Determining the Environmental Optimum Rate of Fertilizer N for Irrigated Crops in the Semiarid Prairies

Nitrous oxide (N₂O) emissions from soils are known to be significantly affected by the timing and intensity of precipitation/irrigation events. Indeed, emission intensity generally follows what has been described as an event-based/background pattern wherein the most intense emissions are associated with some sort of triggering event. The primary triggering events are: snow melt accompanying an increase in temperature during the spring thaw; precipitation events following the application of nitrogen (N) fertilizer; and large, early- to mid-season rainfall events that trigger denitrification of inorganic N in the soil. In general, late-season precipitation events do not trigger large N₂O emissions as, by then, the inorganic N pool has been largely depleted by the growing crop.

Field plots were established at the Canada-Saskatchewan Irrigation Diversification Centre (CSIDC) near Outlook, Saskatchewan in 2014, and continued in 2015, and 2016. The main treatment consists of a split application of urea applied at rates of 0, 55, 110, 165, or 220 kg N/ha, with half the fertilizer applied prior to seeding canola (broadcast and incorporated) and the remainder applied as a topdressing just prior to bolting. Additional treatments included (i) side-banded applications of urea applied at planting at rates of 110 and 220 kg N/ha; and (ii) broadcast applications of urea applied and incorporated prior to seeding at rates of 110 and 220 kg/ha.

In general, N₂O fluxes measured during the 2015 growing season were generally smaller than they were in 2014 and 2016, but still followed an event-based and background pattern as being typical of the semi-arid prairies. Cumulative N₂O emissions were significantly impacted by fertilizer placement in all three years of the study, though the effect was inconsistent (i.e., whereas emissions were lower for the broadcast application compared to the concentrated side-band application in 2014 and 2016, the reverse was true in 2015). This suggests that wetter conditions existing in spring 2014 and growing season of 2016 are required to induce and maintain denitrification activity in the sub-surface band. Cumulative N₂O emissions were significantly impacted by the timing of the fertilizer application, with emissions being lower for the split N application compared to a single urea application at seeding in all years. Likewise, N₂O emissions in all years were positively affected by N application rate. Fertilizer banding is often thought to increase N₂O emissions compared to broadcast applications by enhancing the denitrification potential within the band. It is likely that these inter-annual variations in treatment reflect the interplay of soil environmental conditions and the effects of concentration on N transformations within the fertilizer band.