Evidence of the need for crop-specific N2O emission factors

Crop residues are an important source of N for subsequent crops and contribute to cropping system nitrous oxide (N2O) emissions. Oilseed residues, particularly canola (Brassica napus L.), can instigate higher N2O emissions compared to pulse and wheat crop residues but the reason for this disproportionate emission response is unknown. To determine the quantity and source of N2O emissions, we conducted an incubation experiment (84 d) using N-15 and C-13 labelled residues of canola, wheat, flax, pea and investigated key N-cycling gene abundances, microbial abundance and community structure using PLFA and soil C and N dynamics. Residue addition of all types significantly increased microbial abundance and abundances of denitrification and nitrification genes. Canola residue resulted in significantly greater nosZI abundance. Lower incorporation of canola residue C-13 into PLFA and higher (CO2)-C-13 emissions suggests that canola residue C was used less efficiently (i.e., less for growth and more for respiration), depleting O-2 and stimulating denitrification. The magnitude of N2O emission from residue-amended soils was significantly higher (p < 0.05) than the unamended control soil and differed with residue type: canola > pea = wheat > flax > control. The canola residue emission factor was 1.56% of residue N - significantly higher than that of wheat (0.99%), pea (0.95%) and flax (0.18%). This higher canola emission factor resulted from greater residue-derived (1.47%) N2O as well as residue-induced (0.65%) soil emissions. The combined use of stable isotope tracing of (N2O)-N-15 and (CO2)-C-13 and microbial characterization quantified differences in residue-derived N2O emissions from common crops that were linked to differences in microbial abundance, community structure and activity.