Small-subunit rDNA clone libraries. DNA isolated from the clay fraction of the AM treatment was used to create a 16S ribosomal DNA (rDNA) clone library. 16S rRNA genes were amplified by PCR with the primer 8f (59) under the PCR conditions (59) described above. PCR amplicons approximately 1,500 bp long were ligated into the pGEM-T plasmid vector (Promega) and transformed into Escherichia coli DH5a competent cells. After overnight transformation,single-cell colonies were transferred into 1.5-ml Eppendorf tubes containing 80 ml of TE buffer. Tubes were heated for 10 min at 95°C to lyse cells, chilled on ice, and centrifuged for 2 min at 13,000 rpm. Supernatants were used in subsequent PCRs