C4 plants have higher CO2 assimilation efficiency, therefore minimizing the CO2 concentration in the atmosphere (Brown 1999). Urea is also a contributor of CO2 emission. The CO2 fixed in the industrial production process is lost, when urea is applied to paddy soils. Urea is converted into NH4+, OH−, and HCO3 in the presence of urease enzymes and water. The bicarbonate that is formed in this process eventually evolves into CO2 and water. Most of the C in urea is emitted as CO2. As urea fertilizer contains 12 g C for every 28 g N, this works out to a GWP of 1.6 kg CO2 kg−1 of urea-N applied (Snyder et al. 2009).

Crop management options to mitigate GHG emissions Emission of GHGs in rice fields can be reduced by modifying irrigation pattern and tillage permutations, managing organic additives and fertilizer inputs, selecting suitable cultivar, and cropping regime. The following sections discuss all these in detail with suitable options and possibilities under different agroconditions. Modifying irrigation pattern Water management during rice production is one of the key factors controlling GHG emission. Several water management options like distinct drainage periods in mid-season, alternate wetting and drying of the soil, intermittent irrigation, and controlled irrigation have been reported to minimize GHG emissions as compared with traditional flooded rice (Table 1) and can be opted as a practice under varying soil and climatic conditions without lowering crop yields. Mid-season drainage Mid-season drainage encompasses a distinct period of interrupted irrigation during the crop growth phase. Usually a short-term drainage (5–20 days) is carried out before the maximum tiller number stage to prevent rank growth and to reduce the number of ineffectual tillers, and the duration is regulated by regionally determined conventional methods. At

the beginning of soil aeration, CH4 emission may increase for a short period of time due to the release of soil-entrapped CH4 that is followed by persistently low emissions even when the fields are flooded again. Variations (15–59 %) also exist in efficiency of mid-season drainage to cut down CH4 due to the availability of additional water to reflood the paddy soils (Bloom and Swisher 2010). Mid-season drainage enhances soil oxidative conditions along with N absorption (Inubushi et al. 2001). So, it could be accomplished by reducing the amount of applied water since a net decrease in water will ultimately reduce CH4 emissions. Corton et al. (2000) reported that mid-season drainage reduced CH4 emissions by 43 % because aerobic conditions created by the oxygen flux into the soil were unfavorable to the activity of methanogenic bacteria. A timely managed mid-season drainage appears as a prominent practice to attain net gains in GHG emissions (Wassmann et al. 2000b). On the basis of overall GHG emissions, up till now, many studies have confirmed its suitability in rice fields. Towprayoon et al. (2005) suggested the mid-season drainage as the best option to reduce GHG emission, as they noted 27 % less GWP of mid-season drainage compared with traditional flooding on the basis of CH4 and N2O. Zou et al. (2005) and Itoh et al. (2011) also reported 42 and 72 % less GWP (CH4 and N2O) of mid-season drainage than that of traditional flooding, respectively (Table 1). As GHG emissions are greatly influenced by the duration and timing of the drainage period, therefore, this management practice may further be improved to reduce emissions. Alternate wetting and drying Alternate wetting and drying is the periodic drying and reflooding of the rice field. In contrast to mid-season drainage, the time intervals between dry and wet conditions appear to be too short to facilitate the shift from aerobic to anaerobic soil conditions (Wassmann et al. 2000a, b). Alternate wetting and drying results in a significant reduction of CH4 emission, but N2O emission from this system varies in a broad range. Water drainage and resulting aerobic soil conditions allow the oxidation of CH4 and avoid CH4 production. Katayanagi et al. (2012) reported that alternate wetting and drying has the potential to reduce CH4 emission by 73 % compared with traditional flooded rice. Song et al. (1996) suggested that optimal irrigation according to the crop physiological characteristics at different growth stages can limit the frequency of alternate dry and wet conditions leading to less N2O production and emissions. Nevertheless, further studies are required to solve the offset of N2O emission in this practice. Intermittent drainage Intermittent drainage involves a repetition of free drainage and irrigation. It possess the advantage of ameliorating soil

Environ Sci Pollut Res Table 1 Relative mitigation potential (GHG emissions) of various water management practices as compared to traditional flooding in rice Reference

Suggested practice

GHG

Mitigation potentiala (%)

Yagi et al. (1996) Cai et al. (1997) Corton et al. (2000) Zheng et al. (2000) Adhya et al. (2000) Yu et al. (2004) Minamikawa and Sakai (2005)

Itoh et al. (2011) Yang et al. (2012) Katayanagi et al. (2012)

Intermittent irrigation Mid-season drainage Mid-season drainage Mid-season drainage Intermittent irrigation No flooding (wet) Mid-season drainage Intermittent irrigation Mid-season drainage Multiple drainage Mid-season drainage Intermittent irrigation Mid-season drainage Multiple drainage Mid-season drainage Controlled irrigation Alternate wetting and drying

CH4 CH4 CH4 CH4 CH4 CH4, N2O CH4 CH4 CH4, N2O CH4, N2O CH4, N2O CH4, N2O CH4 CH4 CH4, N2O CH4, N2O CH4

38 50 43 36 15 59 64 26 27 35 42 34 37 41 72 67 73

Pathak et al. (2012) Hou et al. (2012) Feng et al. (2013) Win et al. (2013)

Mid-season drainage Controlled irrigation Intermittent irrigation Water saving irrigation

