Maintenance carbon requirements of actively-metabolizing microbial populations under in situ conditions

For estimating turnover rates of soil microbial populations or for energy balance studies maintenance coefficients of microorganisms grown in vitro have generally been used. To study maintenance carbon requirements under in situ conditions the biomasses of two agricultural soils (Cambisol, Chernozem) and a beech forest soil (Rendzina) were activated metabolically and the CO₂ production rate at 22°C recorded every hour. In soil samples treated with increasing amounts of glucose the concentration was determined that still yielded a maximal initial respiratory response but where the CO₂ rate remained constant over 8–13 h. Decrease of CO₂ production after that time was directly related to glucose exhaustion. The amount of glucose-C which kept the CO₂-C rate at the previous level was regarded as the maintenance ration. Values for the maintenance coefficient m (mg glucose-C mg^?1 biomass-C) were 0.012 h^?1 for the two agricultural soils and 0.03 h^?1 for the forest soil. The metabolic quotient qCO₂ and maintenance values were identical for both agricultural soils and the qCO₂ values were the same in the three soils used. No net growth was observed during the experimental period using bacterial plate counts and nalidixic acid treatment as test measurements.The determined m values reflect all individual maintenance requirements of species belonging to the total active biomass pool in these particular soils and correspond to known values from in vitro studies. The relationship between annual input of carbon and the maintenance requirements of actively-metabolizing biomasses are discussed.