Evidence of the need for crop-specific N2O emission factors

Abstract

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 ¹⁵N and ¹³C 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 ¹³C into PLFA and higher ¹³CO₂ emissions suggests that canola residue C was used less efficiently (i.e., less for growth and more for respiration), depleting O₂ 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 ¹⁵N₂O and ¹³CO₂ 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.