Investigating the link between riparian weed invasion, riparian soil geochemistry and catchment urbanisation

This study examined the effects of increased catchment imperviousness (as a surrogate for the intensity of urbanisation) on riparian vegetation and riparian soil geochemistry in the Georges River catchment in south-west Sydney. Soil sampling was undertaken in the riparian zone of 10 freshwater streams in non-urban and urban catchments in March to June 2013. This study found increased catchment imperviousness was associated with increased weed cover and elevated soil minerals and nutrients. Mean soil salinity increased 11 fold in the highly urbanised catchments. Urban soils had an elevated mean pH and elevated mean phosphorus. A major finding of this study was riparian soil calcium concentrations in highly urbanised catchments were over 2000 times greater than those in non-urban catchments. Changes in these factors are likely to have implications for weed invasion. The most important management message from this research and review of similar studies is that future urbanisation, particularly in catchments that are currently undisturbed and naturally vegetated, is likely to contribute to the contamination of riparian soils which in turn may provide ideal conditions for the colonisation of invasive plant species and therefore degrade riparian vegetation communities. We speculate the primary source of calcium in urban riparian soils derives from concrete materials used in urban stormwater infrastructure (e.g. pipes and gutters).