Effects of low molecular weight organic substances, biochars and temperature on the microbial carbon use efficiency

Microbial carbon use efficiency (CUE) is a key parameter affecting soil C decomposition and affected by substrate, soil amendment and temperature. Moreover, different CUE calculation approaches exist and quantitative comparisons are scarce. Objectives were to investigate the short-term effects of (i) the 13C-labelled low molecular weight organic substances alanine and glucose, (ii) the incubation temperature (15 and 25 °C) and (iii) the presence of the soil amendment biochar, produced from different feedstocks (corn cob, miscanthus and grass silage) on the CUE. Soils from an arable loess site were sampled and were incubated with or without artificially aged biochar and 13C-labelled alanine and glucose at 60% water holding capacity at 15 or 25 °C for 5 days. The sum (∑) of total and substrate-derived (SD) CO2-C and microbial biomass C (MBC) were determined, and CUETP accounting all freshly synthesized microbial non-biomass metabolites that leave the cells and CUEMB solely considering the incorporation of substrate-derived C into MBC were calculated. Analyses of variance showed that ∑CO2-C emission was significantly (p ≤ 0.05) affected by incubation temperature, added substrate and biochar addition, whereas ∑CO2-CSD emission was significantly stimulated only by substrate and temperature. CUETP showed a significant response of substrate type and was reduced at the higher incubation temperature. CUETP, MBC and MBCSD results at both incubation temperatures confirmed the greater importance of glucose for MBC formation compared to alanine, whereas the latter was mineralized to a greater extent. The decrease in CUETP at the higher temperature for both substrates indicated the increasing importance of the substrates for energy production. Biochar, once it is aged as in this study, significantly stimulated ∑CO2-C emission, but not CUETP, ∑CO2-CSD emission, MBC, and MBCSD contents. CUEMB values were significantly lower than CUETP values and were affected differently by the factors studied. Overall, CUETP should be preferably reported over CUEMB values, because the exogenous metabolism of soil microorganisms should be accounted for when calculating CUE values.