Catabolic and genetic microbial indices, and levels of nitrate, ammonium and organic carbon in soil from the black locust (Robinia pseudo-acacia) and tulip poplar (Liriodendron tulipifera) trees in a Pennsylvania forest

Soil samples were collected from the upper soil horizon within 4 m of black locust (Robinia pseudoacacia) and tulip poplar trees (Liriodendron tulipifera) from the same mixed forest in south-central Pennsylvania. The soil samples were analyzed for organic C levels, pH, NO₃^-, NH₄⁺, catabolic diversity (Shannon diversity index; catabolic H), catabolic evenness (Simpson-Yule index; catabolic E), genetic H, and genetic E. The catabolic H and genetic H of microbes in these soils were found to correlate well with the levels of mineralized N, organic C, and pH. Significant variations in these parameters were found between the soils from near black locust and tulip poplar trees. Conditions in the soil near the black locust trees were more favorable to nitrification as indicated by the elevated pH, organic C, NO₃^-, and total mineral N levels, along with lower NH₄⁺ levels. The microbial genetic H and E were greater and the catabolic H and E were lower in the black locust soils than in the tulip poplar soils. This suggests that a more specialized environment exists in the soil near the black locust trees which selects for enhanced nitrification and the use of fewer, but preferred catabolic pathways by a more genetically diverse group of microbes that grow to a greater biomass. Conversely, the soils from near the tulip poplar trees are such that they do not select for some dominant catabolic pathways, rather they allow for the use of a greater variety of catabolic pathways by a less diverse microbial population, which appear to grow to a lower biomass. We believe that the combined application of the microbial genetic and catabolic diversity analyses, microbial biomass estimates, and traditional physicochemical characteristics in soil studies provides information not easily available that can be useful during assessment of soil processes in different terrestrial habitats.