CH4, N2O, CO2 CH4, N2O CH4, N2O CH4, N2O

33 27 54 60

Towprayoon et al. (2005) Zou et al. (2005) Hadi et al. (2010) Tyagi et al. (2010)

GWP (CH4 +N2O+CO2)=(CH4 ×25)+(N2O×298)+(CO2 ×1) a

Mitigation potential was computed over traditional flooded rice system. Mitigation potential of combined gases was calculated on the basis of carbon dioxide equivalents by assuming GWPs for CH4 and N2O as 25 and 298 times the equivalent mass of CO2 over a 100-year period (IPCC 2007)

oxidative conditions by enhancing root activity, higher soil bearing capacity, and ultimately minimizing water inputs that result in anaerobic conditions. It enhances diffusion of oxygen into the soils increasing the aerobic area and reducing the CH4 production. Yagi et al. (1996) stated that intermittent drainage can minimize CH4 emission by 44 % compared with traditional flooding. Adhya et al. (2000) also demonstrated 15 % reduction in CH4 emissions by intermittent drainage with respect to permanent flooding. Emissions of N2O during intermittent irrigation periods strongly depended on the status of water logging in the fields. Different water regimes in rice fields caused a sensitive change in N2O emissions (Zou et al. 2005). Nevertheless, Hadi et al. (2010) and Feng et al. (2013) reported 34 and 54 % less GWP (CH4 and N2O) of intermittent irrigation as compared with traditional flooding (Table 1).

accomplished in the System of Rice Intensification (Sato et al. 2011). Yang et al. (2012) reported that CH4 emission was decreased by 79 %, while N2O emission was increased by 10 % in rice field under controlled irrigation as compared with traditional flooded rice and GWP of controlled irrigation was far less (67 %) than traditional flooded rice. Hou et al. (2012) also reported 27 % less GWP (CH4 and N2O) of this irrigation system as compared with traditional flooding. Furthermore, some authors have reported that deep irrigation (10 cm water depth) and permanent wetness, moist irrigation, and water-saving irrigation can also reduce GHG emission especially CH4 as compared with flooded rice (Yu et al. 2004; Liu et al. 2012; Win et al. 2013) that can be used as a tool in mitigating GHG emission. Tillage permutations

Controlled irrigation Controlled irrigation has also been reported to minimize net GHG emission with respect to transplanted flooded rice (Yang et al. 2012; Hou et al. 2012). Soil in controlled irrigated paddy fields remains dry (60–80 %) during the rice growing season without flooding after the regreening of rice seedlings (Peng et al. 2011), similar to the water management strategy

Tillage has a pronounced bearing on GHG emissions in rice fields originating primarily to alteration in soil properties (soil porosity, soil temperature, soil moisture, etc.) and biochemical processes (Ahmad et al. 2009; Li et al. 2013). Soil disturbance caused by tillage can increase emissions by aerating the soil and mechanically breaking down soil aggregates, causing the release of protected organic C fractions (Jacinthe and Lal

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2005; Sainju et al. 2010). When soils are tilled, it accelerates oxidation of soil C pool to CO2 by improving soil aeration, increasing contact between crop residues and soil, and exposing aggregate-protected SOM to microbial attack (Beare et al. 2009; Khaliq et al. 2013a). Nayak et al. (2013) stated that reducing tillage and soil disturbance in rice-based cropping systems could lead to less GHG emissions. Reicosky and Archer (2007) reported less CO2 fluxes under no tillage (NT) than conventional tillage (CT) after harvesting of spring wheat. So many explanations are made to justify less CH4 emission under NT as compared with CT practices from rice fields (Table 2; Ali et al. 2009; Li et al. 2012; Feng et al. 2013). Harada et al. (2007) found a 43 % reduction in cumulative seasonal CH4 emissions from NT rice fields in Japan compared with the CT. Pandey et al. (2012) indicated that reduction in tillage frequency significantly reduced CH4 fluxes in a rice-wheat cultivation system. Reduction in CH4 emissions under NT is attributed to the increase in soil bulk density resulting in decrease volume fraction of large pores and less decomposition of organic matter. Ahmad et al. (2009) also observed that NT could significantly minimize CH4 emissions from a paddy field compared with CT. Omonode et al. (2007) demonstrated that soil surface compaction due to NT blocked the entry of CH4 into the soil for oxidation, which resulted in less CH4 uptake by soil in dry land farming systems. Similarly, increased soil compaction might prolong the CH4 transfer pathways, so alleviating direct CH4 emission to the atmosphere or reducing CH4 transfer to the rhizosphere and emission through the rice plant (Smith et al. 2001). Studies regarding the effect of NT on N2O emission from paddy soils have reported diverse results. Xiao et al. (2007) observed that N2O emission from NT paddy fields was less than that from CT paddy fields in Ningxiang County of Hunan Province, China. Liang et al. (2007) and Wu et al. (2009) also reported less N2O emission in NT with respect to CT. The impact of NT paddy in mitigating N2O decreased over time and the net effect not only depends on duration of conversion to NT but also on the prevailing climatic conditions (Six et al. 2004). Six and his colleagues observed a clear reduction in N2O emission under NT in humid areas as compared with dry conditions. Rochette (2008) concluded that in a humid region, the impact of NT systems on N2O emissions was negligible in well-aerated soils, while N2O emissions increased by 2 kg N ha−1 year−1 from the soils where aeration was reduced. Some researchers believe that NT practices can increase N2O emission in rice fields (Zhang et al. 2011, 2013; Nyamadzawo et al. 2013). Nevertheless, these practices are capable of offsetting overall GHG emissions because of C sequestration and CH4 mitigation ability. Furthermore, overall GWP of NT is less than CT in rice fields (Ahmad et al. 2009), which suggests that the adoption of NT is beneficial in GHG mitigation and C-smart agriculture and needs to be promoted in rice-based cropping systems.

