aFrom Mikaloff Fletcher et al. (2004) and entries for sinks are the fractionation. bEstimates from global inverse modeling (top-down method).
global atmospheric concentration of CH4 has increased from a pre-industrial value of about 700 ppb to 1,745 ppb in 1998, and to 1,774 ppb in 2005 (IPCC 2007). Once emitted, CH4 remains in the atmosphere for approximately 8 years before removal (Dentener et al. 2003). The major CH4 sinks are oxidation by OH in the troposphere, biological CH4 oxidation in dry soil, and loss to the stratosphere. Oxidation by chlorine (Cl) atoms in the marine atmospheric boundary layer is suggested as an additional sink for CH4, possibly constituting an additional loss of about 19 Tg CH4 yr-1 (Allan et al. 2005).
CH4 sources can be divided into anthropogenic and natural sources. Anthropogenic emissions dominate present-day CH4 budgets, accounting for more than 60% of the total global budget. The anthropogenic sources include rice agriculture, livestock, landfills and waste treatment, biomass burning, and fossil fuel combustion. Natural CH4 is emitted from wetlands, oceans, forests, fire, termites and geological sources. CH4 concentration is higher in the northern hemisphere as most sources are larger. The relative contributions from different source types are shown in Table 6.1.
Cultivated wetland rice soils emit significant quantities of CH4 (Khalil et al. 1998c, Yan et al. 2003). Estimates of rice paddies, as a global source of CH4, range from 20 to 100Tgyr , which is equivalent to 5—28% of the total CH4 from all other anthropogenic sources. Uncertainty in this estimation is mostly due to the complexity of extrapolating measured fluxes on a global scale. Direct measurements result in a wide range of local CH4 fluxes for different climatic conditions, soil types, and cultivation practices of the rice-growing regions of the world.
CH4 from rice is emitted mostly from South and East Asia, where it is a dominant food source (82% of total emissions). In comparison to other CH4 sources, rice paddies are unique because they provide the principal food source for over 50% of the world's population. Thus, methods to mitigate CH4 emissions from paddies must also address the consequences of those methods on crop yield. The ultimate goal is to reduce CH4 emissions while maintaining or increasing rice grain yield.
184.108.40.206 Processes Controlling Production, Oxidation, and Emissions
CH4 is produced when organic materials decompose in oxygen-deprived conditions, notably from fermentative digestion by ruminant livestock, from stored manures, and from rice grown under flooded conditions (Conrad 1996; Mosier et al. 1998). Similar to the conditions of natural wetlands, CH4 is produced by methanogenic archaea from acetate (i.e. the methyl group) and H2/CO2 as precursors in the anaerobic environments of flooded rice fields. The base material, from which CH4 is produced, may be supplied by organic amendments as well as by the roots of the rice plants. Studies show that a large fraction is oxidized by the methanotrophic bacteria before it can reach the atmosphere. These bacteria live in the oxygenated root zone or perhaps the sub-surface region inside the roots. The vascular system of the rice plant facilitates the transport of the remaining CH4 to the atmosphere through the aerenchyma.
The observed seasonally averaged CH4 emissions vary between ~1 and 40mgm-2hr-1. Several known factors determine the flux from any particular rice field or region. The most important factors that control emissions from rice fields over large spatial scales of the size of a country are water management and the supply of organic material. As expected, more organic amendments lead to more emission, and intermittent flooding leads to less emission as the anaerobic conditions are reduced. Chemical and microbiological soil properties control CH4 production in the wetland ecosystems (Mitra et al. 2002).
The relative contributions of two methanogenic pathways to total CH4 production can be quantified by the stable carbon isotopic signatures of CO2 (Conrad 2005). Hydrogenotrophic methanogenesis leads to the production of more negative 12C-enriched CH4 than acetate-dependent methanogenesis (Krüger et al. 2002; Nakagawa et al. 2002). Stable and radiocarbon isotopes become important tools for studying CH4 production and secondary isotope fractionation processes (CH4 oxidation and transportation processes).
