Soil phosphorus enhancement below stormwater outlets in urban bushland: spatial and temporal changes and the relationship with invasive plants

Abstract

Invasion by exotic plant species is a significant problem in urban bushland remnants and is often associated with nutrient enrichment of soils. A major source of nutrient enrichment in urban areas is stormwater runoff, which is transferred from impervious surfaces in urban catchments and discharged at outlets on the residential/bushland interface. We measured the spatial extent of soil total phosphorus (P) enhancement below stormwater outlets on Hawkesbury Sandstone-derived soils in northern Sydney and examined whether total P concentration has increased with time since urban development and extended laterally beyond the stormwater flow path. The average area of soil P enhancement below outlets was 0.24 ± 0.05 ha and was widest 30–50 m downslope from the outlet, where it extended an average 40 m across slope. Catchment area was not significantly related to average soil total P concentration. There was a significant decline in total P across slope from the centre of the flow path and a significant positive relationship between soil total P and proportion of exotic plant cover, with soil P accounting for 77.5% of variation. We found evidence for a build-up in soil total P concentration over time within the run-on zone below outlets, with the rate of enhancement being ~68 mg/kg per decade over a 40-year period. Evidence for lateral transfer of soil P out of the run-on area was more equivocal. There was a significant decline in soil total P across slope from the boundary of the run-on zone, with higher concentrations at distances 0.5 m and 1 m from the boundary compared with >1.5 m. However, this could be due to error in locating the boundary between run-on and non run-on areas. There was no significant relationship between soil P in the non run-on zone and age of development, which would be expected if P was being transferred by biological activity beyond the run-on zone over time. It is clear that the primary areas of concern for management must be the run-on areas below outlets.