What Is the Ecological Issue

Agriculture provides important ecosystem services in the forms of food and fiber, but can also convey many disservices to agroecosystems themselves and to the ecosystems affected by agricultural practices. In particular, agricultural activities contribute substantial amounts of greenhouse gases, including more methane and nitrous oxide than any other human activity. For example, Duxbury (1994) estimated that agriculture contributes 25%, 65% and 90% of all anthropogenic emissions of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), respectively.

Several processes identified below are responsible for greenhouse gas emissions in production agriculture:

• Fossil fuels are oxidized to provide energy for machinery involved in tilling, planting and harvesting.
• Initial cultivation of previously untilled soil results in substantial losses of carbon previously stored in soil organic matter (Robertson and Grace 2004). This occurs because tillage increases oxygen supply to soil organisms and exposes previously protected soil organic matter to decomposers.
• Inputs such as nitrogen fertilizer, irrigation and manure can increase plant productivity and soil carbon sequestration, but don't necessarily result in a net decrease in carbon dioxide emissions due to the fossil fuel energy requirements to provide these inputs (Schlesinger 1999; PDF included).
• Nitrogen fertilization and tillage decrease the amount of CH₄ sequestered by soils because of a decrease in the abundance of methanotrophic bacteria in soil (Goulding et al. 1995).
• Nitrogen fertilization and tillage increase the amount of N₂O given off to the atmosphere through the processes of nitrification and denitrification (Mosier et al. 1991).
• Nitrogen fertilizer is produced using energy from fossil fuels, and applications of nitrogen fertilizer can result in high nitrous oxide emissions.

Certain management activities have been shown to reduce agricultural greenhouse gas emissions after accounting for all inputs and emissions (i.e., Net Global Warming Potential) (Robertson et al. 2000; pdf included). For example, no-till agriculture reduces soil disturbance, thus increasing soil aggregation and decreasing available oxygen for decomposition. Growing winter cover crops increases net primary productivity and inputs of organic carbon to the soil. Perennial plants have expansive root systems and have long growth periods, thus increasing soil carbon storage (Cox et al. 2006).

In this activity, students investigate three sources of greenhouse gas emissions from agriculture, and how different cropping methods, including no-till, organic and perennialization, affect global warming potential. In addition, students will discuss potential trade-offs that limit the broad application of these practices and identify tactics that may aid in the reduction of global warming potential from agriculture. The PDFs of several articles are included as resources with this Figure Set.

These Figure Sets have been developed over a period of time when they were used to teach high school ecology students, incoming first year college students and high school science teachers. We believe that these activities could be used in a range of classes, from high school biology up to graduate level biogeochemistry. Material is presented in a format that can be used directly in class, but instructors may need to modify the Figure Sets to better fit their objectives.

References

• Bouwman, A. F. 1996. Direct emission of nitrous oxide from agricultural soils. Nutrient Cycling in Agroecosystems 46: 53-70.
• Cox, T. S., J. D. Glover, D. L. Van Tassel, C. M. Cox and L. R. DeHaan. 2006. Prospects for developing perennial grain crops. Bioscience 56: 649-659.
• Duxbury, J. M. 1994. The significance of agricultural sources of greenhouse gases. Nutrient Cycling in Agroecosystems 38: 151-163.
• Goulding, K. W. T., B. W. Hutsch, C. P. Webster, T. W. Willison, D. S. Powlson, R. S. Clymo, K. A. Smith and M. G. R. Cannell. 1995. The exchange of trace gases between land and atmosphere. Philosophical Transactions: Physical Sciences and Engineering  351: 313-325.
  Haas, H. J., C. E. Evans and E. F. Miles. 1957. Nitrogen and carbon changes in Great Plains soils as influenced by cropping and soil treatments. Technical Bulletin No. 1164, USDA, State Agriculture Experiment Stations.
• IPCC. 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press, Cambridge.
• IPCC. 2007. IPCC fourth assessment report: the physical science basis. Cambridge University Press, Cambridge.
• Johnson, K. A. and D. E. Johnson. 1995. Methane emissions from cattle. Journal of Animal Science 73: 2483-2492.
• McSwiney, C. P. and G. P. Robertson. 2005. Nonlinear response of N2O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping system. Global Change Biology 11: 1712-1719.
• Mosier, A., D. Schimel, D. Valentine, K. Bronson and W. Parton. 1991. Methane and nitrous oxide fluxes in native, fertilized and cultivated grasslands. Nature 350: 330-332.
• Moss, A. R., J. P. Jouany and J. Newbold. 2000. Methane production by ruminants: its contribution to global warming. Annales de Zootechnie 49: 231-253.
• Neff, J. C., A. R. Townsend, G. Gleixner, S. J. Lehman, J. Turnbull and W. D. Bowman. 2002. Variable effects of nitrogen additions on the stability and turnover of soil carbon. Nature 419: 915-917.
• Post, W. M. and K. C. Kwon. 2000. Soil carbon sequestration and land-use change: processes and potential. Global Change Biology 6: 317-327.
• Robertson, G. and P. Grace. 2004. Greenhouse Gas Fluxes in Tropical and Temperate Agriculture: The need for a Full-Cost accounting of Global Warming Potentials. Environment, Development and Sustainability 6: 51-63.  [PDF]
• Robertson, G. P., E. A. Paul and R. R. Harwood. 2000. Greenhouse gases in intensive agriculture: Contributions of individual gases to the radiative forcing of the atmosphere. Science 289: 1922-1925.
• Roslev, P., N. Iversen and K. Henriksen. 1997. Oxidation and Assimilation of Atmospheric Methane by Soil Methane Oxidizers. Applied. Environmental. Microbiology. 63: 874-880.
• Schlesinger, W. H. 1999. Carbon and agriculture: Carbon sequestration in soils. Science 284: 2095. [PDF]
• Segers, R. 1998. Methane production and methane consumption: a review of processes underlying wetland methane fluxes. Biogeochemistry 41: 23-51.
• Vitousek, P. M., J. D. Aber, R. W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger and D. G. Tilman. 1997. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7: 737-750.