Management of Paddy Soil towards Low Greenhouse Gas Emissions and Sustainable Rice Production in the Changing Climatic Conditions Management of Paddy Soil towards Low Greenhouse Gas Emissions and Sustainable Rice Production in the Changing Climatic Conditions

Climate change is a vital environmental issue for the twenty-first century, which may sig - nificantly affect rice productivity and accelerate greenhouse gas emissions from paddy ecosystem, which is of great environmental concern which is of great environmental concern. Methane (CH 4 ) and nitrous oxide (N 2 O) are the most important greenhouse gases due to their radiative effects as well as global warming potentials (GWPs). CH 4 and N 2 O gases are simultaneously emitted from rice fields to the atmosphere due to their favorable production, consumption, and transport systems. The intensive rice farming system has been creating excessive pressure on rice fields to produce more rice for the expanding world population, thereby deteriorating soil fertility status and rice paddy ecosystem balance by stimulating more CO 2 , CH 4 , and N 2 O fluxes to the atmosphere. The extreme climatic variables such as high light intensity, high water vapor or relative humidity, high temperature, and drought stress may badly suppress beneficial microbial activity, soil nutrients, and water availabil - ity to rice plant; eventually, rice yield may be decreased drastically, and simultaneously, greenhouse gas emissions could be increased significantly. In this situation, conservation tillage, water saving irrigation technique such as alternate wetting and drying, soil amend ments with biochar, vermicompost, azolla-cyanobacterial mixture, recommended silicate slag, and phospho-gypsum with minimum NPKSZn fertilizer (IPNS) should be introduced to the field level farmers for sustainable rice production and mitigating greenhouse gas emissions.

. (a) Schematic diagram of methane production, oxidation, and emission from rice paddy field and (b) schematic diagram of N 2 O, NO, and N 2 emissions from rice paddy field.

Introduction
World rice production especially in Southeast Asia and tropical Asia is highly vulnerable to climate change. Rice production systems contribute to global climate change through emissions of carbon dioxide (CO 2 ), methane (CH 4 ), and N 2 O gases to the atmosphere and simultaneously are affected by the changed climatic variables. Rice is the major cereal crop for more than half of the world's population and its production needs to be increased 40% by the end of the 2030s to meet the increasing demand for the expanding population [1], which may further accelerate CH 4 and N 2 O emissions to the atmosphere [2]. In 2012, worldwide rice production covered 163 million ha of cropland, where approximately 80 million ha were managed under continuous flood irrigation and contributed to 75% of the world's rice production [3]. China and India, most densely populated countries in the world, account for 20.0 and 28.5% of the global rice area, respectively [4]. In China, approximately 90% of the rice fields are irrigated [5], while in India, more than 46% of the rice fields are irrigated [6]. Unfortunately, the irrigated rice farming acts as one of the main sources of anthropogenic CH 4 emission to the atmosphere [7]. Therefore, IRRI is promoting water-saving alternate wetting and drying techniques for improving water use efficiency, while reducing CH 4 production in the rice rhizosphere.
Rice paddy fields act as a source of greenhouse gases such as methane (CH 4 ) and nitrous oxides (N 2 O) depending on soil organic matter status, land use and cropping intensity, irrigation water and drainage management practices, soil microbial populations and their activities, soil properties, and climatic variables. The management practices such as tillage operations, leveling, plant residue incorporation, irrigation frequency and standing water levels, drainage system, and organic and inorganic soil amendments followed in rice farming influence the amount of CH 4 and N 2 O emitted to the atmosphere. Generally, CH 4 gas is produced under flooded or anoxic soil conditions (Figure 1a), while N 2 O gas is produced through nitrification and denitrification processes depending on soil aerobic (oxygenated) and anaerobic conditions (Figure 1b).
Typically, N 2 O emissions are low under flooded fields, while CH 4 emissions are high, a tradeoff relationship observed which is largely dependent on paddy soil water level, redox status, soil organic matter content, and external sources of organic and inorganic soil amendments. Ali et al. reported that the total GWP of CH 4 and N 2 O gases was decreased by 7-27% and 6-34% with calcium carbide, phospho-gypsum, and silicate fertilizer amendments under continuous and intermittent irrigations, respectively [8]. However, biochar amendments increased the overall GWP of CH 4 and N 2 O gases, which simultaneously increased rice yield [8].

