Relationships among nitrogen availability, vesicular-arbuscular mycorrhizae, and Bromus tectorum in disturbed rangeland sites in Colorado

Abstract

This dissertation has three main parts. The first was a study of effects of nitrogen availability and the exotic weed Bromus tectorum L. on the density of vesicular-arbuscular mycorrhizae (VAM) fungi in disturbed land in Colorado. Field and laboratory studies were designed to measure the change in density of VAM fungi in response to available N levels and plant species present. Three nitrogen treatments were established: 1) added nitrogen (N+); 2) control (no added nitrogen. C): and 3) sucrose added to reduce nitrogen availability (N-). Two common grasses, the exotic annual weed B. tectorum and a native perennial Agropyron smithii Rydb., were examined in this study. Soil samples beneath each species were taken from each N treatment plot. Both nitrogen availability and plant species were correlated with change in VAM fungal density. The N+ treatment was associated with decreased VAM fungal propagules in soil beneath B. tectorum. Similarly, there was a general trend but not a significant association between N+ treatment and a decrease in VAM fungal propagules in soil beneath A. smithii. The second part was a study to determine changes in VAM fungal abundance and infectivity following disturbance. Three different methods -1) a bioassay to measure mycorrhizal inoculum potential (MIP), 2) a bioassay to measure most probable number (MPN), and 3) direct spore counts (SC) - were used to determine the VAM fungal populations in the same soil samples collected from patches dominated by B. tectorum and adjoining patches dominated by perennial populations of native species at two different sites (Central Plains Experimental Range and Piceance Basin). All three methods showed significantly reduced VAM fungi population beneath B. tectorum. The success of B. tectorum appears closely tied to its ability to reduce and perhaps change the beneficial VAM fungal population indigenous to an area. The third part was a study to determine the effects of VAM fungi on competition between B. tectorum, and a perennial wheatgrass (Pseudoroegneria spicata (Pursh.) A. Love) and to determine the relative mycorrhizal dependency (RMD) for both species. Both species were grown in monoculture and mixed pots, with (M+) and without (M-) VAM fungal inoculum. Both species developed substantial mycorrhizal colonization when grown on M+ soils. Mycorrhizal colonization increased growth (dry shoot and root biomass) of both species. Competition between the two species resulted in elevated colonization in B. tectorum roots and reduced colonization in P. spicata roots when compared to monoculture pots. B. tectorum had higher RMD than P. spicata. They did not show significant competition between them, except the increased colonization in B. tectorum roots and the reduction in colonization in P. spicata roots during competition. Mycorrhizal inoculum generally did not reverse the competitive outcome, but did confer varying advantages in terms of increased dry biomass to the mycorrhizal B. tectorum. Moreover, the response to mycorrhizal colonization and dry shoot or root biomass in monoculture and mixed pots were not correlated.