Seasonal influences on plant phenology can have a potent influence on soil microbial community composition because it changes the availability of substrate entering the soil and nutrient availability.  Furthermore, topography also can have a significant influence by altering microclimate and soil weathering.  The primary objectives of this study were to determine the seasonal and spatial influences on important soil properties and microbial biomass, composition, and function.  The objectives were tested in acidic mixed-mesophytic forest stands in southeast Ohio.  Thirty-six plots were arranged within six forested stands over a 60 km² area.  Plots were in transects parallel with slope, in groups of three, with four aspect categories: northeast (NE), northwest (NW), southwest (SW), and southeast (SE).  Plots were sampled three times during the winter and the summer.  We analyzed soil for nutrients (C, N, P, base cations), common measurements of soil quality (pH, Al, base saturation, cation exchange capacity), and tree composition.   We evaluated microbial community composition using phospholipid fatty acid analysis (PLFA) and community function with extracellular enzymes. 

Results/Conclusions

We observed that season and aspect were significant (P < 0.01) main effects for all the measures of soil nutrients and quality.  We observed that, typically, soil chemistry was similar between the NE and NW aspects, but SE and SW aspects were different from the other aspects. Enzyme activity was significantly greater (~ 25%) in the summer when compared to winter.  Microbial biomass was 40% greater (P < 0.01) in the winter than the summer, but did not differ due to aspect.  Using non metric multidimensional scaling (NMS), we observed PLFA composition separated significantly and strongly (r² between 0.88 and 0.65) out along a gradient of soil base saturation, CN, pH, N, and total P.  These relationships were similar between winter and summer even though the microbial composition was different (P < 0.01) based on Multi-Response Permutation Procedures (MRPP).  Furthermore, we observed that microbial composition was different (P < 0.01) between the aspects, and this was strongly explained by soil chemistry.  Tree composition weakly explained these observed differences.  Results indicate soil properties change seasonally and are different due to aspect which correlates with microbial composition and function.  These results suggest that in dissected landscapes, it is necessary to account for aspect when trying to link microbial composition to ecosystem level processes.