Forest floor leachate fluxes under six tree species on a metal contaminated site Lotte Van Nevel, Jan Mertens & Kris Verheyen Ghent University Forest & Nature Lab EUROSOIL 2012, Bari 3 July

Campine region, NE Belgium Zn and Pb refineries during ± 100 years 1950: emission 340 kg Cd / day !! 1992: emission 0,04 kg Cd / day Metal pollution problem Cd contaminated soils in Flanders (OVAM, 2007)

Campine region, NE Belgium Zn and Pb refineries during ± 100 years 1950: emission 340 kg Cd / day !! 1992: emission 0,04 kg Cd / day severe historical pollution problem, despite reductions of metal emissions in the last decades => diffuse pollution over 700 km² (Be + Nl); mainly Cd, Zn, Pb poor sandy soils: low buffering capacity => risks of metal leaching and spreading Phytoremediation Metal pollution problem

Definition (Garbisu & Alkorta, 2001): Phytoremediation is a technique that involves the use of plants for the removal of pollutants from the environment or to render them harmless extraction versus stabilization Phytoremediation

Definition (Garbisu & Alkorta, 2001): Phytoremediation is a technique that involves the use of plants for the removal of pollutants from the environment or to render them harmless extraction versus stabilization Phytoremediation ET uptake input via litterfall accumulation litter decomposition leaching

Tree species effects on mobilization of Cd and Zn  possibilities and limitations for phytostabilization  how to minimize the risk of metal dispersion in the ecosystem?  above- and belowground metal dispersion In situ research 6 tree species: oak (Quercus spp.), aspen (Populus tremula), silver birch (Betula pendula), black locust (Robinia pseudoacacia), Scots pine (Pinus sylvestris), Douglas fir (Pseudotsuga menziesii) Research objectives

Study site 203 ha

Study site

Leaf litter  litterfall traps  Cd, Zn, base cations, C, N Methods

Leaf litter  litterfall traps  Cd, Zn, base cations, C, N Forest floor leachates  zero-tension lysimeters  Cd, Zn, base cations, DOC, pH Methods

Leaf litter: Cd & Zn normal Cd range in leaves = up to 2.4 ppm (Alloway, 1995) toxic Cd range in leaves = ppm (Kabata-Pendias & Pendias, 1992) Results

Leaf litter: Cd & Zn normal – toxic Zn range in leaves: ± 400 ppm (Alloway, 1995) Results

Leaf litter: base cations and C/N ratio  litter quality: low versus rich Results a a a a a b a a a a b b

Forest floor leachates  Metal accumulating species => higher metal fluxes in leachate?  Low litter quality => DOC and H + leachate fluxes ? => base cation leachate fluxes ? soil acidification metal mobility risk for groundwater pollution Results

Forest floor leachates: fluxes of DOC, H +, base cations Results

Forest floor leachates: fluxes of DOC, H +, base cations Scots pine & oak:  high DOC & H + fluxes  low base cation fluxes aspen:  extremely low H + fluxes (130 x less than oak)  low DOC fluxes  high base cation fluxes Results metal mobilization

Forest floor leachates: Cd & Zn fluxes Results

Forest floor leachates: Cd & Zn fluxes Total Cd fluxes slightly higher under aspen No tree species effect on Zn fluxes  differences between aspen and other species forest floor leachate << leaf litterfall  high abundance of earthworms under aspen ? Dissolved Cd fluxes  differences between aspen and other species almost disappeared  extremely low H + fluxes (high pH) in aspen leachate Results

Aspen accumulates Cd & Zn in leaves  aboveground dispersion risks Cd & Zn fluxes in aspen leachate lower than expected  belowground dispersion risks lower than expected? DOC, H +, base cation fluxes in leachates: significant tree species effects  Scots pine & oak: metal mobilizing  aspen: metal immobilizing  other species: intermediate Implications for phytostabilization? Further research is essential Conclusions

Thank you for your attention! Questions?