Plant growth controls net emissions by determining how much substrate will be available for either methanogenesis or methanotrophy (Matthews and Wassmann 2003). CH4 emissions correlate strongly with plant growth in a Texas rice field (Sass et al. 1990). Any climate change scenario, that results in an increase in plant biomass in rice agriculture, may increase CH4 emissions (Xu et al. 2004).
There is a considerable disparity in the published results on the rate of CH4 that is oxidized before emitted. Studies, that determine the oxidation as the difference between measured flux and production rates, have shown high rates of oxidation between 60 and 90% (Schutz et al. 1989; Sass et al. 1990; Khalil et al. 1998a). But data from other direct methods in some cases show oxidation rates, as low as 7% in the study by Groot et al. (2003) and < 30% reported by van der Gon and Neue (1996). From the review by Groot et al. (2003), it is evident that oxidation rates varied widely both across methods and within methods. The in situ isotope ratio approach presented by Tyler et al. (1997) and Chanton et al. (1997) was to empirically determine the fraction of CH4 oxidized during the growing season in rice fields by comparing 13C/12C in CH4 dissolved in the below ground production zone that emitted through the plant. Enrichment with both 13C and 12C occurs during CH4 oxidation by the methanotrophic bacteria. Using these methods, studies have determined in situ oxidation in the rice paddies which varied significantly (20-60%) and in general, it increases with the growing season (Tyler et al. 1997; Chanton et al. 1997; Bilek et al. 1999), while Kriiger et al. (2002) reported that CH4 oxidation was quantitatively important at the beginning of the season, but decreased later by combining isotope mass balances and in situ inhibition experiments with difluoromethane (CH2F2), as specific inhibitor of methanotrophic bacteria.
The most obvious limiting factor for methanotrophs is the availability of CH4 and O2. Another important factor for methanotrophs is the availability of N sources (Eller and Frenzel 2001). Recent findings of positive effects of NH+ fertilization on methanotrophs in rice paddies (Bodelier and Laanbroek 2004) are in contrast to the results of other studies, in which NH4+ fertilization had an inhibitory effect on CH4 oxidation in soils (Gulledge et al. 1997) and dryland rice fields (Dubey and
Singh 2000). The elevated CH4 concentrations even in the rhizosphere in a flooded rice field lead to less competitive inhibition of the CH4 monooxygenase by NH+ (Cai and Mosier 2000). This can be further supported by the fast uptake of NH+ and other ions by rice plants and no toxic compounds (e.g. NO-) accumulated in the porewater.
The only known group of bacteria able to grow on CH4 is the aerobic methy-lotrophic bacteria, which oxidize CH4 with O2 to CO2. The methanotrophs are subdivided into two major groups, type I, which dominates at lower CH4 concentrations and type II on higher concentration. A third group type X has characteristics of both of the other groups. The existing methanotrophs are unable to grow on atmospheric CH4, an activity that is obviously accomplished by unknown and as yet un-isolated bacteria in soils (Conrad 1996). CH4 oxidation is also important under anoxic conditions. The data usually show a coincidence of anaerobic CH4 oxidation with sulfate reduction as well as ferric iron reduction in the sub-soil of the rice fields (Miura et al. 1992). Current study shows that an anaerobic oxidation of CH4 can lead to denitrification with microbial mediation (Raghoebarsing et al. 2006; Xiong et al. 2007). However, microorganisms able to consume CH4 in the absence of O2 have not been isolated so far.
The area of rice grown globally is forecast to increase by 4.5% by 2030 (FAO 2003). However, CH4 emissions from rice production are not expected to increase substantially. Instead, there may even be reductions, if more rice is grown under intermittent flooding, or if new rice cultivars that emit less CH4 are developed and adopted (Wang et al. 1997). Emissions during the growing season can be reduced by various practices (Yagi et al. 1997; Khalil et al. 1998d; Adhya et al. 2000; Wassmann et al. 2000; Aulakh et al. 2001). For example, draining wetland rice once or several times during the growing season reduces CH4 emissions (Li et al. 2002; Yan et al. 2003, 2005; Khalil and Shearer 2006). This benefit, however, may be partly offset by increased N2O emissions (Akiyama et al. 2005), and this practice may even be constrained by water non-availability for flooding again. Rice cultivars with low exudation rates could offer an important CH4 mitigation option (Aulakh et al. 2001). In the off-rice season, CH4 emissions can be reduced by improved water management, especially by keeping the soil as dry as possible and avoiding water logging (Cai et al. 2000, 2003; Xu et al. 2003). CH4 emissions can be reduced by adjusting the timing of organic residue additions (Xu et al. 2000; Yan et al. 2005), by composting the residues before incorporation, or by producing biogas for use as fuel for energy production (Wassmann et al. 2000).