Climatic change and greenhouse gas emissions
Greenhouse gases (GHGs), mainly carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O), have been contributing to about 80% to the current global radiative forcing [9]. Agricultural activities contribute to approximately 20% of the present concentrations of atmospheric GHGs [10], especially the emissions of CH 4 and N 2 O from paddy fields [9]. Methane (CH 4 ) and nitrous oxide (N 2 O) are the two most important GHGs from agriculture, with global-warming potentials (GWP) of 25 and 298 CO 2 -equivalents, respectively, on a 100-year time horizon. Apart from the water vapor, CH 4 is a major greenhouse gas contributing 20% toward global warming with almost 25-fold higher global warming potential than carbon dioxide [11]. The concentrations of atmospheric CH 4 and N 2 O have increased from 722 and 270 ppb in the pre-industrial period to 1853 and 328.9 ppb in 2016, respectively [12]. China, the largest rice-producing country, accounts for about 28% of global rice production [4] and . Paddy soil redox status, sequential reduction and oxidation of inorganic nitrogen, manganese, and iron in flooded soil, and methane gas formation [16]. the total CH 4 and N 2 O emissions from paddy fields are estimated to be 6.4 Tgyr −1 and 180 Ggyr −1 , respectively [13]. Although the global estimates of CH 4 emission from rice cultivation vary within 20-150 TgCH 4 year −1 , the global average CH 4 is about 100 TgCH 4 year −1 [14] and may increase further due to the expansion of rice cultivation as well as intensification of rice agriculture for the increasing world population [15]. Therefore, it is very important to understand the mechanism of CO 2 , CH 4 , and N 2 O exchange and their main controlling factors for developing appropriate strategies to mitigate GHG emissions.

Characteristics of paddy soils
Paddy soils are mostly alluvial soils and low humic gley soils (or Entisols and Inceptisols). In addition, vertisols, reddish-brown earths or Alfisols, red-yellow podzolic soils or Ultisols, and latosols or Oxisols are utilized for paddy rice cultivation. Paddy soils are found mainly on alluvial lands such as deltas and flood plains of big rivers, coastal plains, fans, and lower terraces. In general, paddy soils are resistant to erosion when they are terraced and there are ridges around the field, as measures to retain surface water. Paddy fields in the lowlands receive new sediments deposited from run-off that carries eroded topsoil down from the uplands, thus sustaining soil fertility and productivity. The paddy soils have medium to high organic matter (1.5-3.97 g/kg), available phosphorus (11.7-19.9 mg/kg), available potassium (61.6-132.9 mg/kg), and cation exchange capacity (15.5-33.1 cmol/kg). The most common practice in paddy rice cultivation is flooding or temporary water logging of the land surface. Soil redox potential (Eh) or electron activity in soil gradually decreases after flooding, which causes significant methane production at around −200 mV [16], and creates high risk of gaseous N losses through denitrification (Figure 2).
In paddy fields, the kinetics of the reduction processes are strongly affected by the composition and texture of soil and its content of inorganic electron acceptors. After flooding, microbial reduction processes sequentially use NO 3 − , Mn 4+ , Fe 3+ , and SO 4 2− as electron acceptors, accompanied by the emission of the trace gases N 2 O, N 2 , H 2 S, and CH 4 due to reduction-induced increasing pH-NH 3 (Figure 2). Microorganisms drive redox reactions in soil by using organic carbon and electron acceptors for their metabolic activities. Methane is produced at the terminal step under anaerobic decomposition of organic matter and due to the reduction of CO 2 into CH 4 in wetland soils. Soil Eh values decreased rapidly after flooding within 5-7 weeks then stabilized toward −200 to −240 mV and produced significant amount of methane [17]. High concentrations and fluxes of dissolved organic matter (DOM) in paddy soils from plant debris trigger microbial activity and thus the emission of greenhouse gases. Therefore, the objectives of this thematic topic are to highlight the feasible field management practices for sustainable rice production and recommend appropriate strategies to mitigate GHG emissions from paddy soils in the changing climate.

