Chapter 5
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Greenhouse Gas Emissions from Rice Cropping Systems W. R. Horwath* University of California Davis, Dept. of Land, Air & Water Resources, One Shields Avenue, University of California, Davis, CA 95616 *E-mail:
[email protected] Rice cultivation is an important source of greenhouse gases (GHGs) that cause global warming. Rice systems contribute over 25% of total global anthropogenic methane (CH4) emissions currently. In this chapter, a review of rice cropping systems is presented in context of GHG emissions and possible mitigation measures. Increasing atmospheric carbon dioxide (CO2) has been shown in many cases to increase methane emissions in rice. In-season drainage can reduce methane emissions up to 80%. However, practices such as straw incorporation and organic matter amendments can increase methane emissions. Different rice cultivars and hybrids have varying effects on methane emission but results indicate selection to reduce methane emission should be an area of future research. Nitrous oxide (N2O) emissions are generally low compared to methane in irrigated rice. In-season draining to mitigate methane emissions will increase nitrous oxide emissions, but current studies show that the overall warming potential is generally lowered.
Introduction Rice culture can emit greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O). Rice paddies are similar to wetland ecosytems where plants are adapted to flooded conditions. The flooded status produces anoxic or reduced environments that are especially conducive to the production and emissions of CH4. Both CH4 and N2O are active in absorbing infrared radiation causing the © 2011 American Chemical Society
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greenhouse effect associated with climate change (1). These gases originate from both biogenic and abiotic sources. Emissions from biogenic sources account for 70% of total emissions and include wetlands, rice paddies, livestock, waste treatment, landfills and termites (Table I). Non-biogenic sources include fossil fuel exploration, biomass burning, and geological sources. A substantial portion of the daily food requirements of more than 2 billion people is met by rice (2). The rapidly growing world population will require that food production, particularly rice, increase by 50% of 1990 levels (3). However, rice yields have declined or stagnated over the last decade in some areas, especially Asia, where the gains in yield during the green revolution starting in the 1970’s were significant. Global rice production (yield ha-1) has increased by 19% from 1990 to 2009; but for the last 10 years, yield increases have only been 8% (4). Figure 1 shows increases in the human population, rice yields and yields expectations forecast in the year 2000 and 2010 to the year 2050 (2, 3). The more recent yield forecast indicates the need for a linear increase in food production to meet the needs of the growing population. However, the increase in rice yields is slow despite gains in cultivar improvement, genetic modifications and fertilizer technology. The recent yield declines have been attributed to declining soil N availability even though application of N fertilizer have steadily increased over the last 30 years (4). The decline is thought in part to be attributed to the intensification of production using additional rice planting within year and across years leading to extended periods of soil saturation. Extended flooding regimes have been shown to exhibit binding of available N to decomposition products, such as lignin aromatics (5). This may require additional N ferilizer to maintain rice yield potential and could increase N2O emissions. In contrast, in a temperate California rice system, where only one rice crop is grown annually, the addition of rice straw and summer and winter flooding has no long-term effect on N availability. However, this practice results in increased CH4 emissions (6). Climate change will also affect the efforts to maximize rice yield potential. Studies on rice production and GHG emissions show both positive and negative results of intensification. The objective of this chapter is to give an overview of CH4 and N2O processes and the factors and management practices that affect overall GHG emission in rice production.
Global Methane Methane is the most abundant hydrocarbon in the atmosphere. The current global abundance of CH4 is 1775 ppb giving a total atmospheric burden of approximately 5,000 Tg (7). The total global annual emission of CH4 is about 553 Tg. As a GHG, CH4 is responsible for about 21% of the total radiative forcing attributed to the major GHGs or 0.48 W m-2. The total annual global CH4 sink is 537 Tg resulting in an approximate net annual release of 16.5 Tg emission to the atmosphere. The major removal mechanism of methane from the atmosphere involves radical chemistry; when it reacts with the hydroxyl radical 68
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(·OH), initially formed from water vapor broken down by oxygen atoms that come from the cleavage of ozone by ultraviolet radiation. The reaction occurs in the troposphere resulting in a methane lifetime of 8 to 10 years. Upland soils comprise about 6% of the sink for methane facilitated through microbial oxidation to CO2. Observations from the prior decade suggested atmospheric concentration decline growth and stabilization of atmospheric CH4 burden (8). However, recent evidence suggests there is a renewed growth in atmospheric CH4 following almost a decade of decline (9). The biogenic sources of CH4 account for about 60% of its total emission. Approximately 1% of global net primary production (NPP) is converted to CH4, of which half is oxidized to CO2 by methanothrophs in the soil (10). Rice cropping systems account for about 15% of total global CH4 annual emissions.
