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) i s one possible mechanism allowing the coexistence of species in the cold de sert community on the Colorado Plateau. We simulated 25-mm precipitation ev ents and used stable isotope tracers (H-2 and N-15) to follow water and N u ptake patterns in six dominant perennials (Artemisia filifolia, Coleogyne r amosissima, Cryptantha flava, Ephedra viridis, Quercus havardii, and Vancle vea stylosa) at different times of the growing season. Water pulse utilizat ion varied on a seasonal basis and was to some extent different among speci es during the summer. Carbon isotope discrimination was negatively related to both plant use of moisture in upper soil layers and foliar N concentrati on. Species that were similar in water pulse utilization patterns differed in the natural abundances of N-15, suggesting partitioning in N sources. Al l species were able to utilize N pulses after rain events, but there were t emporal differences in the responses among species. We also found that wate r and N uptake in shallow roots do not necessarily occur simultaneously. Ar temisia, Cryptantha, and Quercus showed significant uptake of both water an d N from the upper soil layers. In contrast, Coleogyne and Ephedra showed t he 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 ma y be responsible for the acquisition of water and N. Our results also sugge st that N and water partitioning could contribute to the coexistence of spe cies in highly variable environments such as the Colorado Plateau desert sy stem.