- Root Growth Responses to Soil Amendment in an Urban Brownfield
High metal concentrations in soil can act as a strong abiotic filter in plant communities, limiting the establishment, growth, and reproductive capacity of sensitive species (Adriano 1986). However, some species can successfully colonize soils contaminated by metals, often because they have adaptive traits that minimize deleterious physiological effects. For example, metals may be excluded at the root epidermis (Feng et al. 2013) or sequestered in plant tissue as phytochelatins (Steffens 1990). In ecosystems where metals are abundant, plants tolerant of metalliferous soils can form highly functional assemblages (e.g., with nutrient cycling similar to sites with normal metal loads), despite having atypical developmental trajectories (Gallagher et al. 2011). Once established, these assemblages may provide useful ecosystem services, such as preventing contaminants from transferring out of the site.
Increasing tree cover can be a low cost, yet effective, tool in restoring brownfields (Dickinson et al. 2000). However, tree growth is compromised when soil-induced stress exceeds metabolic thresholds; this is common in sites with high metal concentrations (Dickinson et al. 2000, Gallagher et al. 2008). Low water or macronutrient availability can compound the effects of metal contamination, often preventing or restricting the natural regeneration process to the point that active management is not attempted. Hence, if soil amendments are effective in mitigating the synergistic effects of high soil metals and low soil resources on tree growth, they would be an attractive option for promoting vegetative restoration in brownfields and other urban sites.
Measurements of root productivity and morphological traits provide a means of evaluating the effectiveness of amendments to poor quality soils during the same growing season that the application takes place. To determine the potential for long term greening in urban brownfields whose soils contain high concentrations of trace metals, we investigated the effects of simple soil amendments on root production and morphology in a gray birch dominated assemblage.
The study was conducted in a portion of Liberty State Park in New Jersey, USA (40°42ʹ14ʺ N, 74°03ʹ14ʺ W) that was a rail yard for over a century before closing in 1967. A young forest community dominated by gray birch (Betula populifolia), eastern cottonwood (Populus deltoides), and winged sumac (Rhus copallinum) has spontaneously formed over most of the former rail yard. This study took place in a site with some of the highest total soil metal loads in the park (denoted TP-14 in Gallagher et al. 2008). In addition, the soil texture is very gravelly, and macronutrient levels are low (soils are described in greater detail in Appendix 1 and in Gallagher et al. 2008). Past research at the site showed that metal concentrations exceed a threshold beyond which plant productivity, diversity, and seed viability have been significantly reduced (Gallagher et al. 2008, Gallagher et al. 2011).
Eight gray birch trees (mean height = 7.38 m, mean diameter = 6.15 cm) were randomly selected from among those with diameter at breast height within one standard deviation of the mean for the trees at the site (7.98 ± 3.46 cm; n = 617; Dahle et al. 2014), and which had no neighbor within 2 m. In April 2011, soil cores (ϕ = 5.1 cm, depth = 5.4 cm) were taken at 12 random positions 0.5 or 1.0 m from each tree base, sieved to remove roots (0.5 mm mesh), placed in root ingrowth bags (polyethylene, 6 mm mesh), and reburied. Soil amendments were applied on 4 May 2011; they included mulch (0.15 m3 of shredded pine bark), lime (304 g of powdered limestone), and a combination of both mulch and lime (same application rates). Each of the three amendment types was applied individually to two trees; the remaining two trees served as non-amended [End Page 10] controls. Amendments were applied ≤ 1.1 m of tree bases, covering all ingrowth bags.
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