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 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 glc and AC gln, respectively) in five contrasting soils. The AC estimates represented only very small fractions of the total soil C, with the AC glc:total C ratios ranging between 2.5×10 −3 and 3.5×10 −3 and the AC gln:total C ratios ranging between 1.7×10 −3 and 5.9×10 −3. AC glc and AC gln were greater for a soil which was highly acidic as a result of long-term (NH 4) 2SO 4 addition and which had a small microbial biomass C content compared with near-neutral 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.