There is a need to improve the way in which wastes, such as sewage sludges, are managed and a potential way to proceed would be to transform them into biochar. On the other hand, there is a growing interest in the use of soil biochemical properties as indicators of soil quality because they are sensitive to alterations in soil management. Thus, we have studied the effect of a biochar obtained from sewage sludge on soil biochemical properties in an organic soil using two doses of biochar and comparing these results with the control soil and with soils amended with the same two doses of unpyrolyzed sewage sludge. Microbial biomass C, soil respiration, net N mineralization and several enzyme activities (dehydrogenase, β-glucosidase, phosphomoesterase and arylsulphatase) were monitored. The geometric mean of enzyme activities (GMea) was used as a soil quality index. Individual biochemical properties showed a different response to the treatments, while GMea showed an increase in the quality of soils amended with the high biochar dose and a decrease in those amended with a high sewage sludge dose. The geometric mean of enzyme activities was a suitable index to condense the whole set of soil enzyme values in a single numerical value, which was sensitive to management practices.