Environment
Agriculture, Greenhouse Gases and The kyoto Protocol Part IV
by Don McCabe, Chair, OCPA Research & Technology Committee

First, it is important to note the impact of soil texture on soil processes associated with carbon sequestration and nitrous oxide emission reductions. As the soil particle size distribution (i.e., texture) moves from a coarse sand to a fine clay, the opportunity for various soil processes differs both in rate and amount. In general, the impacts of texture are listed in Table 1.

From the farmer’s perspective, a balance of soil nitrogen on a given soil texture is necessary to ensure that a low level of residual nitrogen after crop growth is attained. This is generally achieved at or near the maximum economic rate of N (MERN). However, enough nitrogen must be present to ensure organic matter retention can occur. That is, soil organic matter usually runs with a soil carbon to nitrogen ratio of 10:1. So, the soil microbes cannot afford to run out of building blocks to build soil organic matter.

Agriculture and Agri-Food Canada scientists have modeled the combined effects of a change in management practice compared to conventional tillage for carbon dioxide (CO2) and nitrous oxide (N2O) emissions based on a 20-year average for the major soil classification orders in Canada. In Table 2, a negative sign means a net reduction in GHG from this management practice on this soil will result. The larger the number, the greater the modeled reduction is per hectare on a yearly basis. These numbers are relative estimates developed from a model and are, therefore, very open to further discussion. However, the table gives a possible indication of trends.

The soils identified in Table 2 are the major soil types found across the country: their relative areas in millions of hectares are provided. In Ontario, most farms are on Gray Brown Luvisol soils (i.e., well-drained sand and silt soils). The clay-textured soils are classified as Gleysolic (e.g., Brookston clay). The other soils listed are found mainly in the western provinces.

Comparing the Gray Brown Luvisol with the Gleysolic indicates more GHG reduction will occur under no-till with the Luvisol. However, with higher than normal fertilization, the Gleysol will reduce more GHGs than the Luvisol.

This indicates the Gleysolic soils are more deficient in nutrients and will respond to fertilization. When underfertilized, the Luvisol soils will produce more GHGs than the Gleysol. In this case, a healthy soil fauna population will break down soil organic matter (i.e., mineralization) to release nutrients for a crop. The wetter, clay soils (Gleysolic) will not have the same extent of mineralization. Therefore, the need for proper balance of nutrients and different soils will be necessary to ensure overall GHG reduction. This underlines the importance of research and proper values to be used in any nutrient management planning to achieve the proper balance for crop growth versus removal and economic return.

The practices that could store more carbon with possible modeled rates are summarized in Table 3. The modeled rates are ranges since the averaged values cover all soil types, soil properties, climates, etc.

For example, reducing tillage on Canadian soils in general will result in a maximum of 0.4 Tonne C per hectare per year (or roughly 0.2 ton C per acre per year).

Reducing N2O emissions primarily involves reducing opportunities for denitrification. Since N2O loss usually occurs only at spring thaw and after the application of excessive amounts of N followed by wet conditions, practices revolve around minimizing these opportunities (Table 4).

Again the values in Table 4 are estimates across all Canadian soils by AAFC scientists.

In summary, the corn farmer’s best option for reducing GHG emissions would be to have a no-till corn crop, properly sidedressed for nitrogen, on well drained, uniform, silt soils with a sound, scientifically based nutrient management plan. So for the rest of us who don’t have the luxury of silt soils from fencerow corner to corner (i.e., 100% of Ontario farmers) to maximize all the benefits, such practices when balanced to our soils, will initate the opportunities to maximize economic return and environmental benefit by reducing GHGs.

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