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

The overall objective of this three-year project is to determine the environmentally optimum rate of fertilizer N under irrigation in the semiarid prairies of western Canada. All site characterization of soils was completed, including initial background nitrate and ammonium concentrations in the unfertilized check plots. Greenhouse gas sampling was conducted as planned, and harvest samples at crop maturity were taken, as well as residual inorganic N levels measured in soil samples taken post harvest. Measurements were collected of crop yields, as well greenhouse gas fluxes of nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂), and residual concentrations of plant-available inorganic N in the soil after harvest. Field plots of spring seeded canola were initially planted in May, 2014, but needed to be replanted in June, due to low post planting precipitation received and poor stand establishment. The experimental plots were established at the Canada-Saskatchewan Irrigation Diversification Centre, near Outlook, SK.

The main treatments consisted of a split application of urea at rates of 0, 55, 110, 165, and 220 kg N/ha, replicated four times, with half of the fertilizer N applied prior to planting, and the other half just prior to stem elongation. Greenhouse gas fluxes of N₂O, CH₄ and CO₂, were measured by automated measurement chambers, allowing six to seven measurements per plot per day. Additional related treatments included at planting side-banded N, and also pre-plant broadcast and incorporated N applications of urea at rates of 110 and 220 kg N/ha. Greenhouse gas measurements of these treatments were made using traditional vented, flow through, non-steady state chambers that were manually sampled two to three times each week, after irrigation or high rainfall events.

Interim observations showed that N₂O emissions were significantly impacted by fertilizer placement, with lower emissions of 8 kg N/ha for the average of broadcast applications of 220 kg N/ha, compared to emissions of 17 kg N/ha for side-banded treatments. The split applications had lower N₂O emissions compared to all N applied at planting. Lastly, N₂O emissions increased with higher rates of N fertilizer application. For example, N₂O emissions averaged 11 kg N/ha at the high N rate, (220 kg N/ha), compared to average N₂O emissions of 2, 2, 5, and 6 kg N/ha, respectively for rates of 0, 55, 110, and 165 kg N/ha, all placement methods averaged together.