Tuesday, November 14, 2006

Modification of bacterial communities in a pasture soil by the feeding activity of earthworms.

Josh A. Chapman, Sherie L. Edenborn, and Alan J. Sexstone. Plant & Soil Sciences, West Virginia University, Morgantown, WV 26506

            The effect of earthworm (Lumbricus terrestris) activity on microbial community composition was investigated in a Gilpin silt loam soil (fine-loamy, mixed, semiactive, mesic Typic Hapludults). Fingerprints of bacterial community structure were constructed using PCR amplicons of bacterial 16S rDNA separated by denaturing gradient gel electrophoresis (DGGE).  Culture-dependent (CD-DGGE) and culture-independent (CI-DGGE) approaches were taken to compare untreated soil; soil that contained worms feeding on manure and/or soil; and worm casts. Analysis of similarity (ANOSIM) established that significant differences existed between community fingerprints within individual gels. ANOSIM R values were as follows: R= 0.93 (P<0.001) for CD-DGGE fingerprint; R= 0.89 (P<0.001) for CI-DGGE fingerprint; and R= 0.95 (P<0.001) for a combined CI and CD DGGE fingerprint. Differences in the similarity data were explored using non-metric Multidimensional Scaling (MDS) and UPGMA.   Ordination plots and cluster analysis separated earthworm casts from soil treatments for both CD-DGGE and CI -DGGE profiles. CD analysis resulted in 73 total bands; 20 were unique to soil treatments while 24 were unique to earthworm casts.  CI analysis resulted in 64 total bands; 17 were unique to soil and 31 unique to earthworm casts.  Community fingerprints obtained from cast materials separated according to the worm's food source. Soils containing both earthworms and manure separated from all other soil treatments.  Differences in community fingerprints between untreated control soil and soil that received only earthworms were not apparent.  The large separation between CD and CI data indicated different bacterial communities were detected by these methods.  Major trends observed above also were evident in separate analyses employing PCR amplification of nirK  for DGGE, and BIOLOG Eco Plates for community level physiological profiles (CLPP).  Data suggest that bacterial communities are modified after ingestion, however resultant changes in the soil bacterial community only occurred when earthworms consumed a soil manure mixture.