We studied the effects of invasions by three plant species: Reynoutria japonica, Rudbeckia laciniata, and Solidago gigantea, on arbuscular mycorrhizal fungi (AMF) communities in habitats located within and outside river valleys. Arbuscular mycorrhizal colonization, AMF abundance and species richness in soils were assessed in adjacent plots with invaders and native vegetation. We also quantified the performance (expressed as shoot mass, chlorophyll fluorescence, and the concentration of elements in shoots) of two common, mycorrhizal native plants, Plantago lanceolata and Trifolium repens, grown in these soils. The invasions of R. japonica, R. laciniata, and S. gigantea influenced AMF communities compared to native vegetation, but the changes depended on the mycorrhizal status of invaders. The effects of non-mycorrhizal R. japonica were the most pronounced. Its invasion reduced AMF abundance and species richness. In the plots of both mycorrhizal plants, R. laciniata and S. gigantea, we observed decreased AMF species richness in comparison to native vegetation. The AMF community alterations could be due to (i) depletion of organic C inputs to AMF in the case of R. japonica, (ii) plant secondary metabolites that directly inhibit or selectively stimulate AMF species, or (iii) changes in soil physicochemical properties induced by invasions. The effect of invasion on AMF abundance and species richness did not generally differ between valley and outside-valley habitats. The invasions affected photosynthetic performance and the concentrations of elements in the shoots of P. lanceolata or T. repens. However, the directions and magnitude of their response depended on both species identity and the mycorrhizal status of invaders.

Heavy metals can cause a decrease in the taxonomic diversity of soil communities. Because of functional redundancy, it remains unclear to what extent different functions performed by the soil microbial communities may be affected by pollution. We studied the impact of metal contamination on soil bacterial and fungal functional diversity, active microbial biomass, and soil respiration rate. Soil samples were collected from 39 sites along three forest and five meadow pollution transects near an abandoned Pb/Zn smelter in Avonmouth (UK) and Ni smelter in Clydach (UK), in a Cu mining and smelting region near Glogów (Poland), and in a Zn/Pb mining and smelting region near Olkusz (Poland). Biolog GN2 and SFN2 plates were used to study the bacterial and fungal functional diversity, which subsequently was expressed as Shannon's diversity index (H'). The active microbial biomass was measured as substrate-induced respiration. We found that the bacterial functional diversity significantly decreased, whereas the fungal functional diversity slightly increased, with increasing metal concentration. We also observed a slight negative effect of metal pollution on the active microbial biomass. No relationship was found between metal contamination and total soil respiration rate. This suggests a higher sensitivity of bacterial functional diversity as an indicator for the effects of metal pollution compared with overall soil respiration. All microbial parameters were affected by nutrient concentrations and/or soil pH.

This study assessed the effects of Reynoutria japonica, Rudbeckia laciniata, and Solidago gigantea invading sites within and outside river valleys on activity, biomass, and composition of soil microbial communities. Microbial properties such as soil respiration, urease and arylsulfatase activities, microbial biomass (based on substrate-induced respiration, or SIR, and phospholipid fatty acids, or PLFA), and community composition (based on PLFA) were determined. R. japonica encroached on sites characterized by the lowest values of microbiological properties and R. laciniata on sites with the highest microbiological quality. The effect of invasion on soil microbial properties depended on the invasive plant species. R. japonica significantly decreased microbial biomass, determined by both SIR and total PLFA, urease activity, fungal PLFA, fungal:bacterial PLFA ratio, gram-negative bacterial PLFA, and soil respiration in comparison to soil under adjacent native plant communities. Microbial community composition also differed between soils under R. japonica and those under native plants. In contrast, R. laciniata and S. gigantea did not influence most microbial properties, though S. gigantea significantly increased fungal PLFA and R. laciniata and S. gigantea increased fungal:bacterial PLFA ratio. The effects of plant invasion on microbial properties were basically similar in soils located within and outside river valleys, probably because initially (i.e., before invasion) soils from the two locations were largely similar in terms of basic properties such as texture, moisture, pH, C:N ratio, and most microbial properties.