Global Nitrous Oxide The N cycle is complex and involving a number of oxidation/reduction (redox) processes that convert dinitrogen (N2) to ammonia (NH3) further to nitrate (NO3-) and back to N2. The various redox of the N cycle support the growth of a wide range of soil microorganisms through supplying alternate electron acceptors and the ability to produce amino building blocks to construct proteins. The incomplete reduction of NO3- produces nitrogen oxides (NOx and N2O). Of the major GHGs, N2O is the most potent in terms of reflecting infrared radiation back into the lower atmosphere. The radiative forcing attributed to N2O in the atmosphere represents about 7% of the major GHGs. It has increased markedly since the preindustrial era from 270 to 319 ppb corresponding to a global burden of about 1510 Tg N (7). The atmospheric burden of N2O continues to increase by 0.25% annually. The main sink for N2O is photochemical destruction or reaction with energetic oxygen produced by photodissociation of ozone in the stratosphere but the process is slow resulting in a mean lifetime of about 300 years. Agricultural activities, such as increased N fertilizer use and biological N fixation represent the largest source of N2O released to the atmosphere today. In addition, N deposition from industrial processes and urban areas undoubtedly contribute more N to soils. In the past, land use change resulting from conversion to agriculture likely released significant N2O from mineralization of soil organic mater (SOM). Today, the emissions of N2O from soil and fertilizer N applications represent more than 60% of total global emissions to the atmosphere (Table I). Approximately 1% of the fertilizer N applied to soils is emitted as N2O (7). In traditional flooded rice systems, N2O emissions are relatively low due to reduced nitrification activity, a process providing both source and substrate for nitrifier denitrification and denitrification. Today’s agronomic practices for intensified rice production often include in-season draining events in order to manage pest and disease, which can also increase nitrification and N2O emissions.
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Table I. Global sources and sinks for methane and nitrous oxidea Tg CH4 yr-1
Sources
Tg N2O-N yr-1
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Natural sources Wetlands
188
Termites
24.5
Oceans
9.5
Geological sources
9.5
Hydrates
4.5
3.7
Soils
6.3
Atmospheric processes
0.6
Total Natural sources
236
10.6
Anthropogenic sources Ruminants
87
Rice production
83
Biomass burning
64.5
Landfills
42
Fossil fuel production/distribution
41
0.6
1.0
Agriculture
4.6
Rivers, wetlands, coastal
1.7
Total anthropogenic
Total
317.5
7.9
553
18.4
Sinks 504.5
Atmospheric removal
32
Soil microbial oxidation Total sinks
536.5
Atmospheric increase (yr-1) a
+16.5
Adapted from Horwath 2007 (16) , and IPCC 2007 (6).
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+18.4
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Figure 1. Projected population and rice yield and production increases to 2050. Filled circles represent projected rice needs in 2050. A 50% increase in yield will be required to meet food requirements of the growing population.
Biogeochemistry of Methane and Nitrous Oxide Production Methogenesis Methane is the end product of the breakdown of organic matter under anoxic or reduced conditions (11). It is formed directly from acetate or through the combination of hydrogen (H2) and CO2 by methanogens. The primary fermentation of complex (polysaccharides) and simple (sugars) organic compounds to alcohols and fatty acids is required to begin the process of CH4 production. Methanogens cannot directly consume the primary fermentation products, which must be first converted to acetate, CO2 and H2 through secondary fermentation by a group of microorganisms called syntrophs (12). These organisms must work in concert or syntrophy with methanogens because they cannot complete the required secondary fermentation in the presence of excess H2. As methanogens consume H2 and CO2 to produce CH4, the syntrophic bacteria can continue to produce secondary fermentation products to drive the formation of CH4 (Figure 2). Another physiologically distinct group of fermenting bacteria called the homoacetogens ferment sugars directly to acetate (12). The fermentation process occurs during the sequential reduction of electron acceptors following depletion of oxygen. The sequential reduction of NO3-, Mn(IV), Fe(III), sulfate (SO42-) and finally CO2 are required before CH4 is produced (Figure 2). The production of CH4 occurs at redox potentials of
