| Interactions between dissimilar organisms form the basis for studies in community ecology, with recent emphasis placed on the role of positive interactions in structuring natural communities. Molecular methods were used to examine the soil microbial community associated with the high salt marsh grass Spartina patens, and to determine the relative importance of plant phenology and vesicular arbuscular mycorrhizae (VAM) colonization on specific microbial populations including the important nitrogen-fixing functional group.; Changes in total microbial and bacterial population size and biomass coincided with plant phenology. However, maximum microbial biomass occurred at a different stage of plant growth than maximum biomass of specific bacterial subgroups suggesting that resource partitioning may occur in these communities. Plant activity had a strong influence, with microbial population size and biomass responding to plant growth and plant species in field studies. Seasonal changes in nitrogen-fixers were apparent in greenhouse studies, with populations during active plant growth substantially different from populations during dormancy and senescence. However, field populations of nitrogen-fixers were unaffected by season or plant species, suggesting that under field conditions, abiotic forcing conditions may minimize plant influence resulting in stable populations of nitrogen-fixers.; VAM suppression reduced early season plant growth and seasonally affected photosynthetic regulation, a consequence of stomatal limitations on photosynthesis. Despite these reductions, VAM did not significantly affect overall microbial population size or biomass, or the community of nitrogen-fixers on samples of combined bulk and rhizosphere soil. However, short-term VAM suppression increased microbial populations, and rhizosphere populations of nitrogen-fixers differed from non-treated control plants. However, phosphorus addition to VAM suppressed plants had no effect. VAM colonization was positively associated with populations of Gamma Proteobacteria in both long-term greenhouse and field studies. This suggests that VAM colonization affect specific bacterial populations, including nitrogen-fixing bacteria affiliated with the root/rhizosphere, probably as a consequence of changes in carbon allocation engendered by fungal symbiosis. These results indicate that soil microbial populations in marsh sediments respond to plant phenological patterns of growth and carbon allocation. In addition, VAM colonization positively affects one subdivision of bacteria, and root/rhizosphere affiliated nitrogen-fixing populations with implications for biogeochemical cycling in marsh sediments. |
