Introduction The structure of plant communities is believed to be governed in part by competition. Allelopathy is a well- documented competitive interaction by which plants chemically inhibit their neighbors (Heisey 1990, Gant and Clebsch 1975). A more recent view on the structure of plant communities is the hypothesis of negative feedback. Negative feedback states that populations growing above ground positively and negatively affect communities growing below ground. Those communities in turn affect those growing above ground (Bever 1994). Few studies have been conducted to determine if this is a mechanism in structuring communities. Additions of nitrogen to the soil decrease species diversity (Bush 2000), and thus it is expected that nitrogen-fixing Robinia pseudoacacia would do the same, producing a negative feedback effect on its neighbors. This experiment investigated the hypothesis that R. pseudoacacia exhibits negative feedback. Known allelopathic and non-allelopathic plants were used to provide evidence of a possible connection between allelopathy and negative feedback. Discussion The hypothesis that R. pseudoacacia would exhibit a negative feedback effect was not supported. R. pseudoacacia did not affect, nor was affected substantially by the treatments. It therefore does not appear to be influenced by soil microbial communities differently from other species. A. altissima, in contrast, appears to be influenced by soil microbial communities, but does not have an inhibitory microbial effect on other species. This indicates that A. altissima’s allelopathy is not microbially linked. More investigation is needed to determine if this is a trend. While this experiment did not provide evidence of negative feedback, more investigation needs to be done to determine if it is a significant factor in community structuring. Literature Cited Bever, J.D Feedback Between Plants and Their Soil Communities in an Old Field Succession. Ecology. 75: Boring, L.R. and W.T. Swank The Role of Black Locust (Robinia pseudoacacia) in Forest Succession. Journal of Ecology. 72: Bush, M Ecology of a Changing Planet. 2 nd ed. Prentice Hall, Upper Saddle River, NJ Gant, R.E. and E.E.C. Clebsch The Allelopathic Influences of Sassafras albidum in Old Field Succession in Tennessee. Ecology. 56: Garland, J. and A. Mills Classification and Characterization of Heterotrophic Microbial Communities on the Basis of Patterns of Community-Level Sole- Carbon-Source Utiliization. Applied and Environmental Microbiology. 57: Heisey, R.M Evidence for Allelopathy by Tree-of- Heaven (Ailanthus altissima). Journal of Chemical Ecology. 16: Acknowledgements Dr. Karl Kleiner, PhD; faculty mentor Benjamin Rogers Abstract Microbial communities might influence plant community structure, but this has not yet been thoroughly researched. Soil fertility is negatively related to the diversity of plant communities (Boring and Swank 1984). Black locust trees are nitrogen- fixing, and would be expected to decrease the diversity of soil. This research examined the effects of the microbial community on the growth of plant species, and vice versa. This soil was tested for microbial diversity using microtiter biolog plates. Four plant species were then grown in the inoculated soil and examined for germination and biomass. The Negative Feedback Effect of Robinia pseudoacacia L. Lindsay Pulliam Department of Biological Sciences, York College of Pennsylvania Methods R. pseudoacacia was planted from seed, while the rest were transplanted seedlings from local areas: Fraxinus excelsior, Juglans nigra, Ailanthus altissima, and Liriodendron tulipifera. Soil in all tree pots was mixed in a 2:2:1 ratio of local soil, peat, and perlite, with the addition of local inoculum. After 2 months of growth, soil from each of the 25 pots was added to 0.85% NaCl. Samples were then centrifuged, diluted to concentration, and pipetted into 25 Biolog microtiter plates (one plate per tree pot). Changes in absorbance were measured using Wallac Software in a spectrophotometer (Garland and Mills 1991). After 4 months of growth, soil was taken from the root area of 25 tree pots (5 from each species); half was autoclaved. Four species were grown in that soil in both treatments: A. altissima, R. pseudoacacia, Lolium multiflorum and Beta vulgaris. After 6 weeks, all were harvested, dried and massed (roots and foliage separately). Results 1. In general, autoclaving resulted in increased germination rates and greater biomass production, but the effect was significant only for A. altissima seeds. 2. The source of soil influenced germination rates of R. pseudoacacia seeds, and the biomass of B. vulgaris (from a two-way ANOVA ). However, one-way ANOVA's revealed no differences among treatments (N=5). 3. A. altissima germination and growth was inhibited by all soil sources except its own. Figure 2c Figure 2c: the effect of treatments on L. multiflorum. No growth difference in autoclaved soil (p=0.1208). No growth difference between soil treatments (p=0.1205). No difference in autoclaving effect between soil treatments (p=0.6872). Figure 2d Figure 2d: the effect of treatments on B. vulgaris. More growth in autoclaved soil (p=0.0006). Difference in growth between soil treatments (p=0.0135). No difference in autoclaving effect between soil treatments (p=0.4088). Figure 2a Figure 2a: the effect of treatments on A. altissima. More growth in autoclaved soil (p<0.0001). No growth difference between soil treatments (p=0.2213). The autoclaving effect between soil treatments varied (p=0.0158). Figure 2b Figure 2b: the effect of treatments on R. pseudoacacia seeds. No growth difference in autoclaved soil (p=0.2196). No growth difference between soil treatments (p=0.6971). No difference in autoclaving effect between soil treatments (p=0.9958). Figure 1a Figure 1a: Autoclaving positively influenced germination rates of A. altissima overall (p < ). Source of soil did not influence germination rates (p =0.6980). There was no significant interaction between tree species and autoclaving (p = ). Figure 1b Figure 1b: Autoclaving did not influence R. pseudoacacia germination (p > ), but source of soil did influence germination rates (p < ). There was no interaction between tree species and autoclaving (p = ).