Microbial respiratory quotient during basal metabolism and after glucose amendment in soils and litter

The microbial respiratory quotient (RQ), defined as the ratio of mol CO2 evolution per mol O2 uptake, was estimated in soils in northern and southern Germany under different land use with and without glucose addition in order to: (i) test the degree of corresponding data of the two procedures, and (ii) evaluate discrepancies with reference to the current eco-physiology of the soil microbiota. The RQ was frequently <1 during basal metabolism when no substrate was added. This indicates relatively high O2 consumption during the current microbial mineralisation of available substrates. Throughout the first 4 h after glucose addition, the RQ values were regularly approximately 1 showing corresponding activity values based on the two procedures. Between 4 and 24 h after glucose addition when microbial growth occurred, the soil RQ was approximately 1.3 or greater but varied significantly depending on land use, soil horizon and soil pre-conditioning. Under such conditions, the RQ value was greater in soils under conventional farming than those under organic farming systems and additionally increased from the L, Of to the Ah horizon in a beech forest. RQ values >1 during the initial period of microbial growth could not be attributed to abiotic soil properties. Thus, the soil microbiota apparently adapt to the degree of complete oxidation and the incorporation of the available substrates. Corresponding measurements of basal and substrate-induced respiration measurements with some litter types also showed RQ values different from 1.