Managing organic additives Organic amendments pose a significant influence on GHG emissions in rice fields. Generally, emission of CH4 increases by the addition of organic materials like straw or organic manure amendment and such an increment depends on quantity and quality of material as well as timing of its application (Denier van der Gon 2000; Naser et al. 2007). Furthermore, organic additives create a pool of readily available N in soil leading to stimulated N2O emissions (Liang et al. 2007). In fact, there are some contradictive lines of evidence asserting the reduction in N2O emissions from rice fields by high straw amendments, which might be due to N immobilization (Zheng et al. 2000; Abao et al. 2000; Aulakh et al. 2001a). There are intricate problems in understanding such interactions and must be dealt with in research priorities under different ecological conditions. Straw/residues management Crop production inevitably leads to the production of huge amounts of straw/residues that are typically left in the field (Khaliq et al. 2013b). As the organic manure application is gradually decreasing, rice soils largely depend upon straw recycling to overcome C losses caused by soil cultivation and harvesting of crop. Although burning of straw ensures the quick seedbed preparation to farmers and also avoids the N immobilization risks during residue decomposition with wider C/N ratio, yet incomplete C combustion generates large amounts of GHGs and adversely affects air quality (Beri et al. 1995; Khaliq et al. 2013b). Furthermore, N oxides and other fire-borne organic compounds results in tropospheric ozone formation. Rice straw consists of many kinds of organic constituents such as cellulose, hemicellulose, lipids, proteins, lignin, etc., and the contribution of each constituent in increasing CH4 emission rates is variable. CH4 emission rates are very sensitive to the mode of straw management into the soil. Watanabe et al. (1993) reported that CH4 emission rate was higher from fresh rice straw with respect to off-crop season incorporation in paddy fields. In a field investigation carried out in Zhejiang Province, China, Lu et al. (2000) found that early straw incorporation at the start of the winter fallow recorded 11 % less GHG emissions as compared with that of conventional straw incorporation method during spring. Similarly, Wassmann et al. (2000b) suggested that residue incorporation during the fallow period (60 days before rice sowing) is beneficial in terms of GHG emission and grain yield as compared to typically applied before transplanting. Abandoning straw application to rice fields could also be an effective measure (Table 3) because straw removal decreased the emission of all three gases as compared with straw incorporation (Liang et al. 2007). Koga and Tajima (2011)

Environ Sci Pollut Res Table 2 Comparison of conventional and no-till practices on GHG emissions in rice Reference

Treatment

CH4 (kg ha−1)

N2O (kg ha−1)

CO2 (kg ha−1)

Liang et al. (2007)

NTa CTa NT CT NT CT NT CT NT CT NT CT NT CT NT CT NT

0.2 3.9 502.0 484.0 429.0 550.0 279.0 381.0 – – – – 63.0 89.0 54.0 69.0 13.8

5.73 5.84 2.56 2.81 4.09 3.10 – – – – 2.36 1.74 – – – – 0.28

2134.0 2843.0 – – 7927.0 7745.0 – – 1740.0 1566.0 – – – – 2447.0 1876.0 3438.0

CT NT CT NT CT

6.5 188.1 228.3 297.0 721.5

0.01 0.51 0.43 – –

2016.0 – – 10,553.0 16,328.5

Wu et al. (2009) Ahmad et al. (2009) Ali et al. (2009) Li et al. (2010) Zhang et al. (2011) Li et al. (2011) Li et al. (2012) Nyamadzawo et al. (2013) Zhang et al. (2013) Li et al. (2013)

NT no tillage, CT conventional tillage a

NT and CT during fallow period in paddy soil

also recorded less CH4 and CO2 emissions in straw removal treatments as compared rice straw return. Nayak et al. (2013) argued that straw addition stimulated CH4 emission by 108 % and inhibited N2O emission by 21 % compared to plots with chemical fertilizer. It also increased soil C sequestration, but the magnitude of its effect on CH4 increase is so high that the GHG benefit with decreased N2O emission or C sequestration is rarely negated. Most of the CH4 production occurs under flooded conditions from the decomposition of rice straw because this decay favors the growth of methanogenic bacteria. Zschornack et al. (2011) indicated that in rice fields, surface retention of straw may decrease CH4 and N2O emission by 69 and 81 %, respectively, as compared to that of straw incorporation (Table 3). In rice-wheat cropping system, wheat straw incorporation results in more emissions as compared to rice straw (Hou et al. 2013). Partial or complete mulching of wheat straw on the soil surface remained effective regarding economic and environmental perspectives. Because along with a negligible effect on N2O emission, mulching significantly reduced CH4 emission compared with incorporation (Table 4; Ma et al. 2009).

