Willow cuttings in this study demonstrated increased growth in treatments of urban water and pristine water exposed to concrete, compared to willows grown in unmodified pristine water. Overall, from all measured growth parameters, willows grown in RCA recirculated water showed the greatest growth, followed by plants grown in urban BMUS water, then those grown in whole concrete pipe recirculated water, and, finally, willows grown in pristine BMUS water. Results suggest that exposure of water to concrete materials contribute to favourable water geochemistry for the growth of the willow, Salix spp., which is regarded as an Australian weed of national significance. Salix spp. is also identified as an invasive weed of concern for the conservation of endangered BMUS ecological communities (NSW Scientific Committee 2007).
These findings are supported by an earlier pilot study, which recorded increased growth of willow cuttings in reference BMUS water circulated through a concrete pipe for two hours, when compared with cuttings in reference water that had not been exposed to concrete (Purdy and Wright 2019). To the best of our knowledge, there are no other laboratory studies investigating how water exposed to concrete may affect the growth of an invasive weed species and, thus, the results of this study and our previous study (Purdy and Wright 2019) appear to be unique. These results add to the growing knowledge of how widespread urban materials, such as concrete, can affect the natural environment.
Urban riparian zones and urban wetlands are frequently dominated by vigorous growth of weed species, often to the detriment of native species (Ehrenfeld and Schneider 1991; Grella, Renshaw and Wright 2018; Leishman, Hughes and Gore 2004). Previous research widely attributes the dominance of invasive plant species in urban riparian zones to elevated nitrogen and phosphorous levels in water and riparian soil (Paul and Meyer 2001). However, in the case of BMUS, weed invasions are prevalent in wetlands of urban catchments that do not have increased nitrogen and phosphorous concentrations in the water (Carroll, Wright and Reynolds 2018). The results of this study highlight other anthropogenic factors, such as higher pH and increased concentration of metals and ions, that contribute to favourable growth conditions for urban invasive weeds.
The other key finding in this study is that water exposed to concrete can increase the growth of invasive weeds. This adds further support to the theory that urban concrete materials in the built environment can be a potential source of modified geochemistry in urban waters and soils and therefore can contribute to the chemical and the ecological degradation of urban streams and wetlands (Belmer, Tippler and Wright 2018; Davies et al. 2010; Grella, Renshaw and Wright 2018; Kaushal et al. 2013; Tippler, Wright and Hanlon 2012; Wright et al. 2011).
Water in the four treatments used to grow willow cuttings in this study had varying pH levels, from a mean of 4.89 in pristine reference BMUS water to 8.91 in the RCA treated water. Urban BMUS water had a pH of 7.53, while water exposed to a whole concrete pipe had a pH of 7.42. This was in accordance with previous studies of pristine non-urban BMUS water that often record pH <5, with urban BMUS generally reporting pH >6 (Belmer, Wright and Tippler 2015; Carroll, Reynolds and Wright 2020). It is well known that pH plays an important role influencing plant growth through the bioavailability and uptake of nutrients and toxicity of metals (Alam, Naqvi and Ansari 1999; Charman and Murphy 2007; Murrmann and Peech 1969; Wang et al. 2006). Previous research has reported that pH differences can be highly influential to plant growth, including weed growth (Buchanan, Hoveland and Harris 1975). For example, an experimental study using tomato seedlings reported that when soil pH of was at both 4 and 8, plant growth was diminished, but at a pH of 6, plant growth was optimal (Kang et al. 2011). Riparian soils have also recorded substantial pH differences in urban versus non-urban catchments (Grella, Renshaw and Wright 2018). Grella, Renshaw and Wright (2018) reported that the mean riparian soil pH was 5.0 in catchments with low coverage of urban development and was 6.49 in catchments with highest coverage of urban development. That study also found that riparian soil pH was the single most highly correlated factor associated with variation in riparian vegetation, with higher pH urban soils having greater coverage by invasive weeds (Grella, Renshaw and Wright 2018). It is likely that pH differences in water treatments in this study contributed to the observed growth differences.
In the current study, elevated potassium and calcium concentrations in urban and both concrete water treatments were also likely contributors to the increased growth of willow cuttings observed in these treatments. Calcium and potassium are both regarded as essential plant nutrients and are integral for numerous functions essential for plant growth (Burstrom 1968; Groffman and Fisk 2011; Schachtman and Schroeder 1994). Potassium is very important for plant growth as it is generally the most abundant cation in plants (Schachtman and Schroeder 1994). Two previous investigations of soils and weeds in sandstone-derived soils in the Sydney area (Grella, Renshaw and Wright 2018; King and Buckney 2000) reported increased weed growth in urban soils with higher calcium and potassium content, compared to naturally forested riparian soils. Another study (Carroll, Reynolds and Wright 2020) tested the major ion contents of plant foliage growing in urban and non-urban BMUS. Potassium accounted for a mean of 3129 mg/kg of dried plant tissue in non-urban BMUS foliage, representing 33% of combined major cations content. The same study found that plant tissue from urban BMUS foliage had substantially (5.5 times more) greater potassium content (mean 17740 mg/kg) than non-urban plants, which accounted for 61% of the combined major cations content.
