Black locust is a tree species considered suitable for afforestation in Turkey because of its rapid growth and ability to fix atmospheric nitrogen in disturbed soil ecosystems. Quantitative indicators of soil health and quality can be usefully derived from a data set of soil physical, chemical and microbial characteristics. In this study changes in soil characteristics after afforestation with black locust were assessed by comparing several afforestation sites with control (no vegetation) sites randomly chosen along the roadside in Ulus-Bartin, the western Black Sea region (Turkey). Results showed that some physical and chemical characteristics of the soil (soil bulk density, clay content, soil organic C and total N) were higher at the afforestation sites as compared with the control sites. Similarly, afforestation sites showed higher values for mean soil microbial biomass C (afforestation: 311.97 mu g g(-1); control: 149.68 mu g g(-1)) and N (afforestation: 43.07 mu g g(-1); control: 19.21 mu g g(-1)), and basal respiration (afforestation: 0.303 mu g CO2-C g(-1) h(-1); control: 0.167 mu g CO2-C g(-1) h(-1)). However, the mean metabolic quotient (qCO(2)) assessed at the control sites was higher (1.47 mg CO2-C g(-1) C-mic h(-1)) than that observed the afforestation sites (0.96 mg CO2-C g(-1) C-mic h(-1)), likely due to difficulties in the utilization of organic substrates by the microbial community. In addition, the correlation between the qCO(2) and Cmic/Corg percentages was negative (r = -0.586, P < 0.01) in both sites. Our results indicated that afforestation with black locust could be advantageous, not only for soil improvement and regeneration, but also for sustainable land management.