Understanding of microbial processes in soil is important for the management of farming systems, particularly those that imply organic inputs of nutrients. In this work the effect of consecutive addition of two mature composts (vegetal and animal compost) at rates of 30 tm ha-1 over a 4-year period under a dryland system on the chemical and biochemical properties of a clay soil (Chromic Haploxeret) was investigated and compared to that of an inorganic fertilization. Soil chemical and biological status was evaluated by measuring the total organic carbon (TOC), humic acids, Kjeldahl-N, bicarbonate-extractable P (Olsen-P), ammonium acetate extractable-K (AAE-K), microbial biomass carbon (Cmic), enzymatic activities (dehydrogenase, protease, glucosidase, alkaline phosphatase) and Cmic/TOC ratio. At the end of the study, soils fertilised with composts showed increases in quantity (TOC) and quality (humic acids) of organic matter compared to inorganically fertilised soil. The nutrient content (Olsen-P and Kjeldahl-N) also showed an increase in the plots fertilised with composts. From the fourth crop cycle the plots fertilised with compost showed more clearly the increase in microbial biomass and enzymatic activities, which can have been related both with leguminous pre-crop and with the organic fertilization. In general, under a dryland system, an improvement of soil fertility was achieved in organically fertilised soils, confirming the positive effect of organic fertilization.
