Effects of perennial neighbors and nitrogen pulses on growth and nitrogen uptake by Bromus tectorum

An experiment was conducted to determine if growth and biomass responses of the annual grass Bromus tectorum are affected by the magnitude and timing of nitrogen (N) pulses and if these responses are influenced by different perennial neighbor species. Nitrogen (NH4:NO3) was applied in three pulse treatments of varying interpulse length (3-d, 9-d, or 21-d between N additions). The total amount of N added was the same among treatments; hence, both the frequency and magnitude of N pulses varied (i.e., the longer the interpulse period, the greater the amount of N added for a single pulse). Bromus showed little response to the different N-pulse treatments. The only characteristic that varied among pulse treatments was specific leaf area (SLA), which was significantly greater when Bromus was grown under the 21-d N pulse than when grown under the 3-d or 9-d N pulses. Bromus height, leaf and tiller numbers, leaf area and aboveground biomass were not affected by the N-pulse treatments nor were tissue-N contents and concentrations. However, Bromus production and tissue-N were significantly different when Bromus was grown with different perennial neighbor species. Tiller production, aboveground biomass, and seed numbers of Bromus were lowest when the perennial neighbor was the tussock grass Agropyron desertorum, intermediate when the neighbor was the evergreen shrub Artemisia tridentata, and greatest when the neighbor was the deciduous shrub Chrysothamnus nauseosus. N contents of Bromus leaves were also lowest when the neighbor was Agropyron. In contrast, root N uptake capacities were greatest for Agropyron-Bromus root mixes and lowest for Chrysothamnus-Bromus root mixes. These results suggest that perennial neighbors affect growth, seed production, and N uptake of Bromus to a greater extent than the timing and magnitude of N pulses.