Biochar application Biochar is a C-rich material produced by pyrolyzing waste biomass under anoxic conditions and high temperature (Lehmann 2007). Highly porous structure, C-rich fine-grained and increased surface area of biochar makes it an ideal soil amendment for C sequestration (Lehmann 2007; Zhang et al. 2013). Nayak et al. (2013) stated that the application of biochar produced with crop straw pyrolysis can increase C sequestration by 22 % and decrease N2O emission by 35 % as compared to plots without biochar application. Higher levels of biochar were more effective in reducing N2O emission from the rice field (Zhang et al. 2010; Liu et al. 2012). Zhang et al. (2010) observed 58 and 74 % reductions in N2O emission by the application of biochar at 10 and 40 t ha−1, respectively. Zhang et al. (2012) and Liu et al. (2012) also recorded significant reductions in N2O emission by the application of biochar. Biochar application with the possible positive impact on soil organic C and significant reduction in N2O emission could be a way forward to decrease GHG emission. However, the long-term impact of biochar on soil physicochemical

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properties and soil organic C sequestration rates needs to be investigated further for a sound conclusion.

(Ali et al. 2012), precise placement of fertilizers into the soil (Jin et al. 2000), avoiding over applications, or eliminating N applications where possible (Zhang et al. 2010; Table 4).

Fermentation of manure Adjusting fertilization and matching N supply with demand Soil incorporation of fermented manures entails a lower emission potential of GHGs because the SOM pool is rapidly depleted during the fermentation process. Applying fermented residues can reduce approximately 60 and 52 % emissions of CH4 as compared to fresh organic amendments and combination of urea and organic amendments, respectively (Wassmann et al. 2000b). Several field studies have evaluated various types of organic amendments with regard of GHG emissions especially CH4. Differences between fresh materials, either straw or manure, have been relatively small; however, a big disparity has been reported between GHG emissions triggered by prefermented and fresh material (Wassmann et al. 1993; Corton et al. 2000). Application of fermented biogas residue increased CH4 emission only by 42 %, while unfermented manure increased CH4 emission by 112–138 % (Wassmann et al. 2000a). With the additional C benefits acquired by displacement of conventional fossil fuel energy with biogas, the use of biogas residue in rice field can provide soil fertility with less CH4 emission. Nayak et al. (2013) concluded that livestock manure application in rice considerably decreased N2O emission while increased CH4 emission and soil organic C sequestration. Zheng et al. (2000) reported that compost application in rice field resulted in 50 % less N2O emissions than that of urea application. However, CH4 emissions during anaerobic composting process could counterbalance the outputs gained after the incorporation into the soil, and aerobic composting techniques can minimize such emissions to a great extent. Corton et al. (2000) observed that organic amendments derived from aerobic composting of rice straw resulted in significantly less emission as compared to fresh straw, suggesting its practical use as an environment friendly approach. Fertilizer management Fertilizer management is a critical component for reducing environmental impacts of rice fields. Fertilizers applied to soil are not always efficiently used by the crops (Galloway et al. 2003; Cassman et al. 2003). Enhancing the fertilizer use efficiency can reduce GHG emissions especially N2O and it can also indirectly minimize CO2 emissions from manufacturing of nitrogenous fertilizer (Schlesinger 1999). Practices that improve fertilizer use efficiency and decrease GHG emission include the following: precise adjustment of application rates according to crop needs (Zou et al. 2005; Shang et al. 2011; Pittelkow et al. 2013), using nitrification inhibitors or slowrelease fertilizers (Ghosh et al. 2003; Linquist et al. 2012b), adjusting application timing and selecting appropriate source

Adjusting N and phosphorus levels to meet the crop requirements benefits the crop yield while controlling GHG emissions. Substantial amounts of the applied N are released to the atmosphere, even under the best possible fertilization practice. In irrigated rice, nearly 48 % of applied N is lost in gaseous form (Reddy and Patrick 1976). The responsible mechanisms for N losses are ammonia volatilization, nitrification, and denitrification. The specific significance of all these processes may differ depending on natural conditions as well as crop management practices (Freney 1997). The rate of fertilizer controls GHG emission, and generally, GHG emission especially N2O increases with increased Ninputs (Gregorich et al. 2005; Pittelkow et al. 2013). A generic strategy to minimize N losses and thus reducing N2O emissions is to avoid excess application of N in space and time. Reducing N fertilizer application rate to a level that would not decrease the crop yield could also reduce demands for N fertilizer ultimately leading to less indirect emission of CO2 during N fertilizer production. IPCC (1997) estimated that 1.25 % of applied N is lost as N2O regardless of N source. Several studies have recorded an instant increment of N2O emissions in rice fields triggered by the application of N fertilizer (Table 4). Zhang et al. (2010) observed 54 % higher N2O emission by application of urea as compared with no N fertilization treatment. Huang et al. (2005) reported that N2O emission in rice fields was increased with the amount of N fertilizer especially at higher rates. Reduction in N fertilizer had no significant effect on CH4 emissions, and 33 % reduction in current average N application rate for rice could result a 27 % decrease in N2O emission (Li et al. 2010). Recent field studies reported that high N rates can roughly decrease net CH4 emissions by 30–50 % from the rice system (Dong et al. 2011; Yao et al. 2012). Aulakh et al. (2001a) reported that the increase in the application of N fertilizer decreased CH4 emission and increased N 2 O emission compared with a control treatment, in which no N was applied. Zou et al. (2005) also recorded 75 % decrease in CH4 and 58 % increase in N2O, when N application was increased from 150 to 400 kg N ha−1. Recent meta-analyses indicated that the response of CH4 emissions may be N rate dependent, where N addition at low rates tends to stimulate CH4 emissions but can potentially mitigate CH4 emissions at high N rates (Banger et al. 2012; Linquist et al. 2012b). Further studies are, however, inevitable to deal the tradeoff of CH4 and N2O in response to nitrogenous fertilizers. Placement of N fertilizer into the soil near the active root uptake zone may decrease surface N loss and enhance plant N

