Element mobility and partitioning along a soil acidity gradient in central Ontario forests, Canada

Abstract

The potential environmental risk posed by metals in forest soils is typically evaluated by modeling metal mobility using soil-solution partitioning coefficients (K(d)), although such information is generally restricted to a few well-studied metals. Soil-solution partitioning coefficients were determined for 17 mineral elements (Al, As, Be, Ba, Ca, Cr, Cu, Fe, Ga, K, Li, Mg, Rb, Sr, Tl, U and V) in A-horizon (0-5 cm) soil at 46 forested sites that border the Precambrian Shield in central Ontario, where soil pH(aq) varied from 3.9 to 8.1. Sites were dominated by mature sugar maple (Acer saccharum Marsh.), white birch (Betula papyrifera Marsh.), balsam fir (Abies balsamea (L.) Mill.) or white pine (Pinus strobus L.). Log K(d) values for all elements could be predicted by empirical linear regression with soil pH (r(2) = 0.17-0.77) independent of forest type, although this relationship was greatly affected by positive relationships between acid-extractable metal concentration and pH(aq) for 13 of the 17 elements. Elements that exhibited strong or moderate (r(2) > 0.29; p < 0.001) relationships with soil pH(aq) in soil water extracts include Al, Ba, Fe, Ga, K, Li, Rb, Tl, V (negative) and Ca (positive). Elemental partitioning in mineral soil was independent of forest type; tree species differed in their response to chemical differences in mineral soil. For example, Rb, Ba, and Sr concentrations in foliage of sugar maple and white birch significantly increased with increasing soil acidity, whereas Rb, Ba, and Sr concentrations in balsam fir and white pine foliage exhibited no response to soil pH(aq). While K(d) values can provide useful information on the potential mobility and bioavailability of mineral elements in forest soils, care must be used when interpreting the relative contribution of solid and aqueous phases to this relationship and the differing responses of vegetation in elemental cycling in forests must also be considered.

Assessment of the potential role of metals in sugar maple (Acer saccharum Marsh) decline in Ontario, Canada

“…These differences appear to be brought about by differences Shaw and Vet (2007) in biogeochemical cycling rather than by differences in trace metal deposition as mineral soil concentrations in the region and a subset of the sites are low and are considered to be background concentrations (Watmough 2008). Furthermore, with the exception of Pb, trace metal deposition has historically been low in southern Ontario (Jeffries and Snyder 1981).…”

“…Differences in metal concentrations in foliage and forest floor among sites are most probably brought about by differences in metal biogeochemistry. Soilsolution partition coefficients (K d ) vary tremendously in Ontario and can be largely explained by differences in soil pH (Watmough 2008). Increased metal mobility in acid soils often results in higher foliar metal concentrations, but this depends upon both the metal and tree species (Watmough 2008).…”

“…Soilsolution partition coefficients (K d ) vary tremendously in Ontario and can be largely explained by differences in soil pH (Watmough 2008). Increased metal mobility in acid soils often results in higher foliar metal concentrations, but this depends upon both the metal and tree species (Watmough 2008). In the present study concentrations of Cd, Mn, Rb, Sr and Zn in sugar maple foliage were all negatively related to soil pH reflecting the increased bioavailability of these metals to sugar maple under acidic conditions.…”

“…Trace metals have been proposed as a contributing factor to forest decline in Europe (Godbold and Huttermann 1985) and Gawel et al (1996) suggested that metals were a factor in red spruce (Picea rubens) decline at high elevation sites in the northeastern US. In general acid-extractable and water-extractable metal levels in mineral soil are low and are considered background levels in the forests of southern Ontario, with concentrations in mineral soil being affected more by soil properties than by trace metal deposition (Watmough 2008). However, foliar metal concentrations are greatly elevated under acidic soil conditions (Watmough 2008), demonstrating enhanced metal cycling occurs under acidic soil conditions and that is not due to elevated concentrations in mineral soil (Watmough 2008).…”

“…In general acid-extractable and water-extractable metal levels in mineral soil are low and are considered background levels in the forests of southern Ontario, with concentrations in mineral soil being affected more by soil properties than by trace metal deposition (Watmough 2008). However, foliar metal concentrations are greatly elevated under acidic soil conditions (Watmough 2008), demonstrating enhanced metal cycling occurs under acidic soil conditions and that is not due to elevated concentrations in mineral soil (Watmough 2008). The impact of enhanced trace metal cycling on sugar maple decline has not been previously assessed.…”