220.127.116.11 Decreasing Emission Trends and Approaches to Reduce Uncertainty
The net rate of CH4 emissions is generally estimated from three approaches: (1) the category based approach-extrapolation from direct flux measurements and observations, (2) process-based modelling (bottom-up approach) (Li et al. 1992a, b, 2004; Huang et al. 1998) and (3) inverse modelling that relies on spatially distributed, temporally continuous observations of concentration, and in some cases, isotopic composition in the atmosphere or aircraft and satellite observations (top-down approach)
(Frankenberg et al. 2005). The "categories" are chosen so that within each category, the emissions are expected to be about the same with some range of uncertainty obtained from direct measurements. In this approach, there are two sources of uncertainty: the reliability of assumption of constant emissions within the category and finding the spatial distribution of each category (Khalil 1992). The process models calculate the expected emissions based on a number of inputs that characterize the area under consideration. The actual field measurements are used indirectly to validate or define the values of the various parameters of the models. There are important overlaps and similarities between these two approaches. The process model requires much of the same environmental information as the category approach, such as organic inputs and water management. Obstacles to extensive application of the top-down approach include inadequate observations and insufficient capabilities of the models to account for error amplification in the inversion process and to simulate complex topography and meteorology (Dentener et al. 2003; Mikaloff Fletcher et al. 2004; Chen and Prinn 2006). Due to isotopic fractionation associated with CH4 production and consumption processes, CH4 emitted from each source exhibits a measurably different 813C value. Therefore, it is possible to constrain further the sources of atmospheric CH4 using mixing models, but such data are even more limited (Lassey et al. 2000; Mikaloff Fletcher et al. 2004).
The global average CH4 concentrations are reaching stable levels and the trends are approaching zero (Dlugokencky et al. 1998,2003). This pattern of slowdown and a persistent fall in the trend over two decades is characteristic of a constant global source and not necessarily a decreasing one (Khalil et al. 2007). For a conversion factor of 2.78 Tg CH4 per ppb and an atmospheric concentration of 1,774 ppb, the atmospheric burden of CH4 in 2005 was 4,932 Tg, with an annual average increase (2000-2005) of about 0.6 Tg yr-1. Total average annual emissions during the period considered here are approximately 582 Tg CH4 yr-1.
Total emissions are not increasing, but partitioning among the different sources may have changed. Bousquet et al. (2006) have argued that the present stable concentrations are merely a temporary condition brought about by a drought that has decreased wetland emissions offsetting increasing industrial emissions. These sources include pipeline losses in natural gas distribution systems, coal mining and drilling for oil and gas. Once the rainfall returns to normal, CH4 concentrations will start increasing again. For rice fields, it seems that both the area of rice harvested and the average emissions from a hectare of rice grown have declined in various rice growing regions (Khalil and Shearer 2006).
It is possible that there are sources that are not yet identified at this time and may still affect the validity of our understanding of the CH4 budget (Frankenberg et al. 2005). Recently Keppler et al. (2006) proposed that terrestrial plants are a major source of 60-240 Tg CH4 yr-1 via an unidentified process. Some reports have shown that the global source from living plants, if it exists, is likely to be 20-50 Tg yr-1 and not as large as originally thought, but has a potential for contributing to future trends (Ferretti et al. 2006; Houweling et al. 2006; Kirschbaum et al. 2006; Parsons et al. 2006; Butenhoff and Khalil 2007). However, Dueck et al. (2007) have presented results that do not show any emissions of CH4 from plants. It may be concluded that while plants may be a source of methane, the contribution to the global source is unknown at present.
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