Influence of carbon amendments on soil denitrifier abundance in soil microcosms

It is known that carbon (C) amendments increase microbial activity in anoxic soil microcosm studies, however the effects on abundance of total and denitrifier bacterial communities is uncertain. Quantitative PCR was used to target the 16S rRNA gene for the total bacterial community, the nosZ functional gene to reflect a broad denitrifier community, and functional genes from narrow denitrifier communities represented by Pseudomonas mandelii and related species (cnorB_P) and Bosea/Bradyrhizobium/Ensifer spp. (cnorB_B). Repacked soil cores were amended with varying amounts of glucose and red clover plant tissue (0–1000 mg C kg^? 1 of soil) and incubated for 96 h. Carbon amendment significantly increased respiration as measured by cumulative CO₂ emissions. Inputs of red clover or glucose at 1000 mg C kg^? 1 of soil caused increased abundance in the total bacteria under the conditions used. There was about an approximate 2-fold increase in the abundance of bacteria bearing the nosZ gene, but only in treatments receiving 500 or 1000 mg C kg^? 1 of soil of glucose or red clover, respectively. Additions of ≥ 500 mg C kg^? 1 soil of red clover and ≥ 250 mg C kg^? 1 of glucose increased cnorB_P-gene bearing denitrifiers. Changes in abundance of the targeted communities were related to C availability in soil, as indicated by soil respiration, regardless of C source. Applications of C amendments at rates that would occur in agricultural soils not only increase microbial activity, but can also induce changes in abundance of total bacterial and denitrifier communities in studies of anoxic soil microcosms.