Environ Sci Pollut Res Table 3 Influence of various straw management practices in paddy soils on GHG emissions Reference

Treatment

CH4 (kg ha−1)

N2O (kg ha−1)

CO2 (kg ha−1)

Zou et al. (2005)

Control WS incorporation Control RS incorporation Control Evenly incorporated (WS) Burying straw (WS) Ditch mulching (WS) Strip mulching (WS) Control RS incorporation Residue incorporation Surface retention Control Urea RS incorporation+urea RS incorporation+green manure

39.0 136.0 230.0 440.0 66.0 432.3 249.5 385.5 185.0 260.0 790.0 356.0 111.0 69.7 92.6 115.4 122.7

4.11 3.33 0.41 0.71 0.58 0.25 0.60 0.51 1.29 0.39 0.39 1.79 0.34 0.23 1.00 0.84 0.72

– – 2137.0 2903.0 – – – – – 741.0 545.0 – – 1100.3 1447.7 1680.6 1858.5

Liang et al. (2007) Ma et al. (2009)

Koga and Tajima (2011) Zschornack et al. (2011) Bhattacharyya et al. (2012)

Naser et al. (2007)

Hou et al. (2013)

Das and Adhya (2014)

Control 80 g RS m−2 105 g RS m−2 190 g RS m−2 219 g RS m−2 Control WS+RS incorporation Only RS incorporation Only WS incorporation Control Urea RS incorporation+urea Compost+urea Poultry manure+urea

40.4

–

–

98.4 90.9 389.0 408.0 162.4 420.3 107.1 450.2 113.4 149.6 207.2 187.2 185.3

– – – – – – – – 0.16 0.76 0.57 0.67 0.79

– – – – – – – – – – – – –

Control no straw application, RS rice straw, WS wheat straw

use efficiency resulting in less N2O emissions. Jin et al. (2000) pointed out that placing chemical fertilizer in the 6–10 cm soil layer can significantly increase the N use efficiency and decrease N2O emissions. Furthermore, splitting of N application at different growth stages of the crop can also enhance the N use efficiency and reduce N losses. Selecting fertilizer/amendment Selection of appropriate fertilizer plays a critical role in GHG emissions. Ammonium-based fertilizer addition has been reported to influence CH4 emissions (Cai et al. 1997; Ali et al. 2012). It has been proposed that high soil NH4-N concentrations may stimulate methanotrophic activity and CH4 oxidation in rice soils, thereby reducing overall CH4 emissions

(Banger et al. 2012). Cai et al. (1997) reported that application of ammonium sulfate resulted in less CH4 than that of urea at an equal rate in paddy fields. In direct-seeded rice systems, urea-N addition increased CH4 emissions approximately 40– 75 % compared to control (without N application) plots (Lindau et al. 1991). Ghosh et al. (2003) recorded less CH4 and N2O emissions by application of ammonium sulfate than those of either potassium nitrate or urea. Although potassium nitrate application resulted in less CH4 emission, it increased the fluxes of N2O emission. Recently, Ali et al. (2012) concluded that application of ammonium sulfate fertilizer recorded 23 % less CH4 as compared to urea. Sulfur is known to enhance the substrate competition between sulfate-reducing bacteria and methanogens, thus reducing CH4 production in anaerobic systems (Denier van der Gon

Environ Sci Pollut Res Table 4 Response of CH4 and N2O emissions to various fertilizer treatments Reference and country

Treatment

CH4 (kg ha−1)

N2O (kg ha−1)

Cai et al. (1997) (Japan)

No N fertilization AS (100 kg N ha−1) AS (300 kg N ha−1) Urea (100 kg N ha−1) Urea (300 kg N ha−1) No N fertilization Urea (120 kg N ha−1) AS (120 kg N ha−1) PN (120 kg N ha−1) No N fertilization 150 kg N ha−1 (urea) 300 kg N ha−1 (urea) 400 kg N ha−1 (urea) No fertilization Compound fertilizer (NPK) No N fertilization 100 kg N ha−1 (urea)

95.3 55.0 38.6 88.4 82.1 24.5 37.3 33.0 28.1 39.0 173.0 73.0 42.0 392.0 588.0 – –

0.15 0.19 1.05 0.18 0.66 0.04 0.17 0.15 0.19 4.11 2.67 2.44 6.17 0.67 6.51 0.46 0.75

200 kg N ha−1 (urea) 300 kg N ha−1 (urea) No N fertilization Urea (300 kg N ha−1) No fertilization Potassium deficit fertilization Phosphorus deficit fertilization Balanced fertilization (NPK) No N fertilization 150 kg N ha−1 (urea+AP) 250 kg N ha−1 (urea+AP) Urea alone (250 kg ha−1) Urea+coal ash Urea+phosphogypsum Urea+silicate slag AS (400 kg ha−1) AS+silicate slag Urea+cyanobacteria+azolla No N fertilization

