Root proliferation characteristics of seven perennial arid-land grasses in nutrient-enriched microsites.

We compared root proliferation in fertilized microsites among seven cultivars of five commonly planted cool-desert perennial grass species that differ in productivity and competitive ability. In a greenhouse experiment on nutrient-limited plants, one soil microsite in each pot received distilled water (control) and a second microsite received a rich, complete nutrient solution (fertilized). Roots in and adjacent to the microsites were mapped on Mylar windows for 22 days after the injections to determine the magnitude and timing of response in root length relative growth rates (RGRs). Because we provided adequate water, used a high level of fertilization in the treatment microsites, and conducted the experiments during rapid vegetative growth, the results provide a measure of the relative capacities and maximal rates of the grasses responses to enriched microsites. Root samples were harvested from control and fertilized microsites at the end of the experiment to determine the morphological basis of the proliferation response. In all seven grasses fine roots proliferated in the fertilized microsites faster than in the control microsites. The grasses did not differ in the timing of their response which showed a peak 7-8 days after injection. Although one species, Pseudoroegneria spicata cv. Goldar, had higher maximum root length RGR and higher RGR ratio (RGR in fertilized to RGR in control microsites) 7-8 days after injection, the seven grasses did not differ significantly in the magnitude of root length RGR response to fertilizer integrated over the 22 day experiment. The grasses also did not differ significantly in root morphological changes in fertilized mocrosites. Compared to roots in control microsites, roots in fertilized microsites had greater specific root length, length of secondary roots per length of main axis, number of lateral and sublateral roots per length of main axis, and mean lateral root length. Root proliferation was mainly the result of increased lateral branching and lateral root growth in all seven grasses. The consistency of root proliferation responses among these seven cultivars suggests that differences in the capacity for, maximum rate, or morphological basis of root proliferation are not directly related to ecological characteristics such as productivity and competitive ability. Other aspects of root response to nutrient enrichment, such as differential responses as a function of microsite nutrient concentration, plant phenology, plant nutrient status, or specific nutrient element(s), may still be important, but further experiments are required to determine whether different responses to enriched soil microsites among species correspond with know species differences in ecological characteristics.