Partitioning Residue-derived and Residue-induced Emissions of N2O Using 15N-labelled Crop Residues

Abstract:

Estimates of N₂O emissions in Canada indicate that 17% of all agriculture-based emissions are associated with the decomposition of crop residues. However, research specific to the western Canadian prairies (including Saskatchewan) has shown that the N₂O emission factor for N sources in this region typically ranges between 0.2 and 0.6%, which is well below the current IPCC default emission factor of 1.0%. Thus, it stands to reason that emissions from crop residues should also be lower than those calculated using the current IPCC emission factor. Current data indicates that residue decomposition, N mineralization and N₂O production are affected by a number of factors such as C:N ratio and chemical composition of the residue, soil type, and soil water content; thus, a bench-scale incubation study was conducted to examine the effects of soil type and water content on N₂O emissions associated with the decomposition of different crop residues. The study was carried out using soils from the Black, Dark Brown, Brown, and Gray soil zones and was conducted at both 50% and 70% water-filled pore space (WFPS); the soils were amended with ¹⁵N-labeled residues of wheat, pea, canola, and flax, or with an equivalent amount of ¹⁵N-labeled urea; ¹⁵N₂O production was monitored using a Picarro G5101-i isotopic N₂O analyzer.

Crop residue additions to the soils resulted in both direct and indirect emissions of N₂O, with residue derived emissions (RDE; measured as ¹⁵N₂O) generally exceeding residue-induced emissions (RIE) at 50% WFPS—with RDEs ranging from 42% to 88% (mean = 58%) of the total N₂O. Conversely, at 70% WFPS, RDEs were generally lower than RIEs—ranging from 21% to 83% (mean = 48%). Whereas both water content and soil type had an impact on N₂O production, there was a clear and consistent trend in the emission factors for the residues; i.e., emissions were always greatest for the canola residue and lowest for the wheat residue and urea fertilizer; and intermediate for pea and flax. Results of this research demonstrate that—under the right environmental conditions—there is considerable potential for both direct and indirect N₂O emissions during crop residue decomposition. Moreover, emission factors for the various crop residues tended to increase in the order: wheat ≤ urea < pea < flax << canola.