Farrell: Identifying the mechanisms responsible for the greater than expected residue-induced N20 emissions from canola and flax
Date: July 2019
Term: 3 years
Status: Completed
Researcher(s): Richard Farrell, J.D. Knight, B.L. Helgason, University of Saskatchewan, Saskatoon SK, and R.L. Lemke, Agriculture and Agri-Food Canada, Saskatoon SK
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: n/a
Project Summary
The carbon intensity of commodities is an important marketing consideration for many crops including canola. Research was conducted to identify the factors contributing to previous observations that nitrous oxide (N2O) emissions from canola and flax residues were greater than those from wheat residues. The overall results from this study show that there is there is significant potential for canola residues to enhance N2O emissions relative to those associated with wheat, flax, and pea residues. The project has also identified factors that influence N2O emissions during residue decomposition, which growers could address through potential mitigation strategies incorporated into Best Management Practices.
The carbon intensity of commodities is an important marketing consideration for many crops including canola. Best Management Practices that minimize the carbon intensity of agriculture crop products need to be developed and continually refined in order for growers to remain competitive in the global marketplace. Researchers in Saskatchewan initiated a three-year study to identify the factors contributing to previous observations that nitrous oxide (N2O) emissions from canola and flax residues were greater than those from wheat residues. The objectives were to identify specific factors that influence nitrous oxide emissions from residue decomposition, and help point the way towards potential mitigation strategies that can be incorporated into Best Management Practices.
In the project, a series of bench- and greenhouse-scale studies were conducted to test the factors thought to contribute to N2O emissions from canola, flax, pea, and wheat residues. The first step was to produce and label plant residues from the four crops: canola, flax, pea and wheat with both 15N and 13C stable isotopes, which enabled researchers to track residue-derived N and C during subsequent experiments.
Two bench-scale experiments using molecular techniques were conducted to determine if physical characteristics, or biochemical composition of the residue, or any compounds within the residue, would influence N2O emissions. In the first experiment, soil was amended with crop residues that had been finely ground to eliminate particle size effects. In the second experiment, fertilized soil was amended with freshly ground canola meal containing known amounts of glucosinolates, which are known to have both stimulatory and inhibitory effects on soil microbial communities, N mineralization and nitrification.
The results of the first experiment showed that residue-derived CO2 emissions were directly related to amount of residue C added to the soil, however, there was no significant correlation between the amount of residue-N added and the residue-derived N2O. Molecular techniques showed that residue addition had no significant effect on the abundance of nitrification genes, but had a significant influence on denitrification genes. Overall, it showed that while all residues promoted N2O production, canola residue also inhibited N2O consumption, that is reduction to dinitrogen (N2), therefore increasing the 'yield' of N2O emitted.
In the second experiment, the results demonstrated that one or more glucosinolate-derivatives strongly impacted microbial N processing dynamics to produce a dramatic increase in N2O emissions. There was also a strong interaction between fertilizer-N addition and the incorporation of glucosinolate-containing seed meal. This finding, together with those of the first experiment, strongly suggest that one or more of the derivatives of glucosinolate decomposition “turns off” a denitrification gene expression, resulting in an increase in N2O yield.
The third experiment involved a greenhouse study in which wheat was grown in soils amended with canola, flax or wheat residues and fertilizer-N. Overall, the total N2O emissions in the wheat trials exhibited no significant residue treatment effect. However, the residue-derived emissions from the trials amended with canola residue were lower than those amended with wheat residue. The growing wheat plants in the trials would have competed for N and kept the soils drier, limiting denitrification. As a result, residue-induced N2O emissions were more influenced by the rate of N release from residues, with wheat turning over more quickly than canola.
Taken together, results from this study show that there is there is significant potential for canola residues to enhance N2O emissions relative to those associated with wheat, flax, and pea residues. This emissions enhancement is a result of canola residues releasing bioactive compounds during their decomposition that influence denitrifier communities in the soil, effectively increasing the yield of N2O by inhibiting its reduction to N2.
As a result of the project, factors that influence nitrous oxide emissions during residue decomposition were identified, and will help point the way towards potential mitigation strategies that may improve nitrogen use efficiency and reduce emissions, which can be incorporated into Best Management Practices. Producers can use information from this study to improve their decision making around N management in oilseed cropping systems, bolster claims of the environmental sustainability of canola, and take advantage of environmental marketing opportunities to improve on-farm profitability.