VII. Share of Global Resources
and
some Environmental Impacts of Agriculture

Table of Contents

• Facts and Figures
• Cited Literature and Links

Facts and Figures

1.Humans co-opt a large fraction of the earth's living space and biological production:

• Estimates of the fraction of land's biological production that has been degraded or transformed by humanity fall in the range 39-50% (figures cited by Vitousek et al., 1997)
• Agricultural systems occupy approximately 34% of global land area and approximately 48% of total land area exclusive of desert, rocks and ice ((see Facts and Figures: 2. World Land Use, Table 1).
• Using a trophic model, Pauly and Christensen (1995) estimated that an average of 8% of global aquatic and marine production is required to support reported harvests of edible marine and aquatic species; the requirements are only about 2% for the open ocean, but range from 24 to 35% in fresh water, upwelling and shelf systems. (World fish catch has levelled off in recent years at about 85 million tonnes per year; in 1993, aquaculture contributed 16% to global fisheries production, up from 8% in 1984 FAO, 1995).
• It is estimated that approximately 80% of the 20 million hectares deforested annually is associated with conversion of forest land to agriculture (Pimentel et al., 1992).
• "Habitat conversion from forests to agriculture and then to degraded land is the single biggest factor in the present biological diversity crisis" (Dobson et al., 1997)

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2. Agricultural is a major reservoir and transformer in global cycles of carbon, nitrogen and water :

• Four recent estimates of soil organic carbon in the upper 1m range from 1220 Pg C to 1555 Pg C; on average the soil contains about 3 times more organic C than the vegetation (approx. 610 PgC), and about 2X as much C as in the atmosphere (750 PgC); this suggest that soil management can have critical effects on global CO2 regulation (Batjes and Sombroek, 1997).
• Overall human activity adds as much fixed N to terrestrial ecosystems as do all natural sources combined (Vitousek et al., 1997):
  • Industrial N fixation in fertilizers has risen from less than 10 Tg/yr 1950 to 80 Tg/yr 1990; production of more than 135 Tg/yr is expected in 2030.
  • Cultivated legumes fix 40 Tg N/year.
  • Fossil fuel combustion releases 20 Tg N.
  • More than 50 Tg/yr is released during land transformations.
  • The natural global rate of nitrogen fixation on land is between 90 and 140 TgN/yr.
• "Humanity now uses 26 percent of total terrestrial evapotranspiration and 54 percent of runoff that is geographically and temporally available (Postel et al., 1996).

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3. Agriculture is a major contributor to greenhouse gases, however, it has the potential to act as a sink:

• Release of soil C associated with land transformation (to agriculture) contributes approximately 20% of current anthropogenic CO2 emissions; fossil fuel now adds 5.5 + 0.5 b metric tons CO2-C to the atmosphere annually; annual net accumulation of CO2 averaged 3.2 b metric tons recently (Vitousek et al., 1997).
• Agricultural activities contribute about 70% of all anthropogenic N2O emissions and about 65% of all anthropogenic CH4 emissions (Delgado and Mosier, 1996).
• An analysis by Kern and Johnson (1993) found that if farmers in the contiguous USA used conservation tillage practices more extensively, the systems of soil and soil manipulation would change from a net source of atmospheric C to a net sink. Schlesinger (2000) notes that carbon costs of fertilizer and pumping water for irrigation have to be taken into account when calculating benefots of different regimes for increasing carbon storage.

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4. Nutrient leakage from agriculture is a prime cause of degradation of groundwater, surface waters and estuarine and coastal marine systems, and via the atmosphere affects other terrestrial systems (Matson et al., 1997):

• Eutrophication, associated with inputs of N and P is a prominent environmental problem in many feshwater systems and in coastal regions; in general N contributes to eutrophication and other problems in estuarine and marine environments, while P dominates eutrophication processes of freshwater systems. Eutrophication of coastal marine environments can cause oxygen depletion of stratified waters leading to loss of valuable fish and shellfish, and can lead to nuisance blooms of algae including toxin-producing forms that cause fish deaths or make fish and shellfish inedible
• Nitrate contamination of groundwater is common in agricultural areas around the world; high nitrate in drinking water is a human health concern and influences health of natural systems .
• Inadvertent fertilization associated with NOx and ammonia transported from agricultural systems and deposited downwind in gaseous or solution form can lead to acidification, eutrophication, shifts in species diversity and effects on predator and parasite systems

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Cited Literature and Links

• Batjes, N.H., and W.G. Sombroek. 1997. Possibilities for carbon sequestration in tropical and subtropical soils. Global Change Biology 3: 161-173
• Delgado, J.A., and A.R. Mosier. 1996. Mitigation alternatives to decrease nitrous oxides emissions and urea-nitrogen loss and their effect on methane flux. Journal of Environmental Quality 25: 1105-1111.
• Dobson, A.P., A.D. Bradshaw, and A.J.M. Baker. 1997. Hopes for the future: restoration ecology and conservation biology. Science 277: 515-522
• FAO. 1995. Dimensions of need. An Atlas of Food and Agriculture. FAO, Rome.
• FAO. 1996. Sixth World Food Survey. FAO, Rome
• Kern, J.S., and M.G. Johnson. 1993. Consequence of tillage impacts on national soil and atmospheric C levels. Soil Science Society of America Journal 57: 200-210
• Matson, P.A., W.J. Parton, A.G. Power, and M.J. Swift. 1997. Agricultural intensification and ecosystem properties. Science 277: 504-509
• Pauly, D., and V. Christensen. 1995. Primary production required to sustain global fisheries. Nature 374: 255-257.
• Pimentel, D. et al. (8 co-authors). 1992. Conserving biological diversity in agricultural forestry systems. BioScience 42: 354-362.
• Schlesinger, W.H. 2000. Carbon sequestration in soils: some cautions amidst optimism. Agriculture, Ecosystems and Environment 82: 121-127.
• Vitousek, O.M., H.A. Mooney, J. Lubchenco, and J.M. Melillo. 1997. Human domination of earth's ecosystems. Science 277: 494-499.