Journal of Methods Microbiological Journal of Microbiological Methods 30 (1997) 43–47 Available carbon in soil determined from substrate utilization kinetics: comparison of substrates and soil amendments a, b * L. Badalucco , D.W. Hopkins a ` Dipartimento di Agrobiologia e Agrochimica, Universita degli Studi della Tuscia, Via S. Camillo de Lellis, 01100 Viterbo, Italy b Department of Biological Sciences, University of Dundee, Dundee DD14HN, U.K. Abstract The fraction of total soil C available to soil microorganisms is difficult to measure accurately but extrapolation from the relationship between added glucose and respiratory activity to the point where respiratory activity is zero has been proposed as an approach to estimate the microbially available C (AC). This approach has been used with glucose and glutamine as substrates to estimate AC (AC and AC , respectively) in five contrasting soils. The AC estimates represented only very glc gln -3 -3 small fractions of the total soil C, with the AC :total C ratios ranging between 2.510 and 3.510 and the AC :total glc gln -3 -3 C ratios ranging between 1.710 and 5.910 . AC and AC were greater for a soil which was highly acidic as a glc gln result of long-term (NH ) SO addition and which had a small microbial biomass C content compared with near-neutral 4 2 4 soils that had been either unamended or had received farmyard manure and / or inorganic NPK. The estimates of AC were, however, dependent on the substrate addition and AC determined using glucose was not consistent with that determined using glutamine across all five soils.  1997 Elsevier Science B.V. Keywords: Available carbon; Glucose; Glutamine; Kinetics; Soil 1. Introduction act as the organic H donor required in the denitrifica- tion process. Sikora and McCoy [1] and Bradley and Soil organic matter is a complex mixture of Fyles [2] have proposed that an estimate of AC may decomposition residues and metabolites from plants, be derived from the Michaelis–Menten relationship animals and microorganisms, all of which may be between the concentration of added glucose and the more or less intimately associated with the mineral short-term rate of CO evolution. The proposed AC 2 component of the soil. A large fraction of the organic parameter is equivalent to the intercept on the x-axis C in soil is not readily available as a potential (added glucose concentration) with the short-term substrate to soil microorganisms because of physical induced respiration rate on the y-axis, i.e. the nega- protection and/or biochemical recalcitrance. The tive substrate concentration when the rate of CO 2 amount of microbially available C (AC) is a key evolution is extrapolated to zero. It is argued that this determinant of many microbial processes in soil; for AC parameter is the glucose equivalent driving basal example, AC affects N availability to plants, because soil respiration [2]. of its influence on the balance between N miner- Bradley and Fyles [2] suggested that testing across alization and immobilization. Furthermore, AC may a range of well-characterised soils which have been subjected to various standard treatments was neces- * Corresponding author. sary before the value of the AC parameter could be 0167-7012 / 97 / $17.00  1997 Elsevier Science B.V. All rights reserved PII S0167-7012(97)00043-2