Exploitation of Springtime Ephemeral N Pulses by Six Great Basin Plant Species

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The ability to exploit short-duration nutrient pulses may be an important factor in the competitive balance of plants and in shaping plant community structure. We investigated the growth responses and biomass production of six Great Basin plant species growing in monocultures in the field following a single pulse of nitrogen applied in early, mid, or late spring. As a control, we applied the same total quantity of N that was in each of the individual pulses as a continuous series of applications at twice-weekly intervals over 10 wk in the spring. Surprisingly, most of the species grown under the control, continuous N supply had lower growth rates, fewer tillers, and less biomass production than plants receiving N in a pulse. At least one of the pulse treatments increased biomass production relative to controls in all but one species. The exception to this pattern was the shrub Chrysothamnus, which responded to all pulse treatments and the control supply with equivalent growth rates and biomass production. Each species responded differently to the set of pulses, with the greatest response occurring early in the growth phase when plants were small and growth rates were high. Thus, phenological stage determined the timing of maximum response. Four of six species not only responded to the early-spring pulse, but also had their greatest response to this pulse, suggesting that the cold-season-adapted species of the Great Basin system are well suited to take advantage of this pulse. The combination of rapid plant growth rates and predictable pulses following snowmelt would likely result in intense competition for nutrients at this time. Our study demonstrates that plants are remarkably capable of utilizing pulses of N, and that pulsed nutrients are potentially important in natural systems. In addition, it suggests that studies conducted under constant nutrient supply may not reflect the responses of plants growing under pulsed nutrient conditions. The plants, instead of benefitting from a season-long continuous supply of N, at certain times during the growing season were able to use pulses of N for significant gains in biomass.

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