Measurement of CH 4 and N 2 O emissions from the paddy field
The static closed chamber technique was used to measure CH 4 and N 2 O emissions during rice growing period [18]. The chambers were made of PVC and consisted of two parts: an upper transparent compartment (100 cm height, 30 cm width, and 30 cm length) was placed on a permanently installed bottom collar (10 cm height, 30 cm width, and 30 cm length). Three replicate chambers were used. Each of these chambers was placed in each plot. Each chamber was installed with a battery-operated fan to homogenize the air inside the chamber headspace, a thermometer to monitor temperature changes during the gassampling period, and a gas-sampling port with a neoprene rubber septum at the top of the chamber for collecting gas samples from the headspace. Gas samples were collected twice daily (Figure 3), sampling during 9.00 am to 12.00 and 12.30 pm to 3.30 pm. A 100-mL plastic syringe equipped with a 3-way stopcock was used to collect gas samples from the chamber headspace 0, 15, and 30 min after chamber deployment. Gas samples were collected twice a day. The collected gas samples were immediately transferred to 100-mL air-evacuated aluminum foil bags (Delin Gas Packaging Co., Ltd., Dalian, China) sealed with a butyl rubber septum and transported to the laboratory for analysis of CH 4 and N 2 O concentrations by Gas Chromatograph (Figure 4).

Determination of CH 4 and N 2 O concentrations in the headspace air samples
CH 4 and N 2 O concentrations in the headspace air samples were determined by a gas chromatograph (Shimadzu GC-2014, Kyoto, Japan) packed with a Porapak Q column (2 m length, 4 mm OD, 80/100 mesh, stainless steel column) (Figure 4). A flame ionization detector (FID) and an electron capture detector (ECD) were used for the determination of CH 4 and N 2 O concentrations, respectively. Helium (99.9% purity) was used as a carrier gas (30 ml min −1 ), and a make-up gas (95% argon and 5% CH 4 ) was used for the ECD. Calibration was conducted with 1.01, 7.99, and 50.5 μl CH 4 l −1 in He and 0.2, 0.6, and 1.0 μl N 2 O l −1 in He (CRM/RM Information Center of China) as primary standards.

Estimation of CH 4 and N 2 O fluxes, GWPs, and greenhouse gas intensity (GHGI)
CH 4 and N 2 O fluxes from the paddy field were expressed as the increase/decrease in CH 4 and N 2 O mass per unit surface area per unit time. CH 4 and N 2 O fluxes were estimated by the following equation [19]:

Estimation of GWPs of CH 4 and N 2 O
To estimate the GWP, CO 2 is typically taken as the reference gas, and an increase or reduction in emission of CH 4 and N 2 O is converted into "CO 2 -equivalents" by means of their GWPs. In this study, we used the IPCC factors to calculate the combined GWPs for 100 years [GWP = (25 × CH 4 ) + (298 × N 2 O)], kg CO 2 -equivalents ha −1 from CH 4 and N 2 O under various agricultural practices. In addition, the greenhouse gas intensity (GHGI) was calculated by dividing GWP by grain yield for rice [20,21].

