Influence of carbon amendments on soil denitrifier abundance in soil microcosms

Abstract

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 (cnorBP) and Bosea/Bradyrhizobium/Ensifer spp. (cnorBB). Repacked soil cores were amended with varying amounts of glucose and red clover plant tissue (0–1000mgCkg−1 of soil) and incubated for 96h. Carbon amendment significantly increased respiration as measured by cumulative CO2 emissions. Inputs of red clover or glucose at 1000mgCkg−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 1000mgCkg−1 of soil of glucose or red clover, respectively. Additions of ≥500mgCkg−1 soil of red clover and ≥250mgCkg−1 of glucose increased cnorBP-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.