Environment
Agriculture, Greenhouse Gases and the Kyoto Protocol
Part III
By Don McCabe, Chair, OCPA Research & Technology Committee

Nitrogen (N) is one of the largest inputs for corn farmers. Measures that cut application rates of N provide direct benefit to the environment as well as to producers’ economic returns. The following article, the third in a series, illustrates how possible nitrogen management practices will help producers in corn production as well as greenhouse gas (GHG) abatement.

Figure 1 outlines the nitrogen cycle. This presentation is to help show the complexity of the entire system, as well as how producers are managing and using this cycle.

Nitrogen gas (Ng) makes up 78% of the atmosphere and is fixed into organic matter by soil microorganisms associated with legume crops. When these crops die, microorganisms break down the residue, eventually producing organic matter that contain ten parts carbon to one part nitrogen. Some nitrogen from the residue breakdown will be released as ammonia (NH3). This process is know as ammonification and is a source of nitrogen for plants. Ammonia with water produces the ammonium ion (NH4+), which is stored in soils by clay minerals for subsequent plant use.

Commercially, the Haber process takes nitrogen gas from the air and hydrogen from methane to produce ammonia for fertilizer. This chemical process requires a great deal of energy. Biotechnology applications may eventually allow the nitrogen fixation of legumes to be ‘transferred’ to corn, thus eliminating the need for this costly process.

Nitrification is the process whereby plant-available ammonium is converted to nitrate ion (NO3-) under aerobic (i.e., oxygen rich) conditions. This requires two different groups of bacteria (Nitrobacter and Nitrosomonas) to complete through an intermediate nitrite ion (NO2-).

Nitrate can undergo denitrification back to atmospheric nitrogen (N2), thereby completing the cycle. Denitrification will occur under anaerobic conditions (i.e., oxygen limiting). Waterlogged or excessive moisture conditions in soils will aid denitrification. Nitrate ion (NO3-) can be leached to groundwater prior to transformation to N2.

Nature is driven by energy transfer. At the atomic level, this means the movement of electrons. The nitrogen atom with its ability to have numerous oxidation states from –3 in ammonia to +5 in nitrate ion with the movement of 8 electrons in between results in numerous nitrogen compounds in nature. Each one of these processes gives microbes energy under aerobic to anaerobic conditions. This is what drives the nitrogen cycle outlined above.

The gas nitrous oxide (N2O) is of particular importance in consideration of the Kyoto Protocol. It is one of the compounds that could be formed part way through the nitrogen cycle process during denitrification. As noted in Part I of this series, the problem with N2O is that one molecule of N2O is 310X the strength in global warming potential to one molecule of carbon dioxide (CO2). Nitrous oxide emissions from agriculture are its biggest contributor to its emission profile – 56% of the total.

Nitrous oxide production occurs both from manure storage and soil-applied commercial fertilizer during denitrification.

Numerous conditions must come together for N2O production to occur: N source, organic matter, appropriate bacteria, pH, moisture level, oxygen level, etc. A very general rule of thumb is that for every 100 units of N available in a system, 1 unit will be lost as N2O.

Agriculture can reduce its N2O emissions and generate carbon credits by these direct emission cuts:
• optimize N timing to crop need to avoid opportunity for loss
• for corn, use of the pre-sidedress N test with credits for previous legume crops, cover crops, carryover (residual N), and manure to avoid over-applications
• avoid application of N-containing materials in the fall
• variable rate placement of N in fields where there is sufficient variability to justify the extra cost and management.

Because of the complexity of the N cycle, it has been very difficult to fully define nitrogen soil tests. It also makes the definition of an N index challenging. Much more research is required before an N-index is a consistently reliable tool for nutrient management planning across the wide range of conditions encountered in Ontario.

OCPA continues to fund numerous N projects to refine our understanding of this issue, with the objectives of reducing costs to corn producers, reducing N2O emissions to the atmosphere, and enhancing both water and soil quality for the benefit of all society.

1