AVAILABLE CARBON IN SOIL DETERMINED FROM SUBSTRATE UTILIZATION KINETICS - COMPARISON OF SUBSTRATES AND SOIL AMENDMENTS

The fraction of total soil C available to soil microorganisms is diffi cult to measure accurately but extrapolation from the relationship bet ween added glucose and respiratory activity to the point where respira tory activity is zero has been proposed as an approach to estimate the microbially available C (AC). This approach has been used with glucos e and glutamine as substrates to estimate AC (AC(glc) and AC(gln), res pectively) in five contrasting soils. The AC estimates represented onl y Very small fractions of the total soil C, with the AC(glc):total C r atios ranging between 2.5 x 10(-3) and 3.5 x 10(-3) and the AC(glc):to tal C ratios ranging between 1.7 x 10(-3) and 5.9 x 10(-3). AC(glc) an d AC(gln) were greater for a soil which was highly acidic as a result of long-term (NH4)(2)SO4 addition and which had a small microbial biom ass C content compared with near-neutral soils that had been either un amended or had received farmyard manure and/or inorganic NPK. The esti mates 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. (C) 1997 Elsevier Science B.V.