Protozoan grazing affects estimates of carbon utilization efficiency of the soil microbial community

Reliable estimates of microbial growth yield efficiency (Y=microbial production/substrate utilization) are needed to quantify and predict soil carbon (C) dynamics. We examined patterns of C utilization in two soils, a Paleustoll (USA) and Rhodoxeralf (Australia), under two levels of protozoan grazing (low vs high) when substrate was not limiting. Soil, either amended with unlabeled or 14C-labeled glucose was incubated at 25°C and glucose-C concentration, CO2-C evolution, and microbial biomass-C were determined over a 12–20 h period. Three approaches were used for estimating Y: Ys=(dSC−ΣCO2-C)/dSC, Yb=dBC/(dBC+ΣCO2-C), and Yc=dBC/dSc where dSC is the change in substrate concentration (substrate utilization), ΣCO2-C the cumulative amount of CO2-C evolved, and dBC the change in microbial biomass (biomass production). Calculation of Ys assumes that all substrate-C utilized, minus that respired, is used for biomass and metabolite production. Calculation of Yb assumes that substrate use equals biomass-C plus respired-C and does not account for biomass production consumed by grazers. Under low grazing, the three estimates of Y were similar with an average value of 0.58 and 0.55 for the Paleustoll and Rhodoxeralf, respectively. Under high grazing, the value of Y varied depending on the calculation used, with values of Yb (0.44) and Yc (0.26) being significantly lower than Ys (0.67). The total amount of glucose utilized did not vary with protozoan grazing intensity, but a high level of grazing increased the rate of glucose use and significantly reduced the amount of measurable biomass C. Substrate-based yield (Ys) provided the most reliable C assimilation efficiency estimate under both grazing treatments.