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 mall fractions of the total soil C, with the AC (glc):total C ratios ranging between 2.5x10-3 and 3.5x10-3 and the AC(gln):total C ratios ranging between 1.7x10-3 and 5.9x10-3. AC(glc) and AC(gln) were greater for a soil which was highly acidic as a result of long-term (NH4)2SO4 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.
- Available carbon