Identification of Factors Determining Nitrous Oxide Emissions for a Cropping System in the Red River Valley, Manitoba.

Nitrous oxide (N₂O) emissions from agricultural lands are affected by variable soil factors and weather conditions, as well as crop management practices. The study differentiated the effects of environment from management practices and identified the key factors determining N₂O emissions from a Canadian cropping system, using 8-yr (2006-2013) data from crop lands at the Trace Gas Manitoba (TGAS-MAN) Long-Term Greenhouse Gas Research Site near Winnipeg in the Red River Valley, Manitoba. The N₂O fluxes were measured using the flux gradient technique and a tunable diode laser analysis system. The soil is heavy clay with high organic matter, and frequently prone to flooding and water logging. The site consisted of four plots of 4 ha each. Two plots were managed as continuous annual crops with a cropping sequence of corn -faba bean - rapeseed - barley - spring wheat - corn - soybean was adopted from 2006 to 2013. The other two plots was adopted with the same cropping sequence except starting 2008 the perennial forage as grass-alfalfa was established, lasted for 4 years and terminated in late summer 2011. Synthetic fertilizers, such as urea, urea-blended fertilizers and anhydrous ammonia, were broadcast-incorporated to annual crops at rates ranging from 98 to 180 kg N ha^-1, except in 2007 and 2013 where no fertilizer was applied to legume crops of faba bean and soybean. Addition of N based fertilizer and spring-thaw were the predominant drivers of N₂O production. The univariate and multivariate analysis such as PCA and stepwise regression analysis were conducted with potential soil and weather parameters. Soil nitrate exposure for the post-fertilizer period, which is an index of soil N availability mainly determined by fertilizer N input, dominated N₂O emissions for both post-fertilizer and spring-thaw emissions. Other parameters such as air temperature, precipitation, soil moisture and freezing degrees had minor contributions to N₂O emissions. The results suggested avoiding fertilizer N beyond optimum rates for crop yield and thus soil residual N accumulation is an effective mitigation practice in cold climates to decrease N₂O emissions from agricultural soils.