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

Abstract

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, CO 2-C evolution, and microbial biomass-C were determined over a 12–20 h period. Three approaches were used for estimating Y: Y s=(dS C−ΣCO 2-C)/dS C, Y b=dB C/(dB C+ΣCO 2-C), and Y c=dB C/dS c where dS C is the change in substrate concentration (substrate utilization), ΣCO 2-C the cumulative amount of CO 2-C evolved, and dB C the change in microbial biomass (biomass production). Calculation of Y s assumes that all substrate-C utilized, minus that respired, is used for biomass and metabolite production. Calculation of Y b 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 Y b (0.44) and Y c (0.26) being significantly lower than Y s (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 ( Y s) provided the most reliable C assimilation efficiency estimate under both grazing treatments.