Water and nitrogen uptake patterns following moisture pulses in a cold desert community

Variation in the ability to utilize pulses of both water and nitrogen (N) is one possible mechanism allowing the coexistence of species in the cold desert community on the Colorado Plateau. We simulated 25-mm precipitation events and used stable isotope tracers (H-2 and N-15) to follow water and N uptake patterns in six dominant perennials (Artemisia filifolia, Coleogyne ramosissima, Cryptantha flava, Ephedra viridis, Quercus havardii, and Vanclevea stylosa) at different times of the growing season. Water pulse utilization varied on a seasonal basis and was to some extent different among species during the summer. Carbon isotope discrimination was negatively related to both plant use of moisture in upper soil layers and foliar N concentration. Species that were similar in water pulse utilization patterns differed in the natural abundances of N-15, suggesting partitioning in N sources. All species were able to utilize N pulses after rain events, but there were temporal differences in the responses among species. We also found that water and N uptake in shallow roots do not necessarily occur simultaneously. Artemisia, Cryptantha, and Quercus showed significant uptake of both water and N from the upper soil layers. In contrast, Coleogyne and Ephedra showed the capacity to utilize the water pulse, but not the N pulse. Vanclevea only took up N. The results indicate that different parts of the root system may be responsible for the acquisition of water and N. Our results also suggest that N and water partitioning could contribute to the coexistence of species in highly variable environments such as the Colorado Plateau desert system.