We have examined impacts of alien invasive plants on soil chemical properties, primary productivity and nutrient cycling in the plant / soil system. Specifically, we tested if impacts follow a general pattern across sites and species or, alternatively, if they are entirely idiosyncratic. The study first focused on 36 sites in Belgium invaded by one of the 7 most invasive plant species in NW Europe (Solidago gigantea, Fallopia japonica, Senecio inaequidens, Heracleum mantegazzianum, Impatiens glandulifera, Prunus serotina and Rosa rugosa). We compared invaded to adjacent uninvaded plots for selected parameters. Primary productivity and nutrient uptake were always higher in invaded stands compared to uninvaded plots. Magnitude and direction of impacts on soil chemical properties strongly varied depending on site. However, impacts followed a general pattern, being predictable from soil chemical properties prior to invasion. Thus, in sites with low soil nutrient contents, invasion tended to increase available nutrient pools in the topsoil while the opposite trend was observed in soils initially rich in nutrients. This suggests that exotic plant invasion could lead to the homogenization of soil nutrient concentrations across invaded landscapes.
Later on, we selected two species (Solidago gigantea and Fallopia japonica) to study in details the mechanisms of the impacts on soil properties.
In soil invaded by S. gigantea, soil phosphorus availability was increased. Higher turnover rates of phosphorus in belowground organs and mobilization of soil sparingly soluble P forms through rhizosphere acidification may be involved in the observed differences in soil P status between invaded and uninvaded plots.
In grassland invaded by Fallopia japonica, the carbon and nitrogen cycling were deeply modified. Due to its higher lignin/N ratio compared to resident vegetation, Fallopia litter decomposed much more slowly and immobilized a large amount of inorganic N, reducing the availability of this element in soil. On the other hand, the internal cycling of N in Fallopia was found exceptionally efficient. Indeed, about 80 % of the N present in aboveground biomass in summer is translocated to the rhizomes before leaves abscission. This process makes the plant relatively independent from soil N mineralization and possibly contributes to the high productivity and invasive success of the species. In addition, F. japonica also impacted soil fauna communities. The density of invertebrates under the canopy of F. japonica was reduced and the composition of the community shifted from a typical grassland community to typical forest groups. These changes may be explained by a reduction of food diversity, a change in soil microclimate and in organic matter quality.
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