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Nitrous oxide emissions and carbon credits

Nitrous oxide (N2O) is a potent greenhouse gas that reportedly contributes to climate change. According to the Intergovernmental Panel on Climate Change’s 2007 synthesis report, the current increase in global atmospheric N2O concentration has been caused primarily by agricultural activities such as soil management and nitrogen fertilization. The good news is that farmers can decrease N2O emissions through changes in management practices. At the same time, they may eventually be able to obtain carbon credits, or offsets, for these emissions reductions and sell them in the carbon market for additional revenue.

Automated measurements of nitrous oxide emissions were taken around the clock, in all seasons. Photos courtesy of Claudia Wagner-Riddle. Advertisement

“N2O is produced in soils as a natural part of the nitrogen cycle,” explains Dr. Philippe Rochette of Agriculture and Agri-Food Canada (AAFC) at Quebec City. A small amount is produced during nitrification, which is the conversion of ammonium (NH4) to nitrate (NO3) by microbes and requires oxygen. Most N2O, however, is produced during denitrification, which occurs when oxygen is not available to microbes, for example, in wet soils. In denitrification, soil microbes use nitrate for respiration, converting it to nitrite (N2O), then to nitrogen oxide (NO), N2O, and finally nitrogen gas (N2). “Since N2O is a free intermediate, it is released in the soil environment and some of it can escape,” says Rochette.

For denitrification to occur, there must be a nitrate source, wet soil conditions in which very little oxygen enters the soil, and a carbon source for the microbes, says Dr. Craig Drury, soil scientist with AAFC at Harrow, Ontario. Although denitrification occurs in all natural ecosystems, specific agricultural practices that affect the soil nitrate, moisture, and carbon contents can enhance denitrification and therefore N2O emissions. Scientists such as Rochette and Drury of AAFC and Dr. Claudia Wagner-Riddle of the University of Guelph are trying to tease apart the complexities of practices that affect N2O emissions. Their data are used to improve annual estimates of emissions and emissions reductions.

Many soil factors affect denitrification and N2O emissions
No-till can be a beneficial practice because it improves soil water conservation and may promote carbon sequestration. However, do increases in soil moisture cancel out the emissions reductions achieved via carbon sequestration because of increases in N2O emissions? In a September 2008 publication in the Soil Science Society of America Journal, Rochette and his colleagues studied the effects of no-till on N2O emissions on a heavy clay soil and a well-aerated sandy loam soil planted to barley at the AAFC Harlaka research farm near Quebec City. “In the two soils that we studied, no-till had no effect on N2O emissions on the sandy loam because even if it increased the bulk density and water content a bit, it didn’t really decrease the aeration to a point where denitrification was really enhanced. But on the clay soil, which was already more dense, wetter, and less aerated, the practice of no-till decreased aeration below a threshold where denitrification was enhanced and N2O was produced,” says Rochette.

Rochette has also examined 25 published studies that directly compared emissions from no-till and tilled soils. His results suggest that N2O emissions increase under no till in poorly aerated soils, but not in well-aerated soils. “So if you practise no-till, for example, in eastern Canada on a clay soil where the climate is wet, what I am proposing is that this is likely to increase N2O emissions. If you adopt no-till in the prairies where the climate is dry and the soil is well aerated, there will be little effect of no-till on N2O,” says Rochette.

High N applications a factor
Likely the major factor affecting N2O emissions in agricultural systems is the application of nitrogen fertilizer far beyond the levels available in natural ecosystems. “Agroecosystems are a significant source of N2O, and that has to do with this addition of nitrogen,” says Wagner-Riddle. “Almost every study shows a significant effect of fertilizer additions on N2O emissions.” At a research site at Elora, Ontario, she and her colleagues compared a conventional plowing and nitrogen fertilizer regime with a best management practice of no-till plus reduced nitrogen fertilizer on a corn-soybean-wheat rotation. To reduce the amount of nitrogen applied, they gave nitrogen to young corn seedlings as a sidedress according to soil tests taken during planting. They also gave a nitrogen credit to the legume rotation and reduced the subsequent amount of nitrogen applied, and added a red clover cover crop in the rotation after winter wheat. This reduced the amount of nitrogen applied from 150 to 50–60 kg/ha (134 to 45–54 lbs/ac) for corn and from 90 to 60 kg/ha (80 to 54 lbs/ac) for wheat. “The combination of all of those practices told us that the emissions can be reduced significantly,” says Wagner-Riddle.  “Averaged over the five years of the study, the mean annual N2O emissions decreased by 0.79 kilograms of nitrogen per hectare, or 35 percent,” by using the best management practice. “This corresponds to 1.2 kilograms of N2O per hectare or 366 kilograms of CO2equivalents per hectare.”

Interestingly, only 20 percent of the emissions reduction occurred during the growing season, and only for corn. “The majority of the reduction (30–90 percent) was associated with the spring thaw effect,” says Wagner-Riddle. “It’s a beneficial side-effect of no-till that we hadn’t predicted when we started the experiment.” She explains that with conventional tillage, the soil lacks the insulating blanket of residues and trapped snow that moderates soil temperatures, so soil freezing is more intense. This freezing releases carbon that can be used by the microbes and causes the soil to be wetter during thaw, resulting in greater N2O emissions. So the moderating effect of no-till can depend on the coldness of the winter and the amount of residues.