Results and discussion
The mean rice statistics and cumulative methane emissions in the rice producing countries are presented in Table 1.
It is known that China, India, Indonesia, Bangladesh, and Vietnam have been playing a dominating role in total rice production ( Table 1), which may impose a threat to the environment by stimulating CH 4 emissions. Phospho-gypsum, a by-product of the phosphate fertilizer manufacturing industry, is a feasible soil amendment to supplement mainly calcium and sulfur for rice cultivation. The high content of sulfate in phospho-gypsum might prevent CH 4 formation as well as CH 4 emissions due to stronger competitor for substrates (hydrogen or acetate) than methanogens. It was reported that silicate slag and phospho-gypsum in combination with Azolla-cyanobacteria significantly decreased CH 4 flux while improved rice rhizospheric redox status ( Figure 5) [18,23]. Ali et al. reported that biochar amendments in paddy soils of Japan and Bangladesh decreased seasonal cumulative N 2 O emissions by 31.8 and 20.0%, respectively, followed by 26.3 and 25.0% reduction with biochar plus Azolla-cyanobacteria amendments ( Table 2) [18]. Seasonal cumulative CH 4 emissions and global warming potentials were significantly decreased due to Azolla-cyanobacterial inoculation with phospho-gypsum and  silicate slag amendments [18]. Combined effects of blast furnace slag and revolving furnace slag amendments also showed decreasing effects on GWPs ( Table 2).
Silicate slag and biochar amendments in different soils of Japan also reduced cumulative CH 4 flux, while increased rice growth and yield parameters ( Table 3) [23].
Among the amendments, biochar significantly decreased N 2 O emission rates ( Figure 6); however, it increased CH 4 emission rates (Figure 1). Silicate slag and phospho-gypsum amendments lowered N 2 O emission rates compared to control treatment (NPK), although no significant differences were observed (Figure 6) [18].

Climate change and threats to rice production
The IPCC 4th Assessment Report (IPPC) states that Southeast Asia is expected to be seriously affected by the adverse impacts of climate change [11]. The frequency of floods, drought, cyclones, tornadoes, thunderstorm, and earthquake increased during the last 5 years, which badly affected the natural vegetation and forest covers, wild animals, wetlands, and land resources, and ultimately, agricultural productivity declined. In Indonesia, the Philippines, Thailand, and Vietnam, the annual mean temperatures are projected to rise by 4.8°C by 2100, and the global mean sea level will increase by 70 cm during the same period [24]. It has been reported that in Southeast Asia, small changes in the annual rainfall are expected to continue up to 2040 [25], and there will be an increase in the occurrence of severe weather including heat waves and precipitation events. Increases in tropical cyclone intensities by 10-20% are  anticipated, and temperatures are projected to continue to increase by about 0.7-0.9°C [25]. Furthermore, sea levels have risen by 1-3 mm/year, marginally higher than the global average [24]. Rice production systems of this region have become increasingly threatened by the effects of climate change as a large portion of the rice-growing areas are located in especially vulnerable regions. A decrease of 10% in rice yield has been found to be associated with every 1°C increase in temperature [24], while the yield of dry-season rice crops in the Philippines decreased by as much as 15% for each 1°C increase in the growing season mean temperature. These temperature and aggravating climate change effects may cause a decline in the world rice production, which have already shown negative effects on agricultural production. By 2100, Indonesia, the Philippines, Thailand, and Vietnam are projected to experience a potential fall of about 50% in rice yield due to the occurrence of extreme climatic events [24,26]. Furthermore, rice yield would be affected severely due to sea level rise and intrusion of saline water in the coastal area, which will hamper rice growth and yield. Rising sea levels in association with heavy monsoon rainfall will create serious waterlogging and prolonged stagnant floods in major rice-growing, low lying mega-deltas in Southeast Asia, which ultimately deteriorate rice production in the deltas since only a few low-yielding rice varieties have evolved to withstand such conditions [27]. The recent natural disasters such as flash floods caused by water of Indian Meghalaya state and excessive rainfall in the low lying ha or areas of Bangladesh [28] badly affected the only cultivated rice crop, the Boro rice (winter rice), at this region; however, due to rice duck mixed farming, the overall productivity and net profit were recovered to some extent ( Table 4). 6. Management of rice paddy ecosystem to cope with climate change and sustainable rice production

Soil conservation with suitable cover crops and minimum tillage or no tillage
Soil conservation practices such as suitable cover crops, mulches, and minimum tillage (one or two plowing with proper leveling) may be introduced in rice farming not only to control soil erosion and land degradation, while reduce production costs to sustain rice productivity. In addition, conservation tillage will improve environmental quality by lowering GHG emissions (less air pollution) through decreasing the use of diesel fuel and nonburning of rice residues [29]. It has also reported that the no-tillage system in Korean paddy field with silicate fertilization decreased total seasonal CH 4 flux by 53 and 36%, while maximizing grain yield by 18 and 13% over the control tillage and control no-tillage systems, respectively [30]. Soil properties were also improved with silicate fertilization under the no-tillage system. It was [31] reported that tillage (after the harvest of late rice) with the incorporation of stubble (3.5 t ha −1 ) in the winter fallow season significantly decreased both the net GWP and the GHGI while maintained a high grain yield (13.0-13.3 t ha −1 yr −1 ) in the double-cropping rice system.

