Agriculture provides both sources and sinks of greenhouse gases (GHGs). The global intensification of food and fibre production is an important factor influencing GHG emission. More than 97% of the world's food supply is produced on land that emits GHGs when intensively tilled and fertilized, and/or grazed by animals. While US agriculture is generally thought of as a minor source of GHGs, the increasing world population dictates a challenge to increase agricultural production without increasing the risks of GHG emissions and degrading environmental consequences. This review will attempt to put GHGs from agriculture in perspective, and briefly address fossil fuel in agriculture, soil carbon (C) loss from intensive tillage, emissions associated with fertilizers, emissions from animal production and manure management, and emissions associated with rice production. It has been estimated that 20% of the greenhouse effect (radiative forcing) is related to agricultural activities (Cole et al., 1996). Other recent reviews on agriculture's contribution to GHGs and global change were presented by Houghton et al., 1996; Cole et al., 1997; Lal, 1997; Lal et al., 1997a,b, 1998; Paul et al., 1997; Paustian et al., 1997a, 1998; and Rosenzweig and Hillel, 1998. Since the industrial revolution, the inflow and outflow of carbon dioxide have been disturbed by humans; atmospheric CO2 concentrations ([CO2]) have risen about 28% - principally because of fossil fuel combustion, which accounts for 99% of the total US CO2 emissions (Houghton et al., 1996). Agricultural activity, such as clearing forest for fields and pastures, transforming virgin soil into cultivated land, growing flooded rice, producing sugarcane, burning crop residues, raising cattle, and utilizing N fertilizers, are all implicated in the release of GHG into the atmosphere. The radiative forcing of GHGs and their relative amounts are shown in Fig. 3.1. Although CO2, methane (CH4), and nitrous oxide (N2O) occur naturally in the atmosphere, their recent build-up is largely a result of human activities. Since
©CAB International 2000. Climate Change and Global Crop Productivity (eds K.R. Reddy and H.F. Hodges)
the 19th century, the atmospheric concentration of these greenhouse gases has increased by 30% for CO2, 145% for CH4 and 15% for N2O (Houghton et al, 1996).
The concept of global warming potential (GWP) has been developed by the Intergovernmental Panel on Climatic Change (IPCC) (Houghton et al., 1996) to compare the ability of GHGs to trap heat in the atmosphere relative to CO2. The GWP of a greenhouse gas is the ratio of radiative forcing from a unit mass of the gas to a unit mass of CO2 over a 100-year period. The GWP for CO2=1, for CH4 = 21, and for N2O = 310 (see Chapter 2, this volume). Man-made gases such as hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride have significantly higher GWP, but are not from agricultural sources. To quantify the relative amounts of GHGs, IPCC (Houghton et al., 1996) has chosen to express the GHGs in units of million metric tons of carbon equivalents (MMTCE), calculated as the products of the mass of gas (in teragrams Tg) x GWP x 12/44. The value of 12/44 is the ratio of the mass of C to the mass of CO2. For consistency throughout this chapter, GHG units will be expressed as MMTCE. For the amount of CO2 equivalent, the quantities can be multiplied by 3.67.
The global C cycle is made up of large C reservoirs (or pools) and flows (or fluxes) important to agriculture. The reservoirs of C are interconnected by pathways of exchange through various physical, geological and biological processes. Hundreds of billions of tons of C, in the form of CO2, are absorbed by the oceans and by living biomass (through plant photosynthesis), considered to be C sinks. Comparable amounts are emitted to the atmosphere through natural and man-made processes, considered to be C sources. When the system is at equilibrium, the C fluxes or flows among the various pools are roughly balanced. Estimates of these pools are provided in Table 3.1. The largest pool is the oceans, which contain 38 million MMTCE. A large amount of soil C is stored as soil organic matter (SOM) in agricultural production systems. Over most of the earth's land surface, the quantity of C as SOM ranges from 1.4 million to 1.5 million MMTCE and exceeds, by a factor of two or three, the amount of C stored in living vegetation, estimated to be 560,000 MMTCE (Schlesinger, 1990; Eswaran et al., 1993). The contribution of CO2 released to the atmosphere from agricultural land represents 20-25% of the total amount released due to human activity (Duxbury, et al., 1993). The amount of organic C contained in soils depends on the balance between the inputs of photo-synthetically fixed C that go into plant biomass and the loss of C through microbial decomposition. Agricultural practices can modify the organic matter inputs from crop residues and their decomposition, thereby resulting in a net change in the flux of CO2 to or from soils.
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