Bacterial Communities of Physical Soil Fractions

Blackwood, C.B. and E. A. Paul

Presented at the All Scientist Meeting (1999-07-20 to 1999-07-21 )

Two hypotheses for how the immense diversity in soil bacterial communities is maintained are 1. high habitat diversity 2. low competition due to dormancy of cells. We hypothesized that the light fraction (LF) is a unique microbial habitat analogous to the rhizosphere. LF is a rapidly-cycling soil carbon pool isolated through density separation and sieving. Soil samples were taken from KBS LTER alfalfa plots and Living Field Lab continuous corn, conventional management plots. Samples were sieved to isolate 1. rhizosphere 2. shoot residue >2mm and 3. bulk soil. Bulk soil was then separated into LF and heavy fraction (HF, soil aggregates and other minerals) by centrifugation in water. Bacterial T-RFLP fingerprints were obtained by DNA extraction from each fraction, followed by PCR amplification with fluorescently-labeled eubacterial primers, and restriction with HhaI. Total bacterial population sizes and populations within cell size classes were determined by digital image analysis of soil smears stained with DTAF.Soil bacterial populations were highest in soil fractions where freshly deposited substrate is concentrated (carbon hotspots, Figure 1). This phenomenon is more general than the rhizosphere-effect, including LF and shoot residue. The communities of hotspots contained an increased number of large cells, indicative of increased activity. But the proportion of CTC-active cells was constant across soil fractions. Further studies need to be performed to determine the level of activity of soil bacterial cells and the location of the most active cells. Substrate availability had a large impact on bacterial community composition (Figure 2). Hotspot communities were very different from bulk soil communities. In addition, LF and shoot residue communities were nearly identical but distinct from the rhizosphere. Cropping history also caused divergence of communities within each habitat type. A wide diversity of habitats is likely to be found in soils, particularly those with more heterogeneous plant assemblages.Soil bacterial populations were highest in soil fractions where freshly deposited substrate is concentrated (carbon hotspots, Figure 1). This phenomenon is more general than the rhizosphere-effect, including LF and shoot residue. The communities of hotspots contained an increased number of large cells, indicative of increased activity. But the proportion of CTC-active cells was constant across soil fractions. Further studies need to be performed to determine the level of activity of soil bacterial cells and the location of the most active cells. Substrate availability had a large impact on bacterial community composition (Figure 2). Hotspot communities were very different from bulk soil communities. In addition, LF and shoot residue communities were nearly identical but distinct from the rhizosphere. Cropping history also caused divergence of communities within each habitat type. A wide diversity of habitats is likely to be found in soils, particularly those with more heterogeneous plant assemblages.

Back to meeting | Show |
Sign In