Biocides provide a source of carbon and nitrogen directly to surviving microbes and indirectly through a pulse in microbial necromass

Biocides have been frequently used to understand the roles of fungi and bacteria on soil carbon (C) and nitrogen (N) cycling. However, addition of biocides to soil can result in unwanted temporary increases in C and N supply to surviving microbes due to a pulse in microbial necromass, and where biocides can directly be used as sources of C and N. We assessed temporary dynamics in microbial biomass C (MBC) and N (MBN), soil respiration and gross N mineralization (GNM) during a 22-day laboratory incubation after addition of fungicides (captan, cycloheximide) and bactericides (bronopol, oxytetracycline) in a grassland soil. We also assessed whether captan and bronopol were a C source for remaining microbes based on δ13C measurements in respired CO2. As expected, fungicides decreased and bactericides increased fungi:bacteria ratios, while all biocides decreased MBC and MBN. However, respiration (except for oxytetracycline) and GNM (particularly for bronopol) increased after biocide addition, likely because of a pulse in necromass causing increased substrate supply to the surviving microbes. We further detected biocide-derived CO2 up to 10 days for captan, and 22 days for bronopol, suggesting that they can be an important source for C by surviving microbes. However, soil-derived CO2 remained higher without biocide, indicating that necromass was the most important source for higher soil respiration after biocide addition. We recommend repeated additions of biocides to suppress regrowth of target microbes, and use of 13C and 15N labeled substrates, when studying effects of fungi and bacteria on C and N dynamics.