– – 90.2 81.1 310.2 341.1 323.9 396.6 220.5 136.2 111.9 117.3 102.5 97.5 92.6 95.5 89.5 102.9 81.0

1.07 1.48 0.64 1.39 0.06 0.23 0.18 0.13 – – – – – – – – – – 0.05

80 kg N ha−1 (aqua ammonia) 140 kg N ha−1 (aqua ammonia) 200 kg N ha−1 (aqua ammonia) 260 kg N ha−1 (aqua ammonia) No N fertilization Urea (120 kg N ha−1) Rice straw (30 kg N ha−1)+urea (90 kg N ha−1) Compost (30 kg N ha−1)+urea (90 kg N ha−1) PM (30 kg N ha−1)+urea (90 kg N ha−1)

116.0 118.0 121.0 104.0 113.4 149.6 207.2 187.2 185.3

0.07 0.20 0.39 0.84 0.16 0.76 0.57 0.67

Ghosh et al. (2003) (India)

Zou et al. (2005) (China)

Ahmad et al. (2009) (China) Liu et al. (2010) (China)

Zhang et al. (2010) (China) Shang et al. (2011) (China)

Dong et al. (2011) (China)

Ali et al. (2012) (Bangladesh)

Pittelkow et al. (2013) (USA)

Das and Adhya (2014) (India)

In potassium and phosphorus deficit fertilization treatments, K and P were reduced to one-third and two-thirds of balanced fertilization treatment, respectively AS ammonium sulfate, AP ammonium phosphate, PN potassium nitrate, PM poultry manure

Environ Sci Pollut Res Table 5 Variability in rice cultivars regarding GHG emissions Country

Variation (%)

GHG under study

No. of cultivars

Reference

Japan

15–55

CH4

8

Watanabe et al. (1995)

USA

7–110

CH4

6

Lindau et al. (1995)

Philippines

18–150

CH4

3

Wang et al. (1997)

China

64–414

CH4

4

Kesheng and Zhen (1997)

Germany

35

CH4

2

Butterbach-Bahl et al. (1997)

India

8–74

CH4

6

Mitra et al. (1999)

Philippines

5–338

CH4

9

Wassmann et al. 2002

South Korea Taiwan

11–102 6–44

CH4 CH4

8 2

Shin and Yun (2000) Liou et al. (2003)

Indonesia

56–151

CH4

4

Setyanto et al. (2004)

Thailand Japan

16–52 31–72

CH4 CH4

4 3

Kerdchoechuen (2005) Jia et al. (2006)

India Japan

44 5–14

CH4 CH4, N2O, CO2

2 2

Das and Baruah (2008) Koga and Tajima (2011)

Japan

8–1310

CH4, N2O

4

Riya et al. (2012)

USA South Korea

7–28 25–140

CH4, N2O, CO2 CH4

14 8

Lyman and Nalley (2013) Gutierrez et al. (2013)

Variations among cultivars in different studies were calculated on an average basis regardless of other factors

et al. 2002). Therefore, soil amendments geared toward lowering CH4 emissions could be accomplished by application of sulfates. Nayak et al. (2013) suggested that replacing urea with ammonium sulfate reduces CH4 emission and increases N2O emission, assuming that replacement of urea with ammonium sulfate has no impact on C sequestration potential, and the overall technical potential (−1.12 t CO2 eq. ha−1 year−1) of this management practice justifies it as a good mitigation measure. Soil application of gypsum (CaSO4) may offer some potential for mitigating GHG emissions as it reduced CH4 emissions by 37 % in Louisiana (Lindau and Bollich 1993) and by 62 % in a field study conducted in the Philippines (Denier van der Gon and Neue 1994). Furthermore, potassium application can also effectively reduce CH4 emissions from flooded anaerobic paddy soils and can be used as an effective mitigation option especially in potassium-deficient soils. Babu et al. (2006) recorded 49 and 56 % reductions in cumulative and yield scale emissions, respectively, by the application of 30 kg K ha−1. Potassium amendments are known to inhibit drop in redox potential and reduce the activity of methanogens and stimulate methanotrophic bacteria. Use of nitrification inhibitors or slow-release fertilizers Nitrification inhibitors or slow-release N fertilizers are capable of decreasing both CH4 and N2O emissions from rice fields. Nitrification inhibitors reduce the emission of N2O directly by inhibiting nitrification and indirectly by lowering the

availability of NO3 for denitrification. Previous studies have concluded that dicyandiamide, hydroquinol, nitropyrimidine, and benzoic acid can significantly decrease N2O emissions (Tenuta and Beauchamps 2003; Linquist et al. 2012b). Similarly, the application of urease, dicyandiamide, and hydroquinol to the soil was shown to significantly reduce CH4 emissions (Tenuta and Beauchamps 2003). Xu et al. (2002) have recorded less N2O and CH4 emissions during rice growth using a combination of dicyandiamide and hydroquinone. There are some plant-derived organics such as neem cake, neem oil, and karanja seed extract which can also act as nitrification inhibitors. Nitrification inhibitors have been recognized to possess the potential of inhibiting nitrifiers, CH 4 oxidizers, and methanogens. Previous experiments have concluded that encapsulated calcium carbide which slowly releases acetylene into the soil is effective in reducing CH4 emissions and increasing rice yields (Wassmann et al. 1993; Yan et al. 2005). The application of nitrapyrin and dicyandiamide (DCD) reduced the N2O emission from ammonium-based fertilizers by 52 and 64 %, respectively (McTaggart et al. 1994). Linquist et al. (2012b) concluded that up to 25 and 97 % reduction in CH4 and N2O can be achieved by the use of different nitrification inhibitors. Mycorrhiza fungi also possess the ability to improve nutrient use efficiency of crop plants by assisting soil nutrient uptake and lead to higher crop biomass. Mycorrhiza also trigger the conversion of more atmospheric CO2 into assimilates and increase C sequestration by increment in soil C stock

