EFFECT OF RATE OF NITROGEN-FERTILIZER ON THE ABOVEGROUND AND BELOWGROUND BIOMASS OF IRRIGATED BROMEGRASS IN SOUTHWEST SASKATCHEWAN

A 9-yr study was conducted on an alluvial clay loam at Swift Current, Saskatchewan to determine the effect of annual applications of urea-N (46-0-0) fertilizer on production, nutrient quality, N use efficiency, root mass, and root distribution of bromegrass (Bromus inermis Leyss.). Nitrogen was applied annually at the rates of 0, 50, 100, and 200 kg ha-1 N to an established stand of bromegrass. Plots were flood irrigated. Forage DM yields increased linearly with rate of N applied. In all years, the slope of the response was similar and averaged 24 kg DM kg-1 N. The average v intercept value was 1794 kg DM ha-1. In years 1-4, the N concentration in the grass was depressed at low N rates but after 5 yr the N concentration increased at all N rates. Tissue NO3- N levels over 100 ppm occurred at the 200 kg N rate after 3 yr. Applications of N reduced plant P in all years; the extent depended on N rate. Uptake of N increased with increasing N rate as did the apparent N recovery. Nitrogen rate had no effect on root mass or distribution. Root mass totalled 13 888 kg DM ha-1 to 105-cm depth. Approximately 36% of the root mass was in the top 7.5 cm, 11.9% in the 7.5- to 15-cm depth and 16.9% in the 15- to 30-cm depth. Estimates of the soil space occupied by roots indicate that they would occupy a large proportion of the available pore volume. It was concluded that producers growing bromegrass under irrigation on medium- to heavy-textured soils in southern Saskatchewan can consistently expect considerable increases in hay yield of good quality with N fertilizer at rates up to 200 kg N ha-1. While forage production increased linearly in response to N fertilization, root accumulation remained at a constant level. Further studies are needed to establish maximum yields and economic rates of N application.