Nitrous Oxide Emissions from a Clay Soil Receiving Granular Urea Formulations and Dairy Manure

Soil N2O emissions vary with N source. A study was undertaken on a clay soil in the Red River Valley, Manitoba, Canada, to determine the effect of granular N fertilizers and dairy manure on N2O emissions from a field cropped to rapeseed (Brassica napus L.) in 2009 and spring wheat (Triticum aestivum L.) in 2010. Treatments included an unamended control, granular urea, controlled-release urea (ESN), stabilized urea (SuperU), and solid dairy manure added at rates to achieve a total of 140 kg available N ha(-1) (product plus soil N test). The N fertilizers were broadcast and shallowly incorporated each spring before planting; the manure was broadcast incorporated the previous fall. Nitrous oxide emissions were monitored from planting to freeze in fall and during spring thaw in 2011 using static-vented chambers. In both years, N2O emissions occurred within 4 to 5 wk of planting but not in fall after manure application. Area-scale cumulative N2O emissions (Sigma N2O, kg N ha(-1)) from planting to freeze were control < ESN = manure < urea = SuperU. Nitrous oxide emission factors were 0.017 kg N2O-N kg(-1) available N added for urea and SuperU and 0.007 kg N2O-N kg(-1) available N for ESN. Seventy-eight percent of the variation in Sigma N2O could be explained by NO3- intensity, an integration of soil NO3- concentrations during the study periods. Greater Sigma N2O were also associated with higher yields. These findings suggest that N release rates, as indicated by NO3- intensity and yield, determined N2O emissions. The results highlight the challenge of meeting crop demand yet reducing N2O emissions by selection of an N source.