Evaluation of Soil Microbial Communities Between Black Locust (Robinia pseudoacacia) and Tulip Poplar (Liriodendron tulipifera) in a Pennsylvania Forest Cramer, Sherry A.* and Karl W. Kleiner, Department of Biological Sciences, York College of Pennsylvania, York, PA ABSTRACT Soil microbial communities are strongly linked with the diversity in plant communities. However, there can be species-specific plant effects on soil microbe populations. We compared the diversity of microbial communities from the soil around tulip poplar (Liriodendron tulipifera) and black locust (Robinia pseudoacacia) trees in a mid-successional forest in York County, Pennsylvania. Black locust is a nitrogen-fixing tree species that is known to contribute substantial amounts of nitrogen to the soil. We hypothesized that there would be lower microbial diversity around the nitrogen-fixing black locust tree when compared to tulip poplar. We evaluated the functional diversity of microbial communities using Biolog EcoPlates. There was no difference in functional evenness, functional richness, and functional diversity. INTRODUCTION Traditionally, abiotic factors and competition have been responsible for influencing plant growth (Westover et al. 1997; Bever 1994). Interactions between soil microbes and plants have been shown to reciprocally influence their community structure. Plants can change the soil microbial community and soil microbes can affect the growth rate of a plant population (Bever et al. 1997). In a given community, positive and negative feedback can be present. Negative feedback occurs when a plant species decreases its own growth rate due to the soil microbial community it supports. This feedback, although negative, is essential for encouraging community diversity and stability (Bever 1994). Black locust (Robinia pseudoacacia) hosts a nitrogen fixing bacteria that returns significant amounts of nitrogen back in the soil (Kleiner et al. 2002). Nitrogen has also been shown to decrease the amount of plants around the tree and among the plant community (Redding 2001). Our hypothesis was: Since black locust trees add nitrogen back into the soil, there should be a lower microbial diversity around the nitrogen-fixing black locust trees when compared to the non-nitrogen fixing tulip poplar (Liriodendron tulipifera) species. METHODS Soil was collected at two distances (4 m and 8 m) from around 11 black locust and 11 tulip poplar trees (N = 5 for each distance) located in a York County, Pennsylvania forest. All five samples from one distance were pooled together to create one sample. Yellow poplar trees stood at least 10 meters away from a black locust tree. Average well color development was measured using Biolog microtiter plates (Garland and Mills 1991). Absorbance was read using the Wallac Victor 2 Microtiter Plate reader. 1.Nitrate nitrogen amounts were quantified using the Cadmium Reduction analysis with the HACH water testing kit. 2.Functional richness and functional evenness were calculated. Functional diversity was calculated using the Shannon Diversity Index. (Cox 1996). 3.Phenolics were analyzed using a colorimetric Prussian Blue Assay (Price and Butler 1977). Phenolics are known to have antimicrobial and antifungal properties. LITERATURE CITED Bever, James, D Feedback between plants and their soil communities in an old field community. Ecology 75(7): Bever, James D., Kristi M. Westover, and Janis Antonovics Incorporating the soil community into plant population dynamics: the utility of the feedback approach. Journal of Ecology 85: Cowan, Marjorie Murphy Plant Products as Antimicrobial Agents. Clinical Microbiology Review 12(4): Cox, George W Measurement of Species Diversity. Pages Lab Manual of General Ecology. Wm. C. Brown Publishers, Dubuque, IA. Garland, Jay and Aaron Mills Classification and Characterization of Heterotrophic Microbial Communities on the Basis of Patterns of Community- Level Sole-Carbon-Source Utilization. Applied and Environmental Microbiology 57(8): Horst, E.J MUWIC Weather Newsletter. September 3, Kleiner, K.W., J. M. Mann, and W.D. Eaton Does it matter who your neighbor is? The effect of black locust (Robinia pseudoacacia) on the growth of neighboring trees. Journal of the Pennsylvania Academy of Science 73:162 (abstract). Price, M.L. and L.G. Butler Rapid visual estimation and spectrophotometric determination of tannin content of sorghum grain. Journal of Agricultural Food Chemistry 25: Redding, Mike The Effect of a Nitrogen-Fixing Tree (Robinia pseudoacacia) on the Productivity and Diversity of the Understory Shrub Communities. Journal of the Pennsylvania Academy of Science 74:146 (abstract) Schimel, Joshua P., Jay M. Gulledge, Joy S. Clein-Curley, Jon E. Lindstrom, and Joan F. Braddock Moisture effects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga. Soil Biology and Chemistry 31: Westover, Kristi M., Ann C. Kennedy, and Steven E. Kelley Patterns of rhizophere microbial community structure associated with co-occurring plant species. Journal of Ecology 85: ACKNOWLEDGEMENTS Thanks to Mike Quigley for his many hours helping me clean up soil in the lab. DISCUSSION Our hypothesis that there would be lower soil microbial diversity around the nitrogen-fixing black locust trees was not supported. Trends were lower in functional richness and evenness in the soil surrounding black locust, but diversity, nitrogen, or phenolics did not differ. The lack of a difference in the microbial communities may be due to one or more of these possibilities: 1.As expected, the trend of lower functional richness and functional evenness are present in black locust, but not significant. This may be due to a small sample size. 2.In August 2002, our time of sampling, Pennsylvania had 1.5 inches of rain in a seven week period and had an annual deficit of 10.7 inches (Horst 2002). The control of microbial community structure primarily comes from the amount of soil moisture (Schimel et al. 1999) and may have minimized differences between the species. 3.Our soil sample was a heterogeneous mix of mineral soil and organic matter, which may have contributed to high variation among the samples. RESULTS 1.As expected, functional richness (Figure 1) and functional evenness (Figure 2) were lower in black locust compared to tulip poplar, but the trends were not significant. 2.There was no significant difference between the species in abiotic factors, nitrogen (Figure 4) and phenolics (Figure 5). 3.There was no relationship between either nitrogen (Figure 6) or phenolics (Figure 7) and functional diversity in black locust or tulip poplar. The effect of tree species and distance were analyzed using a two-way ANOVA (SPSS).