Continuing Impacts of Selective Inhibition on Bacterial and Fungal Communities in an Agricultural Soil.

Selective inhibition (SI) has been routinely used to differentiate the contributions of bacteria and fungi to soil ecological processes. SI experiments typically measured rapid responses within hours since the addition of inhibitor, but the long-term effects of selective biocides on microbial community composition and function were largely unknown. In this study, a microcosm experiment was performed with an agricultural soil to explore the effectiveness of two bactericides (bronopol, streptomycin) and two fungicides (cycloheximide, captan), which were applied at two different concentrations (2 and 10 mg g-1 ). The microcosms were incubated for 6 weeks. A radiolabeled substrate, [1,2,3,4,4a,9a-14 C] anthracene, was spiked to all microcosms, and the derived CO2 was monitored during the incubation. The abundance and composition of bacteria and fungi were assessed by qPCR and Miseq sequencing of ribosomal rRNA genes. It was demonstrated that only 2 mg g-1 bronopol and cycloheximide significantly changed the bacteria to fungi ratio without apparent non-target inhibition on the abundances; however, community shifts were observed in all treatments after 6 weeks incubation. The enrichment of specific taxa implicated a selection of resistant or adapted microbes by these biocides. Mineralization of anthracene was continuingly suppressed in all SI microcosms, which may result in biased estimate of bacterial and fungal contributions to pollutant degradation. These findings highlight the risks of long-term application of selective inhibition, and a preliminary assessment of biocide selection and concentration is highly recommended.