Abstract The hydrolysis of the fluorescein diacetate
(FDA), related to several soil hydrolases, has been utilised
to estimate the potential microbial activity of soil freshly
amended with a wide range of organic amendments and
compared to the size and activity of soil microflora,
measured by the microbial biomass C (BC) and CO2
evolution, respectively. Three different composting mixtures
at different phases of the composting process were
added to a semi-arid soil and incubated for 2 months under
laboratory conditions. The addition of the organic amendment
immediately increased BC and both measures of
microbial activity (FDA and CO2 evolution). Highly
significant correlations were found between FDA hydrolysis
and BC for soil amended with the three composting
mixtures (r=0.81–0.96; P<0.01), regardless of the origin,
composition and degree of stability of the organic amendments.
FDA hydrolysis, conversely to CO2 evolution, was
unaffected by the disturbance caused by the soil amendment,
indicating that the two parameters probably reflect
