Combined effect of black locust invasion and fire on soils of Mediterranean shrublands and pine forests

The combined action of alien plants invasion and fire occurrence can strongly alter local biodiversity and soil functions especially in the Mediterranean area. In this context, the goal of the study was to assess the effects of the invasive black locust (Robinia pseudoacacia L.) spread on soil characteristics, especially soil microbial community responses, in some Mediterranean shrublands and pine forests subjected to both a slow natural invasion and a fast fire-induced invasion. The soil sampled in all stands, regardless type of black locust invasion, showed increasing values of pH, water holding capacity and bulk density as compared to soil of pure shrublands and pine forests. Soil organic matter content approximately doubled in stands invaded by black locust trees (from 4.3 and 9.0 % in shrublands and from 5.3 and 14.5 % in pine forests after fire respectively). A similar trend was observed for the soil nitrogen content with values of 2.9 and 8.2 mg g⁻¹ in pure and invaded shrublands after fire respectively and values of 3.8 and 5.4 mg g⁻¹ in pure and invaded pine after fire. Moreover, the combined effect of black locust invasion and fire occurrence improved soil content and availability of the investigated elements (Ca, Cu, Fe, K, Mg, Mn, Na). In addition, the invasion seemed to stimulate microbial growth, by promoting especially the bacterial component regardless of the plant cover and fire occurrence. A significant decrease of microbial respiration was observed only in invaded pine forests (0.18 mg CO₂ g⁻¹ d⁻¹) as compared to pure forests (0.32 mg CO₂ g⁻¹ d⁻¹). On the other hand, in all invaded shrublands a reduced efficiency of carbon resource utilisation and a high stress condition of the microbial community were evident. However, the combined impact of black locust invasion and fire occurrence does not show specific changes in the investigated stands. Fire confirms and, sometimes, promotes the changes due to the spread of black locust in both shrublands and pine forests soils.