This study adds to the growing recognition that urbanisation can influence the environmental geochemistry and ecology of urban water and soil (Chambers et al. 2016; Kaushal et al. 2013). Many researchers have recognised the weathering of urban concrete material, such as concrete paths, drainage pipes and gutters, as a potential source of elevated pH and ions; for example, calcium and potassium (Borris et al. 2017; Davies et al. 2010; Kaushal et al. 2005; Kaushal et al. 2013; Moore et al. 2017; Wright et al. 2011). Such anthropogenic modified geology has been termed as ‘urban karst’ (Chambers et al. 2016; Kaushal et al. 2013). The current study adds further support to the hypothesis that a substantial proportion of the elevated pH, calcium and potassium detected in urban BMUS and other urban soils and waterways across the Sydney and Blue Mountains is due to mineral leaching of concrete materials (Davies et al. 2010; Grella, Renshaw and Wright 2018; Tippler, Wright and Hanlon 2012). The exposure of concrete materials (concrete pipe, RCA fragments) to non-urban BMUS water, used as two of the treatments to grow willow cuttings in this study, resulted in higher pH and elevated concentrations of calcium and potassium, both of which are comparable to research reported for urban BMUS (Belmer, Wright and Tippler 2015; Carroll, Reynolds and Wright 2020; Purdy, Reynolds and Wright 2020).
The strontium and barium contents in tissue samples from willow cuttings grown in the three urban/concrete water treatments (urban, concrete pipe and 20 mm RCA) were more elevated than those grown in the reference treatment. This was similar to a previous pilot study for the current research (Purdy and Wright 2019). The previous study also grew willow cuttings for 7 weeks in two water treatments. One was water from a non-urban BMUS and the other treatment was a sample of water from the same BMUS that was recirculated through an unused concrete pipe. The major difference between this study and the previous study was that the water in the previous study was recirculated for 120 minutes, double the 60 minutes used in the current study. The previous study also recorded a large increase in both barium and strontium in willow tissue grown in the concrete pipe treatment. Another study of water quality in urban and non-urban BMUS also measured barium and strontium at much higher concentration (barium 4.5 times and strontium 30 times) in urban compared to non-urban locations (Carroll, Reynolds and Wright 2020). The water quality and bioaccumulation results suggest that barium and strontium may be a useful as a marker of concrete exposure, based on the results of this study and previous research (Carroll, Reynolds and Wright 2020; Purdy, Reynolds and Wright 2020; Purdy and Wright 2019).
Urban concrete materials are likely to contribute to a substantial proportion of strontium and barium in urban waters. Strontium particularly is often one of the most abundant trace elements in many concrete materials (Graham, Goguel and St John 2000; Long, Zhang and Luo 2019) depending on the source and formulation of ingredients used. Strontium is likely to be in many Australian concrete products due to the widespread use of coal-derived fly-ash in Australian concrete materials. It has been estimated that Australian concretes commonly contain up to 5% fly-ash (Ash Development Association of Australia (ADDA) 2009). Fly-ash itself often has a substantial strontium content, with United States fly ash reported to contain an average of 1334 ppm of strontium (Straughan et al. 1981). Trace metal analysis of Australian fly-ash revealed that strontium was the second most abundant metal (360 ppm), second to barium (490 ppm) (Ward et al. 2009). Our suggestion that strontium may be a marker of urban and concrete exposure is not completely novel. It has previously been suggested that strontium can be used as a soil, water, and plant marker of coal combustion residue, particularly from fly ash (Hurst, Davis and Elseewi 1991; Straughan et al. 1981). Several studies have revealed that plants are able to bioaccumulate strontium into leaf and root tissue, sometimes damaging the plant (Burger and Lichtscheidl 2019). Straughan et al. (1981) has also reported that strontium can be readily bioavailable, as demonstrated when plants grown in soil containing an 8% fly ash mixture accumulated strontium in leaf tissue at up to 300 ppm. That was about 2.5 times greater than the maximum strontium content of 81 ppm recorded in the current study, in the willow leaf tissue grown in the RCA water treatment.
The current research builds on previous studies and highlights the likely contribution of concrete urban materials to the geochemistry of urban waterways (Davies et al. 2010; Kaushal et al. 2013; Cook et al. 2020). Weed invasion is one of the major environmental degradation issues in urban waterways (Paul and Meyer 2001) and in BMUS wetlands within urbanised catchment areas (Benson and Baird 2012; Fryirs, Freidman and Kohlhagen 2012). About one-third of the BMUS wetlands are exposed to urban development, which reduces their integrity through erosion, sedimentation, modified hydrology and weed invasion (NSW Scientific Committee 2007). In the current study, the contribution of concrete materials to increased growth of Salix spp. was probably enhanced by the naturally acidic and soft water of BMUS, which has underlying geology containing negligible calcium and potassium (Belmer, Wright and Tippler 2015; Carroll, Reynolds and Wright 2020; Purdy, Reynolds and Wright 2020). It is probable that urban development in landscapes containing karst geology probably has lesser geochemical impacts (water and soil) when contrasting natural versus urban catchments.
We recommend that more research is needed to investigate the contribution that concrete can have on urban water, urban soil and on invasive species in urban riparian environments. It would be beneficial to conduct further testing on how water exposed to concrete affects other invasive weed species and a range of native species, similar to the experimental exposure of phosphorus to native plants by Leishman and Thomson (2005). It would also be valuable to extend the length of plant studies for longer periods and across different plant growth stages. Further research on topics such as this will be vital in many environmental settings, as concrete is a ubiquitous urban material and will continue to be part of the built environment in an increasingly urban world (Cooke et al. 2020; Kaushal et al. 2013).