Description
Competition and co-existence are strong drivers of community structure. Competition for limited resources can promote species specialization into different niches, which reduce competitive pressure and help maintain biodiversity. Interactions between plants and the microbial communities in their rooting zones create a mixture of positive and negative feedbacks that can have significant impacts on plant growth and survival. Invasion by exotic plant species, a global problem, creates additional competition for resources and can alter the composition, structure and functions of the soil microbial community. These changes can in turn alter the availability and cycling of important nutrients such as nitrogen (N) at both local and ecosystem levels. Southern California's coastal sage scrub (CSS) vegetation is habitat for numerous rare taxa but is threatened by disturbance, development and fragmentation. The native CSS vegetation is dominated by long-lived woody shrub species. Despite CSS ecosystems often occurring on N-poor soils, these shrubs tend to grow in tight clusters with overlapping crowns and root systems, rather than being separated to reduce competition for scarce resources. Open patches between shrubs are often dominated by a dense cover of exotic annual herb species. I hypothesized that the coexistence of native shrub species was due to differentiation in N use strategies, and that the state of the soil microbial community would be an important factor influencing shrub growth. In contrast, I hypothesized that the exotic annuals, established invaders with very different life histories than the shrubs, would be generalists with rapid uptake of all forms of N. I also hypothesized that growth by exotic species would be much less dependent on the state of the soil microbial community, a factor which could help explain their invasive success. I used the same plant species for all analyses and experiments. These included three of the most common native shrubs, Artemisia californica (California sagebrush, Asteraceae), Eriogonum fasciculatum (California buckwheat, Polygonaceae), and Salvia apiana (white sage, Lamiaceae); and three of the most common exotics, the annuals Brassica nigra (black mustard, Brassicaceae), Bromus madritensis (red brome, Poaceae), and Centaurea melitensis (Malta star-thistle, Asteraceae). In Chapter 1 I conducted numerous analyses of N cycling processes in rhizosphere soil and plant tissue samples collected from a CSS field site. Soil analyses included measurements of the pool sizes and flux rates of different forms of N (nitrate, ammonia, organic N) in the surface soil; ___N signatures of the soil at different depths to 30 cm; ___N of the different forms of N in the soil; and rhizosphere rates of potential mineralization, nitrification and protease activity. Plant tissue analyses included natural abundance ___N signatures in the leaves, direct uptake of __C-glycine by plant roots, nitrate reductase activity in leaves, and glutamine synthetase (ammonia assimilation) activity in roots. The native species showed less niche differentiation than expected, while the exotic species showed more than expected. Overall, the exotic species had significantly higher rates of N uptake and immobilization than the native species. Only one species, E. fasciculatum, showed strong evidence of positive feedbacks with soil microbial community. In Chapter 2 I grew seedlings of all six species in separate pots in a greenhouse, with a factorial combination of nitrogen and soil treatments. One third of each species vii was fertilized exclusively with ammonia, one third with nitrate, and one third with glycine, a simple amino acid. Additionally, half of each species was grown in soil that had been fully autoclaved to sterilize the soil microbial community, while the other half was grown in soil containing an inoculum of native CSS microbes. Artemisia californica had no differences in growth among the different N treatments, but had significantly reduced growth in sterilized soil. Eriogonum fasciculatum grew most when fertilized with ammonia, and had reduced growth when planted in sterilized soil. Salvia apiana had no growth differences in response to N fertilizer type or the state of the soil microbial community. The growth of B. nigra also did not differ in response to N or soil treatments. Bromus madritensis had the greatest growth when fertilized with ammonia, but no differences in growth between sterilized and unsterilized soil treatments. Centaurea melitensis had lower growth with glycine than with ammonia or nitrate, and lower growth in unsterilized soil than in sterilized soil. In Chapter 3 I applied solutions of ammonia, nitrate and glycine that had been labeled with heavily enriched __N to clusters of native shrubs and exotic annuals in the field. After one month of cycling through the microbial community and uptake by the plants, I harvested leaf tissue to see which species had taken up the __N label. No species showed a significant difference in the amount of label taken up from the different surface N treatments, but A. californica took up significantly less N from nitrate applied at 15-30 cm than from any N type applied at the surface. There were significant differences in __N enrichment among species. Among the natives E. fasciculatum was significantly less enriched than the other two shrub species, while among the exotics B. nigra was less enriched than the other annuals. Finally, I used uptake rates from the labeling experiment viii to create a simple model of N uptake in a landscape covered by either A. californica or B. madritensis. This model predicted that the exotic species represent a significant N sink across the ecosystem, taking up over three times more N per unit area than the native species. Among the native shrub species, A. californica appears to be a shallow-rooted N generalist; E. fasciculatum showed some of the strongest, positive plant-soil feedbacks, may promote the retention of plant organic matter, and may specialize in using organicderived N; and S. apiana may be adapted to re-colonize quickly after fire or other disturbance, rapidly taking up inorganic N and being relatively insensitive to changes in the soil microbial community. Among the exotic annual species, B. nigra appears to be an N generalist, but with lower rates of N uptake the other exotics; B. madritensis appears to also be an N generalist, but able to rapidly take up N from the soil; and C. melitensis with high rates of uptake may preferentially use inorganic N, but can also take up organic N when it is available. Overall, the exotic species represent a significant source of competition for N, may comprise a larger N sink than the native species, and have the potential to significantly alter N cycling processes in coastal sage ecosystems
