Trace Metals and Metalloids in Forest Soils and Exotic Earthworms in Northern New England, USA

Abstract

Trace metals and metalloids (TMM) in forest soils and invasive earthworms were studied at 9 uncontaminated sites in northern New England, USA. Essential (Cu, Mo, Ni, Zn, Se) and toxic (As, Cd, Pb, Hg and U) TMM concentrations (mg kg^-1) and pools (mg m^-2) were quantified for organic horizons (forest floor), mineral soils and earthworm tissues. Essential TMM tissue concentrations were greatest for mineral soil-feeding earthworm Octolasion cyaneum. Toxic TMM tissue concentrations were highest for organic horizon-feeding earthworms Dendobaena octaedra, Aporrectodea rosea and Amynthas agrestis. Most earthworm species had attained tissue concentrations of Pb, Hg and Se potentially hazardous to predators. Bioaccumulation factors were Cd > Se > Hg > Zn > Pb > U > 1.0 > Cu > As > Mo > Ni. Only Cd, Se Hg and Zn were considered strongly bioaccumulated by earthworms because their average bioaccumulation factors were significantly greater than 1.0. Differences in bioaccumulation did not appear to be caused by soil concentrations as earthworm TMM tissue concentrations were poorly correlated with TMM soil concentrations. Instead, TMM bioaccumulation appears to be species and site dependent. The invasive Amynthas agrestis had the greatest tissue TMM pools, due to its large body mass and high abundance at our stands. We observed that TMM tissue pools in earthworms were comparable or exceeded organic horizon TMM pools; earthworm tissue pools of Cd were up 12 times greater than in the organic horizon. Thus, exotic earthworms may represent an unaccounted portion and flux of TMM in forests of the northeastern US. Our results highlight the importance of earthworms in TMM cycling in northern forests and warrant more research into their impact across the region.

Keywords: bioaccumulation; heavy metals; invasive earthworms; lead; mercury; selenium; trace elements; unpolluted soil.

Figure 1

Sampling site locations.

Figure 2

(A) Earthworm density per square meter at stands when observed. (B) Earthworm dry weight per individual. (C) Earthworm dry weight biomass per m² when observed.

Figure 3

A logarithmic comparison of mean bioaccumulation factors for each earthworm species ± 1 S.E. Bioaccumulation factors were calculated as earthworm concentration (mg kg^-1) divided by soil concentration (mg kg^-1). Epigeic and epi-endogeic earthworms have gray symbols, endogeic earthworms have white symbols and anecic earthworms have black symbols.

Figure 4

A logarithmic comparison of mean organic horizon TMMs pools (mass per unit area) at stands with and without earthworms to epigeic+epi-endogeic TMM tissue pools when observed. Average values are given in μg m^-2 ± 1 S.E. The non-parametric Kruskal-Wallace was used to compare the three pools for each TMM. Letters (a, b, c) are used to group the three pools for each metal (P < 0.05) and (n.s.) denotes no significant difference.