Introducing direct seeded rice (DSR) and puddle rice transplanting (PRT) methods
Direct seeded rice (DSR) is a process of establishing a rice crop which is done by seeds sown in the field rather than by transplanting seedlings from the nursery. The practice of direct seeding instead of transplanting resulted in a 16-54% reduction in CH 4 emission [32]. CH 4 emission was more significantly reduced under dry-direct seeding compared to wet-direct seeding. However, grain yield in direct seeded rice (DSR) was found lower than Puddle transplanted rice (PTR), probably due to poor crop stand, high percentage of panicle sterility, and higher weed and root-knot nematode infestation [33]. It was also observed that grain yield of direct-seeded rice (9.0 Mg ha −1 ) was identical to grain yield of transplanted-flooded rice [34]. Average yield penalty of around 10% was observed for the direct seeded rice (DSR) compared with puddle transplanted rice [34]. It was reported [35] that over the rice-growing season, cumulative CH 4 emissions were significantly higher in puddle transplanted rice compared to the direct seeded rice production system.

Water management for sustainable rice production and minimizing GHG emissions
Water management influences rice yield and CH 4 and N 2 O emissions from rice cultivation systems. Irrigated rice fields are an integral part of the rice production system in Asian countries, which contribute about 75% to global rice production. Single or multiple drainages during a rice growing season (e.g., AWD) are reported to reduce CH 4 emissions by 48-93% compared to those observed under continuous flooding systems [36,37]. Mid-season drainage and intermittent flooding were found effective for increasing productivity and quality of rice as well as reducing methane emissions in Japan [38]. The AWD field showed the same yield as continuous flooded field, but saved 16-24% in water costs and 20-25% in production costs. Most farmers in China, Japan, and South Korea have been practicing this mid-season drainage (5-7 days dry out) to increase rice yield and decrease GHG emissions. Mid-season drainage and intermittent irrigations may reduce methane emissions by about 50%. It was also reported [39] that the AWDI treatment (irrigation applied when water level in the pipe fell 15 cm) showed superiority for the rice yield performance and seasonal CH 4 emission reduction, water savings, and maximum water productivity index. However, the AWD irrigation technique increased the N 2 O emission by 97%, especially in DS [40].

