Goodfriend, W.L. and K.L. Gross
Presented at the All Scientist Meeting (1999-07-20 to 1999-07-21 )
The structure of microbial communities in naturally vegetated systems has been assumed to be driven by bottom-up forces; i.e., the quality, quantity and availability of plant-derived resources. We tested the hypothesis that plant composition, in particular the identity of the dominant species, plays a larger role in determining microbial community structure than plant diversity. Three community types on the KBS LTER were sampled in 1997; successional oldfields (T7) and poplars (T5) with and without (herbicide treated) herbaceous weeds. Herbaceous plant composition and biomass, poplar woody biomass increment, and annual leaf litter biomass was used to assess plant community similarity and total annual net primary productivity (ANPP). Soils were sampled in six replicate successional plots in mid-August at two spatial scales; across the entire 1 ha plot and within a 1m x 10 m strip located in the interior of the plot. Soil samples in the weedy and weed-free poplars were collected from microplots (approx. 15m x 15m)in mid-September. Soils were sampled to a depth of 15 cm and sieved to 4mm prior to analysis. The soil microbial communities were differentiated based on Fatty Acid Methyl Ester (FAME) profiles and the community potential use of 23 single carbon substrates (Biolog).In 1997 ANPP varied among the plant communities ranging from273-820 g/m2/yr. The weedy poplar community had the highest total productivity (wood, leaves, herbaceous biomass), and the weed-free poplars the lowest. Although herbaceous species richness was higher in the successional old fields than the weedy poplars (19 vs 13), plant composition was similar. This result suggests that the majority of difference between the successional oldfields and weedy poplars is the presence of poplar trees and roots. Soil microbes in these three plant community types tended to differentiate based on their FAME profiles, with the weedy poplar community intermediate to that of the successional and weed-free poplar communities. Microbial communities in weedy and weed-free poplars had distinct patterns of potential C-source utilization from the community in the successional oldfields. This difference, based on the relative utilization of two amino acids, L-asparagine and putrescine, suggests that the microbial community is responding to rhizosphere products and exudates of the poplar trees.For the plant community types examined, the soil microbial communities reflected differences in plant species composition, particular the presence of a biomass dominant. There were some differences in microbial composition between the weed-free poplars (a monoculture) and weedy poplars (species richness =14), suggesting that species diversity may have played a role as well in structuring the microbial communities.
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