Corn & The Environment

Corn & Climate Change

As a user of atmospheric carbon dioxide, fossil fuels, and nitrogen fertilizers, corn does affect global net emissions of greenhouse gases. These gases are considered by many to represent a potential threat to global climate.

During a full growing season, an average hectare of corn in Ontario removes 22 tonnes of carbon dioxide from the air (Cemcorp, 1992). The one million hectares of corn grown in Ontario will remove an annual quantity of carbon dioxide equivalent to that produced in burning about 9 billion litres of gasoline (about 75% of annual Ontario gasoline consumption).

Carbon dioxide is released to the atmosphere in manufacturing inputs (fertilizer, fuel, pest control products, equipment) used in growing corn. It can be estimated that about 1.3 tonnes of carbon dioxide are released during the production of a hectare of corn. This includes input manufacturing and all transportation costs. (Calculations are based on data published by Unnasch, 1990, adapted for Ontario conditions.) The ratio of carbon dioxide absorbed to carbon dioxide released is, thus, about 17:1.

The ultimate carbon balance with corn production depends upon the fate of the organic matter produced by corn during a growing season. With grain corn, some of the organic matter produced may be reconverted back into carbon dioxide relatively quickly, for example in the production and digestion of food, or the manufacture and combustion of fuel ethanol. With other uses, for example the production of paper, glue and wallboard, the reconversion may not occur for many years, decades, or centuries.

As outlined earlier, at least 60% of the dry matter produced by a grain corn crop is returned to the soil after harvest. With conventional tillage methods, the rate of oxidation of soil organic matter was at least as great as the rate of organic matter addition, with the result being no net, long-term absorption of atmospheric carbon dioxide. Past research has shown that the balance depended on yield level, with corn providing a net addition to soil organic matter (with conventional tillage) when the amount of corn stalk dry matter returned to the soil exceeded 6 tonnes/ha (Larson et al., 1972). Depending upon assumptions made as to the relative contribution of corn root, stalk, leaf and cob residues to soil organic matter, this figure equates to about 6 tonnes/ha of grain corn yield (about 100 bu/acre). The Ontario average corn yield is now about 7 t/ha, having increased steadily from a level of about 3.6 t/ha 30 years ago.

The trend to minimum tillage and no tillage can be expected to have even greater effects on the net carbon dioxide balance with corn production. The 19.8 tonnes/hectare increase in soil organic matter content of the topsoil of "no-till" versus conventionally tilled corn plots measured after 18 years of experimentation at the University of Guelph (see reference in section entitled Corn and Soil Organic Matter) equates to about 42 tonnes of carbon dioxide per hectare of soil surface. This is equivalent to that quantity of carbon dioxide released in combusting over 18,000 litres of gasoline.

The U.S. Environmental Protection Agency (1991) has calculated that a shift from 27% conventional tillage in U.S. agriculture in 1990 to 76% conservation tillage by 2010 would mean a net increase of about 272-444 million tonnes in the soil organic carbon content (equivalent to 1.0- 1.6 billion tonnes of carbon dioxide) of U.S. farm fields by the year 2020. The same report concludes, "The release of soil carbon globally from agriculture has been estimated at 800 Tg [1 Tg = 1 million tonnes] C per year. If alternative tillage practices could prevent this loss, then approximately 16% of the annual global fossil fuel emissions [5,300 Tg C per year] would be offset without considering sequestration of carbon in soils with management practices such as no- till agriculture. Other benefits would accrue since [soil organic carbon] has a positive influence on soil fertility, structure, water infiltration and heat flow, water holding capacity, aeration, size and distribution of water-stable aggregates, and reduce soil erosion."

Wallace et al. (1990) stated that "a no-till cropping system may be a most important procedure for solving the world greenhouse problem."

As corn yield levels continue to increase and as tillage and input usage decline, the importance of corn as a "sink" for carbon dioxide should increase substantially.

Nitrous oxide (N2O), a greenhouse gas, is released to the atmosphere during the production of nitrogen fertilizer. Nitrous oxide may also be released by soil micro-organisms during the natural reduction or oxidation of organic and inorganic nitrogen fertilizer materials. Environment Canada (Jaques, 1992) has estimated that about 12 thousand tonnes of N2O are released annually in Canada as a result of fertilizer manufacturing and usage. If it is assumed (based on Agriculture and Agri-Food Canada data) that about 10% of Canadian nitrogen fertilizer usage is for corn, this equates to about 1 kg of N2O per hectare of land seeded to corn. One kg of atmospheric N2O has the same "global warming potential" as about 290 kg of carbon dioxide (Intergovernmental Panel on Climate Change, 1990). This is equivalent to only about 1% of the carbon dioxide removed from the air annually by an average Ontario corn crop. Nevertheless, opportunity exists to reduce N2O losses through increased efficiency of usage of nitrogen fertilizer.[Top]