Date of Award:

1996

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Natural Resources

Department name when degree awarded

Rangeland Ecology

Advisor/Chair:

Martyn M. Caldwell

Abstract

For this dissertation, I examined the ability of field-grown plants to capture N presented in enriched patches or in whole-plant pulses. I assessed root proliferation in N-enriched patches when Agropyron desertorum plants had been previously fertilized or shaded. All plants responded with increased root growth rates in N-enriched patches. However, root proliferation by shaded plants was 50% less than unshaded plants. Unexpectedly, plants with higher N status had greater root growth rates in enriched patches than plants that had not received N supplement. I concluded that plants already under competitive pressure above ground for light and below ground for nutrients should be less able to respond to opportunities presented in nutrient patches.

I then examined plant growth responses and biomass production of six Great Basin species (Bromus tectorum, Taeniatherum medusae, Agropyron desertorum, Pseudoroegneria spicata, Artemisia tridentata, and Chrysothamnus nauseosus) following a pulse ofN applied in the early, mid, or late spring. An equal quantity of N, applied continuously, was a control. Surprisingly, most of the species grown under the continuous supply had lower growth rates and less biomass production than plants recieving an N pulse. The exception was Chrysothamnus, which responded equivalently to all treatments. Generally, the greatest response occurred in early phenological stages. Four of the six species had their greatest response to the early-spring pulse, suggesting that these cold-season species are well-adapted to take advantage of early spring nutrient pulses. This study demonstrated that instead of benefitting from a season-long supply of N, there were times during the growing season when plants were able to use pulses of N for significant gains in biomass. I also investigated the root properties (root biomass, specific root length [the ratio of root length:root mass], and root uptake capacity) that determined plant response to pulses. Despite considerable temperature differences and changes in plant phenological stages, root uptake capacity remained remarkably constant throughout the season. However, this consistency did not explain the differences in productivity during the season. Root biomass also did not explain these growth responses to pulses. Instead, I suggest that the quantity of actively growing fine roots, plus the ability to effectively exploit the soil volume in the early spring, results in capture of early nutrient pulses.

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