Invasive Quercus rubra negatively affected soil microbial communities relative to native Quercus robur in a semi-natural forest.

Invasive tree species can exert a profound effect on soil properties and ecosystem processes. Quercus rubra is a Northern American species that has an invader status in many European countries. However, the direction and magnitude of its effect on soil physicochemical and microbial properties relative to native tree species in forests are largely unknown. The aim of this study was to investigate the influence of invasive Q. rubra on physicochemical and microbial properties of soil organic and mineral horizons in comparison to native Quercus robur in a semi-natural forest. The study was performed on 23 Q. rubra and 10 Q. robur stands in the Niepołomice Forest (southern Poland). A number of soil parameters were assessed, namely moisture, water holding capacity, electrical conductivity, pH, organic C, total N, respiration, bacterial and fungal biomass and community structure (phospholipid fatty acid and ergosterol analyses). As soil properties are influenced by the quality of leaf litter deposited by trees, senesced Q. rubra and Q. robur leaves were characterized in terms of C, Ca, Mg, K, N, P, total phenolics and condensed tannins concentrations. It was found that total microbial and bacterial biomass was significantly lower under Q. rubra than Q. robur in both soil horizons. Microbial community structure of organic horizon also differed between the two Quercus species. In contrast, no differences were found in fungal biomass and soil physicochemical variables. The reduction in microbial and bacterial biomass beneath Q. rubra may be associated with the quantity and quality of its litter. Senesced Q. rubra leaves were characterized by significantly higher C/N and C/P ratios relative to those of Q. robur. Preliminary data indicate that although they had lower concentrations of phenolics and condensed tannins, the pools of these compounds supplied to the soil were higher due to higher litter production by Q. rubra.