Microbial activities, numbers, and biomass are key parameters that can be used to elucidate the dynamics of the composting process. In the present study, four different biochemical parameters (dehydrogenase activity, ATP content, arginine ammonification, and nitrification potential) were measured (1) to monitor the dynamics of yard trimmings composting; and (2) to relate these parameters to changes in microbial numbers, physico-chemical properties, and maturity (stability) of yard trimmings compost. The initial yard trimmings was a mixture of leaves, grass clippings, and shredded bark (1:1:1 v/v). Three windrows, each having a dimension of 1.5 m (height) x 4 m (length) x 0.6 m in (width) were established. The temperature profile showed a rapid self-heating of the compost mass from ambient temperature of 20 to 70 degreesC in the first 24 h of composting. This thermophilic temperature was sustained until day 14 and then dropped to ambient level towards the end of composting (day 63). The maturation of yard trimmings compost was accompanied by a decline in pile temperature to ambient level, increases in bulk density, (NO3- + NO2-)-N, population sizes of actinomycetes and fungi, drop in C:N ratio to 20: 1, and decreases and stabilisation of biochemical properties (dehydrogenase activity, ATP content, and nitrification potential) to low levels. Dehydrogenase activity, ATP content, and nitrification potential depended on the changes in C and N content during composting. These parameters were correlated with the populations of either total aerobic heterotrophs or actinomycetes, indicating the usefulness of these parameters as indicators of microbial activity and dynamics of the composting process. Arginine ammonification was dependent on the change of neither C nor N. It also did not correlate with microbial numbers, suggesting that this parameter may not be suitable for evaluating microbial activity and dynamics of yard trimmings composting. (C) 2002 Elsevier Science Ltd. All rights reserved.