Mercury, Pb, Cu and Zn concentrations in forest soils along three chronosequences in the northeastern US

Justin Richardson1, Andrew Friedland2, Chelsea Vario3
1 Department of Earth Sciences, Dartmouth College
2 Environmental Studies Program
3 Department of Ecology and Evolutionary Biology

O 8.4 in Trace element and metal biogeochemistry

14.07.2014, 12:15-12:35, H20

Understanding how the cycling of trace metals in forests responds to clear-cutting on short-term (< 50 yrs) and long-term (> 50 yrs) timescales is important for soil fertility and considering impacts on downslope aquatic ecosystems. Mercury, Pb, Cu and Zn have been widely dispersed across the region but have very different biogeochemical cycles. In 2011 through 2013, we sampled 5 soil depths along three clear-cut chronosequences at three US study areas: Adirondack Ecological Center (AEC) NY, Bartlett Experimental Forest (BEF) NH, and Harvard Forest (HF) MA. In each chronosequence, 4 to 5 forest stands with known ages and similar site characteristics were sampled.

Soil properties (% Clay, % SOM and soil pH) were similar among forest stands for each soil depth at each study area but varied among study areas. Forest floor Hg, Pb, Cu and Zn concentrations ranged from 0.11 – 0.42, 23 – 75, 32 – 166, and 1.3 – 10 mg kg-1, respectively. Mineral soil Hg, Pb, Cu and Zn concentrations ranged from 0.31 – 0.03, 3 – 66, 16 – 137, and 3 – 36 mg kg-1, respectively. Trace metal concentrations varied significantly among study areas, likely due to different parent materials and deposition rates.

Stands were grouped as Cut Stands (harvested < 50 yrs ago) and older stands (harvested > 50 yrs ago). Averaging across all study areas and soil depths had 32% higher concentrations of Cu but 16% lower Hg concentrations. Zinc concentrations were not consistently different between Cut and Older stands, but Pb concentrations from 0 – 10 cm were 45% greater at Cut stands. Two-Way ANOVA and MANOVA were used to examine differences in trace metal concentrations between Cut and Older stands potentially masked by differences among study areas. The results confirmed that Hg, Pb and Cu were different between Cut and Older stands while Zn was not.

The change in Hg, Pb, Cu and Zn concentration was linearly regressed  with stand age for all study areas grouped together. Overall, there were no significant trends in trace metal concentrations due to the wide variation among study areas. When regressed for each study area separately, Cu and Zn concentrations exhibited a negative trend (-4 and -3% yr-1) while Hg had a positive trend (+4% yr-1) for all soil depths. However, changes in trace metal concentrations through time may not be linear and would be better explained with a higher order polynomial regression.

We conclude that the change in Hg, Pb, Cu and Zn concentrations are dependent on the metal of interest and the study area. The results suggest that in general Hg has been lost after clear-cutting, Pb has been redistributed in the soil profile, and Cu and Zn have increased after clear-cutting. Additional research is required to determine the underlying mechanisms for Hg loss and Cu and Zn inputs.

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last modified 2014-04-01