UC Davis

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 (ΣN2O, 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 ΣN2O could be explained by NO3– intensity, an integration of soil NO3– concentrations during the study periods. Greater ΣN2O 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.