Environ Sci Pollut Res Fig. 3 Emission of GHGs from traditional puddled rice and direct-seeded rice systems (source: Pathak et al. 2012)

(Lal 2003; Smith et al. 2008). The use of a leaf color chart (LCC) or photometer is also ideal for determining timedependent N demand and lowering GHG emission. Demand-driven N use using LCC could reduce N2O emission and GWP by about 11 % (Bhatia et al. 2010). Furthermore, the consequent use of site-specific nutrient management in irrigated rice can reduce N2O emissions and increase yields as compared to conventional practices (Dobermann et al. 2002). An integrated nutrient management system (INMS) based on targeted yield should be the ultimate target toward having higher nutrient use efficiency. Selection of rice cultivar Selection of suitable cultivar has been known as a promising strategy to minimize GHG emissions particularly CH4 in paddy soils. The species Oryza sativa (and its relatives) constitutes an enormously diversified crop (Leon and Carpena 1995) comprising more than 90,000 accessions in the IRRI gene bank, varying on the basis of morphophysiological traits and adaptation to a wide range of environmental parameters (IRRI 1999). Furthermore, there exists a large variability regarding GWP of rice cultivars. A number of studies (Jia et al. 2006; Das and Baruah 2008; Riya et al. 2012; Lyman and Nalley 2013; Gutierrez et al. 2013) conducted under controlled and natural conditions have revealed the variability of rice type as well as cultivars regarding CH4 emissions (Table 5). Differences among cultivars in CH4 emission have been attributed to the variation in CH4 production, oxidation, and transport capacities (Aulakh et al. 2002; Lou et al. 2008). Soil Eh has been known to mainly control the CH4 production rate of rice soils, with a threshold level of −150 mV (Yu and Patrick 2004). Soil Eh is reported to be influenced by root respiration and exudation (Han et al. 2013), aboveground

biomass, and status of rice plant development over the entire rice growing season (Denier van der Gon et al. 2002). All these factors have often been used as promising traits to extrapolate the CH4 budget in rice fields (Yan et al. 2003). Variations in these factors and dynamic changes in soil Eh are well linked with CH4 production potential and emission from rice soils. Rice plants exhibit different CH4 oxidation with variable O2 diffusion through the aerenchyma into the rhizosphere mainly because they (cultivars) differ in gas conductance which is related to O2 release in the rhizosphere (Ma et al. 2010; Li et al. 2013). The flux rates of gases through the aerenchyma are influenced by concentration gradient, diffusivity and internal structure of the aerenchyma, tiller density, root biomass, rooting patterns, total biomass, and metabolic activity (Wang et al. 1997; Aulakh et al. 2002). A welldeveloped aerenchyma system ensures the oxygen availability to the rhizosphere for maintaining aerobic metabolisms, restricting movement of potentially toxic substances into plant roots by oxidation (Armstrong and Armstrong 1988), enhancing the CH4 oxidation, and hence, mitigating its emission into the atmosphere (Kludze et al. 1993). Furthermore, it also serves as conduits for CH4 from the rhizosphere into the air driven by concentration and/or pressure gradients (Aulakh et al. 2000; Win et al. 2010). Jiang et al. (2013) observed significantly lower CH4 emission in super rice than that in traditional rice and argued oxidation as the main contributor to the lower CH4 emission rather than production. Rice varieties with a stronger root system can release more oxygen into the soil, enhance resistance to environmental stresses, and increase crop yield (Mei et al. 2009, 2012; Li et al. 2013). Zhang et al. (2009) also reported that root oxidation activity was significantly greater in super rice than traditional varieties before and at heading time.

Environ Sci Pollut Res Fig. 4 Map diagram of rice management package for attenuating GHG emissions based on existing data. M, N, and C represent the targeted GHG as methane, nitrous oxide, and carbon dioxide, respectively

Transportation of CH4 and O2 is also regulated by aerenchyma in rice plants, which provide substrates for methanogenic bacteria and methanotrophs through root exudates and/ or dead root cells (Kerdchoechuen 2005; Win et al. 2010). Decomposition of newly exuded organic matter from rice roots and the amount and type of root exudation significantly differ among rice cultivars (Aulakh et al. 2000, 2001b). The CH4 emitted by plant-mediated transport accounting for the total emissions (Inubushi et al. 2003; Tokida et al. 2013) suggested a huge potential to mitigate CH4 emission by rice cultivar selection. Cultivar-specific differences regarding CH4 emissions can be masked by crop management practices (Mitra et al. 1999) and may vary from season to season (Lu et al. 2000). Setyanto et al. (2000) observed that rice cultivar harvested after 3 months recorded less CH4 emissions than the cultivar harvested after 4 months, suggesting shorter season length as the conclusive criterion for selecting low-emitting cultivars. In crux, selection of rice cultivars with low CH4 emission and higher resource use efficiency seem a promising strategy for minimizing GHG emissions in rice paddy soils. However, the mechanism of exudate and aerenchyma effects under field

