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.