From Cell to Tree: the Soil

Growth chamber park (right bottom), inside of one growth chamber (left top) and one lysimeter with set-up for soil solution sampling.

The phytostabilisation of metal contaminated soils by afforestation is investigated in a large lysimeter experiment. In this subproject, the chemical properties of the soil and how they affect roots and microorganism are studied.

Collaboration

This is part of the large lysimeter experiment “From Cell to Tree” led by the WSL research unit “Forest Ecosystem Processes” (Madeleine Goerg-Günthard). In this subproject, the WSL research unit “Soil Sciences” collaborates with the ETH research group “Soil Protection” (Rainer Schulin, Bernd Nowack); a large part of the work was performed by the former Ph.D. student David Rais.

Problem

Phytostablisation by afforestation promises to be a sustainable means to remediate metal contaminated soils. However, because the various processes in the root zone including root-soil-microbe interactions and soil organic matter transformations can have different and contrasting effects on metal mobility in the soil, there is insufficient knowledge on the success of the phytostabilisation and about the metal effects on tree roots and microorganisms.

Research questions

• How does the chemical form and mobility of heavy metals (Cu, Zn, Cd, Pb) added to the topsoil change with time (seasonal variations, changes over the entire experiment)?
• How are plant roots and microorganisms affected by metal contamination in the topsoil and does this change with time?

Results 1: Long-term changes in the soil

(Nowack et al., 2004; Voegelin et al., 2005; Nowack et al., 2006; Luster et al., 2008)

• Sequential extractions, EXAFS spectroscopy and DGT speciation revealed that in the contaminated topsoils, rapid transformations of Zn phases occurred for half a year while for Cu stable conditions were reached only after 1.5 years.
• Monitoring of the chemical composition of the drainage water revealed two distinct soil conditioning phases that could be attributed to the conditioning of the soil refilled into the lysimeters. During an initial phase of about a year enhanced mineralisation strongly affected soil solution properties like nutrient and dissolved organic matter (DOM) concentrations. These effects were significantly larger in lysimeters with calcareous subsoil than in those with acidic subsoil. The second phase was characterised by smaller gradual changes, mainly in the acidic subsoil. These chemical changes had only little effects on the concentrations of copper and zinc in the drainage water.

In conclusion, both conditioning of the refilled soil material and transformations of the added metal phases occurred mainly during the first year of the experiment and, thus, did not affect the results obtained from the second to the fourth year.

Results 2: Soil solution changes with time and depth

(Rais, 2005; Nowack et al., 2006; Rais et al., 2006)

• While metal concentrations in the contaminated topsoil solution were larger than in the control, there was only little transport of added metals into the subsoils. There was some detectable Zn translocation into the acidic subsoil, mainly in winter and spring.
• Seasonal variations of the heavy metal concentrations in the topsoil were small with copper correlating mainly with DOM, while Zn and Cd were negatively correlated with phosphate.
• Comparison with plant-free columns indicated that the plants influenced metal concentrations and speciation via nutrient uptake (e.g. phosphate) and direct or indirect effects on DOM composition.
• The metal effects on root density and microbial activity in the contaminated topsoils affected the soil solution concentrations of DOM (reduced) and nutrients like phosphate (reduced) and nitrate (increased) when compared to the control.

In conclusion, although the added heavy metals did not leach out of the lysimeters, and thus were successfully kept within the plant-soil system, a major “positive” plant effect could not be establshed.

In addition, a detailed study of the methodology to sample soil solution demonstrated that suction cups based on nylon membranes exhibit minimal sorption of trace metal cations and DOM in mineral soils and that the presence of DOM further reduced the sorption of trace metals.

Results 3: Heavy metal effects on roots and microorganisms

(Menon et al., 2005; Frey et al., 2006; Brunner et al., 2008):

• Metal pollution reduced the root density strongly in the topsoil but not in the subsoil. Under sufficient availability of water, the roots extracted water mainly from the topsoil. Under water stress conditions, root water uptake was shifted into the subsoil, especially with calcareous subsoil which was much more fertile than the acidic subsoil.
• Fine roots of spruce and poplar accumulated large amounts of metals from the contaminated topsoil within the first vegetation period, mainly in the epidermal cell walls; however, their morphological properties were only little affected by the metal pollution.
• The exposure to the added heavy metals in the contaminated topsoils strongly reduced soil microbial activity and led to long-term changes in the microbial community structure.

In conclusion, the metal contamination affected root growth and microorganisms during the entire experiment. This indicates that planting the soil did not lead to an effective reduction of metal availabilty within four years.

Publications

• Brunner, I.; Luster, J.; Günthardt-Goerg, M.S.; Frey, B. 2008. Heavy metal accumulation and phytostabilisation potential of tree fine roots in a contaminated soil. Env. Poll. 152: 559-568
  
• Luster, J.; Menon, M.; Hermle, S.; Schulin, R.; Goerg-Günthardt, M.S.; Nowack, B. 2008. Initial changes in refilled lysimeters built with metal polluted topsoil and acidic or calcareous subsoils as indicated by changes in drainage water composition. Water, Air Soil Poll. Focus 8: 163-176
  
• Frey, B.; Stemmer, M.; Widmer, F.; Luster, J; Sperisen, C. 2006. Microbial activity and community structure of a soil after heavy metal contamination in a model forest ecosystem.. - Soil Biol. Biochem: 38: 1745-1756.
• Nowack, B.; Rais, D.; Frey, B.; Menon, M.; Schulin, R.; Günthardt-Goerg, M.S.; Luster, J. 2006. Influence of metal contamination on soil parameters in a lysimeter experiment designed to evaluate phytostabilization by  afforestation. Forest, Snow, and Landscape Research 80: 201-211.
• Menon, M.; Hermle, S.; Abbaspour, K.C.; Günthard-Goerg, M.S.; Oswald, S.E.; Schulin, R. 2005. Water regime of metal-contaminated soil under juvenile forest vegetation. Plant Soil 271: 227-241.
• Rais, D. 2005. Soil solution chemistry in a heavy metal contaminated forest model ecosystem. ETH Zurich, Diss. ETH Nr. 16091.
• Voegelin, A.; Pfister, S.; Scheinost, A.C.; Marcus, M.A.; Kretzschmar, R. 2005. Changes in zinc speciation in field soil after contamination with zinc oxide. Environ. Sci. Technol. 39: 6616-6623.
• Nowack, B.; Köhler, S.; Schulin, R. 2004. Use of diffusive gradients in thin films (DGT) in undisturbed field soils. Environ. Sci. Technol. 38: 1133-1138.