Ratio of non-growing season to growing season N2O emissions in Canadian croplands: an update to national inventory methodology

National inventory reporting of agricultural nitrous oxide (N2O) emissions in Canada is based primarily on measurements obtained using static chambers. In regions with cold winters and an accumulated snowpack (including Canada), these measurements tend to focus on the growing season (typically May-Oct). However, research has shown that emissions continue throughout the non-growing season (NGS) and that these account for a significant proportion of annual emissions. In the Canadian National Inventory NGS emissions currently are assumed to be adequately captured in western Canada, while they are accounted for in eastern Canada by multiplying the growing season emissions by a correction factor of 1.4. a value that was derived based on a limited number of measurements. Here we use recent Canadian studies to validate this correction factor. We collected data from available Canadian studies that measured soil N2O emissions from agricultural systems for the entire year, and determined the proportion of these emissions that occurred during the NGS. The proportion of annual N2O emissions that occurred during the NGS varied widely, ranging from -4 to 119% with a mean of 35.5%, compared to the previous estimate of 30%. Due to high variability, few differences were observed between means associated with climatic, soil and management variables. To correct for NGS N2O emissions from Canadian agricultural soils we suggest that the current correction factor for converting growing season to total annual emissions be changed from 1.4 to 1.55 and that this be used for all agricultural soils in Canada rather than just eastern Canada.