Among various toxic substances that contaminate the soil, the effects of heavy metals are particularly problematic on different aspects of soil-plant system. Accumulation of heavy metals in the topsoil can adversely affect establishment of seedlings and forest productivity. A number of possible reasons can be implicated, such as: altering catalytic functions of enzymes, damaging cellular membranes, inhibition of root and shoot development, reduction of net carbon dioxide assimilation, and decreasing stomatal conductance and transpiration, which in turn, can affect soil water regime. Knowledge of water regime is the first step to understand other soil-plant relations such as uptake of nutrients and toxic contaminants.
The issue of heavy metal toxicity in a forest ecosystem was being addressed comprehensively by the Swiss Federal Institute for Forest Snow and Landscape Research (WSL) in a project titled “from cell to tree”. The first objective of this work was to experimentally investigate and model the water regime of a heavy metal contaminated soil under young forest ecosystems. The second objective was to visualize the root growth in polluted soil using the non-invasive neutron radiography.
The water regime of young forest vegetation consisting of Norway spruce (Picea abies), willow (Salix viminalis), poplar (Populus tremula) and birch (Betula pendula) was monitored from 2001 to 2003 at WSL in 32 lysimeters. The treatments were applied in a latin square factorial design (contaminated vs. uncontaminated topsoil, acidified rain vs. ambient rain) to 16 open-top chambers, with 4 replicates per factorial combination. Each of the open-top chamber contained two lysimeters, one with a calcareous, and the other with acidic subsoil.
In heavy metal contaminated topsoil, water extraction, evapotranspiration and root growth were significantly reduced. Acid rain did not affect water regime of the young forest ecosystem. The four tree species responded differently with respect to leaf area and biomass to heavy metal pollution and different subsoil types. In the last growing season, leaf area was significantly reduced by heavy metal pollution and acidic subsoil, except for P. tremula. Heavy metal contamination significantly reduced above ground biomass of B. pendula and S. viminalis, however, no such effect was found on P. tremula and P. abies. Compared to the other species, P. tremula also produced highest biomass in acidic subsoil. Heavy metal application did not significantly change the overall water use efficiency of the forest ecosystems. The model MACRO was used to calibrate the water regime of one control and heavy metal contaminated lysimeter.
The root growth imaging studies proved that neutron radiography could be effectively used to study dynamic root growth as well as root responses to heavy metal contamination. Root growth was significantly reduced in boron and zinc contaminated zones. Preliminary experiments with different materials showed that, even though visibility of roots was better with quartz sand than soil, root growth was found to be poor in quartz sand possibly due to higher bulk density.