conditions should be explored for prior assessment of cultivars. Modifying cropping regime: direct-seeded rice technology The traditional puddled transplanted rice (TPR) is a major source of GHG emission, while direct-seeded rice (DSR) has been thought as a feasible alternative to TPR having a potential to mitigate GHG emission and adapt to climate risks (Fig. 3; Pathak et al. 2012; Liu et al. 2014a, b). Ko and Kang (2000) proposed that DSR is a water-saving technology which can significantly reduce GHG emission especially CH4 without decreasing yields. They argued that reduced soil disturbance and shorter flooding period are the major reasons for less CH4 production and emission in DSR as compared to TPR. Corton et al. (2000) found 18–54 % less CH4 emission in DSR, respectively, as compared to that in TPR. Wassmann et al. (2004) reported that CH4 emissions may be suppressed by up to 50 % with mid-season drainage in DSR fields. However, changing water regimes may result in a tradeoff between CH4 and N2O fluxes, so that a decrease in CH4 may be offset by enhanced N2O emissions in the dry

Environ Sci Pollut Res

DSR relative to TPR fields (Zou et al. 2009; Shang et al. 2011; Zhang et al. 2011). Hou et al. (2000) stated that N2O production increases in DSR when redox potentials exceeded 250 mV. They suggested managing water applications in such a way that soil redox potential can be kept at an intermediate range (100 to 200 mV) to reduce the emissions of both CH4 and N2O. This range is high enough to prevent CH4 production and low enough to stimulate N2O reduction to N2 as the critical soil redox potential identified for N2O production is 250 mV (Hou et al. 2000). Although the DSR system offsets N2O emission, it is still a more promising cropping regime on the basis of its GWP. Pathak et al. (2012) observed that the average GWP regarding three GHGs (CO2, CH4, and N2O) in DSR was 53 % less than that of TPR (Fig. 3). Ahmad et al. (2009) reported that GWP of DSR can be further decreased by switching toward NT practices in rice fields. Liu et al. (2013) also proposed that the DSR cropping regime has a great potential to mitigate GHG emissions from rice paddies. Less GWP and high productivity suggest that the DSR cropping regime would lower CH4 and N2O emissions in terms of per unit of rice grain yield. However, more detailed studies involving simultaneous measurements of GHGs under the influence of different factors like water management, tillage, nutrients, etc. are needed for suggesting more appropriate DSR production package that also mitigates environmental concerns.

Conclusions The burgeoning population and increasing demand for rice in the future has induced tremendous concerns to stabilize GHG emissions for minimizing anticipated global climate change. Here we synthesized the existing data to provide a comprehensive review of suitable crop management practices in rice for attenuating GHG emissions (Fig. 4). Due to data limitations, we could not deal all gases in each segment, but we analyzed the feasibility and potential of different possible practices on the basis of their GWP regarding GHGs especially CH4 and N2O. We found that crop management interventions can overcome anticipated global climate change due to rice cultivation. For instance, relative to traditional flooding irrigation, midseason drainage, intermittent irrigation, and controlled irrigation system possess the mitigation potential of 27–64, 34–54, and 27–67 %, respectively, considering CH4 and N2O together. The adoption of NT and conservation tillage practices instead of CT is beneficial in GHG mitigation and C-smart agriculture as these practices are capable of reducing overall GHG emissions. Managing straw by surface retention or mulching and making biochar/compost instead of either burning or incorporation have the potential to offset GHG emissions in rice fields. Likewise, the use of organic manures after

fermentation can be a viable mitigation option. Adjusting fertilization according to crop needs, precise placement, replacing urea with ammonium sulfate, addition of potash, and the use of nitrification inhibitors are efficient approaches to lower GHG emissions. Selection of a suitable cultivar with less CH4 emission and higher resource use efficiency also depicts a huge scope in this regard. Furthermore, DSR has appeared to be the most promising cropping regime and the best alternative to TPR in terms of less GWP. Adaptation of all these proposed options for mitigation of GHG emissions is likely to sustain rice productivity or at least improve input use efficiency without decreasing productivity. Nonetheless, for successful implementation of these practices, all social, economic, educational, and political barriers should be removed. Future work may focus on the verification of these practices in various geographical zones with feasibility in varying circumstances to provide site-specific mitigation package. Moreover, combining the geographic information, yield and GHG emission models, and socioeconomic information will also help in this regard. The GIS database can be used for screening of cultivars with less GHG emission. There is a need to estimate GWP across a wide range of agricultural systems. Ideally, a standard method should be established for calculating GWP. In the context of global climate change and agriculture, there is a growing consensus that other factors including cultural significance, the provision of ecosystem services, food security, and human health must also be considered. We hope that in the future, our attempt will be fruitful due to its practical implications and it will motivate and guide further research work. Keeping in view the issue of food security, all the future suggestions to avoid disastrous climate change should be in accordance with sustaining or rather increasing rice production. Acknowledgments This work is supported by the National Natural Science Foundation of China (Project No. 31371571); the Open Project Program of Key Laboratory of Crop Ecophysiology and Farming System, Ministry of Agriculture (Project No. 201301); the National Science & Technology Pillar Program (2013BAD20B06); and the Fundamental Research Funds for the Central Universities (Project No. 2013PY109).

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