Other factors affect emissions
To further complicate matters, different combinations of the various factors that affect N2O emissions can have different effects. Drury and colleagues looked at the interaction between three types of tillage and two depths of nitrogen placement in the soil (160 kg/ha, or 143 lbs/ac) for the corn phase of a corn-soybean-wheat rotation at Woodslee, Ontario, during the growing season. When nitrogen was applied shallowly, at two centimetres (less than one inch) depth, there was no difference in growing season N2O emissions among tillage types. However, when nitrogen was applied at 10 centimetres (four inches) depth, zone till had the lowest growing season N2O emissions. “Over that three-year study period, N2O emissions from zone-till were 20 percent lower than from no-till and 38 percent lower than from conventional mouldboard plow tillage,” says Drury. This was unexpected because one might think that the emissions from zone-till would be intermediate to those from no-till and conventional-till. This is where the effect of the carbon source becomes noticeable. Drury explains that zone till has the advantages of both no-till and conventional-till: the soil is more aerated than with no-till, but compared to conventional-till, “the carbon residue is still on the surface, not where the most of the nitrate is located.” Both of these factors contribute to minimize N2O emissions.

These and other factors that affect N2O emissions allow for many ways that growers might tweak their management practices to reduce emissions. “It is complex, but it also gives you the opportunity to reduce emissions by placing nitrogen shallower in the soil, using zone-till or no-till, or using crop rotation,” says Drury. “All of these management practices can help you reduce the amount of N2O that’s being lost from the soil. And hopefully, more of that nitrogen would then go into the crop to enhance growth and yield, which is where you really want it.”

Potential for carbon credits
Because of its lifespan and effects in the atmosphere, the estimated global warming potential of one tonne of N2O is 296 times that of one tonne of carbon dioxide emitted. A carbon offset is standardized as a reduction in one tonne (1000 kilograms) of carbon dioxide emitted, which is equivalent to 3.38 kilograms of N2O emitted. Theoretically, a grower could adopt specific nutrient management practices that would reduce N2O emissions and then voluntarily obtain and sell the resultant carbon credits, if an emissions trading system were in place that allowed regulated industries to purchase carbon offsets, and if nutrient management were an approved emissions reduction practice. 

Nitrous oxide emissions were reduced by using specific best management practices to reduce the amount of nitrogen fertilizer applied to cropland (BMP) compared to conventional practices (CONV).  Source: Claudia Wagner-Riddle and colleagues, University of Guelph; research funding provided by the Ontario Ministry of Natural Resources and the Natural Sciences and Engineering Research Council of Canada.

Alberta is currently the only province or territory that has mandatory emissions reductions, an emissions trading system, and carbon offsets as a compliance option. Growers in Alberta have already participated voluntarily in this system under an approved protocol for reduced-tillage practices. To date, more than 1.9 million tonnes of offsets have been created in the Alberta market, and half are from reduced-tillage projects. Currently, there is no accepted protocol for nitrogen fertilization reduction in Alberta. 

However, according to Karen Haugen-Kozyra of Climate Change Central, the Canadian Fertilizer Institute is sponsoring the development of such a protocol based on the application of the right form of nitrogen fertilizer with appropriate placement, timing, and rate. Average N2O reductions will be determined for various levels of nitrogen management. A technical seed document based on scientific investigations such as those described here is currently in development as the second step in an 11-step process to seek provincial government approval for the protocol.

In Ontario, the Ministry of the Environment is developing a cap-and-trade program that could be in place as early as January 2010, says Heather Pearson, manager of the Air Policy Instruments program. This development is part of the Provincial–Territorial Cap and Trade Initiative memorandum of understanding signed by Ontario and Quebec in June 2008. One challenge to implementing a cap-and-trade system at this time may be the number of different organizations that are undertaking the same task, but in a slightly different manner. For example, the Canadian federal government, the Western Climate Initiative, the Midwestern Greenhouse Gas Reduction Accord, and the Regional Greenhouse Gas Initiative all count Ontario as a member or observer. “It is a very fluid time with respect to cap-and-trade programs, both within Canada and on a broader level across North America, so we need to be very flexible in terms of what we do and propose,” says Pearson. “Our interest is to make sure that we have a program in place that will provide very broad access to Ontario industry in terms of linking to other systems.”

This summer, 24 Ontario corn producers participated in a pilot project with the Ontario Ministry of Agriculture, Food, and Rural Affairs to evaluate aspects of the nitrogen fertilizer reduction protocol. The pilot project “is about making sure that that protocol works the way that it’s supposed to, that the recordkeeping is adequate to substantiate the reductions,” says Pearson. The analysis should be completed in early 2009, and the results “should be helpful in informing the development of an offset program.” 

The pilot project did not involve the creation of carbon offsets. “I would be cautious with respect to raising expectations,” says Pearson. “We can’t presume what the mandatory cap-and-trade program will look like at this point in time.” However, even in the non-mandatory market, administrated by the Chicago Climate Exchange, nitrogen fertilization reduction is not yet a method by which producers may receive carbon credits. Until such protocols are developed and approved, producers may be consoled by the fact that with the high costs of fertilizer, any switch to a management practice that reduces the amount of nitrogen required while maintaining yield will still result in financial savings. These are early days yet for carbon credits.

For further information on the current science underlying nitrogen fertilizer management and nitrous oxide emissions, see: Snyder, C.S., T.W. Bruulsema, and T.L. Jensen. 2007. Greenhouse gas emissions from cropping systems and the influence of fertilizer management – a literature review. International Plant Nutrition Institute, Norcross, Georgia, USA. Available online at:  www.ipni.net/ghgreview