Abstract
This study examined the temporal and spatial variation in the populations of bacteria from a Chihuahuan desert bajada grassland that could grow on nearly nitrogen-free medium, a nitrogen-efficient guild (NEG). A Curtis similarity index of 0.876 and nearly identical diversity and equability indexes (H' = 1.22, J = 0.46 and H' = 1.27, J = 0.48 for the crown edge and interplant samples, respectively) indicated that there was no qualitative difference between the NEG assemblages isolated from samples taken at Bouteloua eriopoda plant crowns and in nonvegetated areas 45 cm from crowns. The difference in NEG populations between the crown and interplant samples was only 2-fold, despite a 9.5-fold difference in root biomass between the sites. These differences, while consistent, were statistically significant at only 50% of the sampling times. There was over an order of magnitude difference in NEG numbers in root-associated soil and in bulk soil from the crown or intercrown sites. The typical trend for temporal variation in NEG numbers was that they increased in the spring, fluctuated dramatically over the summer, and declined at the summer's end. The pattern of soil moisture change was the only abiotic variable which showed the same fluctuation pattern as NEG numbers.