Diverse farm management practices, soil amendments, and rice cropping systems
Feasible management approaches based on agroecosystem have to be adopted to sustain agricultural productivity in the changing climatic conditions. For example, the ground cover rice production system (GCRPS), through which paddy soils are covered by thin plastic films to conserve soil moisture nearly at saturated status, is a promising technology to increase yields with less irrigation water. However, increased soil aeration and temperature under GCRPS may cause more CH 4 to N 2 O emissions compared to conventional techniques. Yao et al. [41] reported that the GHG emissions for the ground cover rice production system (GCRPS, i.e., paddy soils being covered by thin plastic film) were found significantly lower (1973 kg CO 2 eq ha −1 ) than that of traditional cultivation (4186 kg CO 2 eq ha −1 ). Total seasonal CH 4 emissions under GCRPS were on average 80% lower as compared to the traditional rice cultivation. The yield-scaled GHG emissions from GCRPS were further reduced from 377 to 222 kg CO 2 eq Mg −1 as N 2 O emissions greatly decreased while yields increased. The system of rice intensification (SRI), an agro-ecological methodology, could be a feasible technique to sustain rice productivity by changing the management of plants, soil, water, and nutrients. Successful application of SRI of increased paddy yield by 50-100% while using less inputs, in particular water, (farmers were able to reduce their water requirements by about 25-50%) [42] has already been reported. Suitable rice cropping patterns, rotations, and mixed rice-duck-fish farming hold the potential scope to sustain agricultural productivity and controlling GHG emissions in the changing climatic conditions. For example, in the Philippines, fish or ducks have been raised with rice as well as legumes such as mung bean (Vigna radiata), groundnut (Arachis hypogaea), and soybean (Glycine max) after two rice dropping. Rotation of crops that have their most drought-sensitive phase in different phases of the growing season may prove a valuable adaptation to limited water resources. Haque et al. reported that the nonrice-based cropping patterns had lower GWPs than the rice-rice-based cropping patterns [43]. Ali et al.
reported that CH 4 emissions from wetland paddy ecosystems were significantly decreased by integrated rice duck farming [28]. It has been reported that azolla application in rice field increased CH 4 emission, probably due to the exudation of azolla root and decomposition of dead azolla. In contrast, reverse report on CH 4 emission was also found from rice soil ecosystems, probably due to the increase in redox potential in the root region and dissolved oxygen concentration at the soil-water interface. Azolla cover increased N 2 O emission from rice paddies due to N-fixation by azolla providing a source for N 2 O production through nitrification and de-nitrification, especially when the azolla died [44]. CH 4 emissions have been reported to increase when crop residues are incorporated prior to planting due to higher amounts of readily available carbon stimulating soil microbial activity. Sander et al. reported that incorporation of rice residues immediately after harvest and subsequent aerobic decomposition of the residues before soil flooding for the next crop reduced CH 4 emissions by 2.5-5 times and also improved nutrient cycling in paddy field [45]. It was also reported that residue incorporation accelerated CH 4 and N 2 O emissions from irrigated rice field compared to residues (ryegrass and serradella) left on the soil surface. The open burning of crop residues emits CO 2 , CH 4 , and N 2 O. Ali et al. [17] reported that silicate slag and phospho-gypsum amendments with nitrogenous fertilizer in rice cultivation significantly decreased seasonal CH 4 flux by 16-20% and increased rice yield by 13-18% in Korean paddy soil, whereas 12-21% reduction in total seasonal CH 4 flux and 5-18% increase in rice grain yield were found in the upland rice paddy soils of Bangladesh [46]. Seasonal cumulative CH 4 and N 2 O emissions, GWPs, and yield scaled greenhouse gas emissions were decreased by combined application of Azollacyanobacterial mixture with silicate slag, phospho-gypsum, and biochar amendments in rice paddy soils of Japan, Korea, and Bangladesh ( Table 2) [18]. Site-specific nutrient management (SSNM) for rice developed by IRRI (2006) in Asia [47] enables rice farmers to tailor nutrient management to the specific conditions of their fields, and provides a framework for nutrientbased management practices for rice. The increase in annual grain yield with use of SSNM in on-farm evaluation trials averaged 0.9 t/ha in southern India, 0.7 t/ha in the Philippines, and 0.7 t/ha in southern Vietnam [48]. Climatic stress tolerant rice cultivars such as drought, salt/ saline, and submergence tolerant rice cultivars have to be developed to cope in the real field stress situation. It was reported [49] that indica-type rice cultivars had significantly higher yield-scaled GWP (1101 kg CO 2 equiv. Mg −1 ) compared to Japonica (711 kg CO 2 equiv. Mg −1 )type rice cultivar. It was also reported that AWD irrigation practice reduced CH 4 emissions by 24-41%, 26-48% compared with continuous flooding, however, an increase in N 2 O emission was observed in both seasons [50]. It was also reported that biochar application in paddy soil significantly decreased N 2 O emission, while increased CH 4 emission [51].

Conclusion
In the context of global climate change, environment friendly agricultural management practices such as conservation tillage, rice seedling transplanting or direct line seeding, alternate wet and dry irrigation (AWDI), mid-season drainage, soil amendments with biochar, vermicompost, silicate slag and phospho-gypsum, site specific rice based cropping patterns and integrated plant nutrients system (IPNS) should be followed to ensure food security, while mitigating greenhouse gas emissions and global warming potentials. Furthermore, Azollacyanobacterial dual cropping with rice, introducing N-fixing legumes and duckling rearing with flood water rice cultivation could be practiced to sustain overall agricultural productivity and minimizing greenhouse gases intensity in the changing climatic conditions.