Glucosinolates in canola residue could influence nitrogen release

Previous research found a higher potential for nitrous oxide (N₂O) emissions from decomposing canola and flax residues compared to wheat or pea residues. This project aims to identify the reasons for this and provide guidance for future studies to develop and test strategies to minimize N₂O emissions from oilseed residues and retain more residue-derived N in the soil for subsequent crop growth.

The overall study consisted of a series of lab and greenhouse studies undertaken to test the factors thought to contribute to N₂O emissions from canola, flax, pea and wheat residues.

In the first experiment, soil from a canola field was amended with residues from all four crops. These soils were then moistened and incubated at room temperature to promote nitrification and denitrification. Results suggests that soil incorporation of crop residues promotes N₂O production. It also found that for the soil amended with canola residue, N₂O emissions went up while abundance of the nosZ gene abundance—the gene that codes for the soil-bacteria enzyme that reduces N₂O to N₂ — went down. Thus, while all residues promoted N₂O production, canola residue also inhibited N₂O consumption thereby increasing the “yield” of N₂O emitted.

The second experiment, also lab-based, want to test an old theory that glucosinolates in canola residue somehow increased N₂O emissions. The experiment involved spiking fertilized soil with freshly ground seed meal that either did (rapeseed meal) or did not (pea meal) contain glucosinolates. Results demonstrated that glucosinolates strongly impacted the N processing dynamics in such a way as to produce a dramatic increase in N₂O emissions. Moreover, there was 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” the expression of the nosZ gene, the soil-bacteria gene known to promote denitrification of N₂O into the benign N₂. The result is an increase in N₂O yield.

The third experiment involved a greenhouse study in which wheat was grown in soil from a field that had been in a cereal/pulse crop rotation for four years and was amended with canola, flax or wheat residues and fertilizer-N. Unlike in the lab-based experiment one, which did not include plants, this experiment showed no significant treatment (residue) effect on total N₂O emissions, while residue-derived emissions from the soil amended with canola residue were lower than those from the wheat-amended soil. These results likely reflect differences in the experimental conditions; e.g., the presence of plants in the soil is expected to exert a significant influence on both N and water availability—reducing both and limiting the formation of conditions favouring denitrification (the process most influenced by glucosinolates).

Conclusion: Taken together, results from this study show that (1) there is there is significant potential for canola residues to enhance N₂O emissions relative to those associated with wheat, flax, and pea residues; and (2) 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.

Future studies will need to determine appropriate mitigation strategies to reduce emissions associated with canola residues. These strategies may include the use of a nitrification inhibitor (applied either alone or as an enhanced efficiency fertilizer) or the use of a fall cover crop to reduce fall nitrate-N levels in the soil.