Tree species effects on Cd and Zn mobility after afforestation of contaminated soils in the Campine region (northern Belgium)

This thesis was executed in the context of the historical metal pollution in the Campine (Kempen) region in north-eastern Belgium. Due to metal refining activities in the past, an extended area of about 700 km² is diffusely polluted with mainly cadmium (Cd) and zinc (Zn). The historical soil pollution is still causing human-toxicological and ecotoxicological risks, through metal leaching to groundwater and accumulation in the food chain. Moreover, the Campine region is characterized by poor sandy soils, aggravating the risks for metal dispersion in the environment. Conventional soil sanitation techniques are technically and financially inadequate to tackle the pollution problem, because of its spatial extent and the relatively moderate contamination levels. Hence, alternative remediation strategies, such as phytostabilization, are to be investigated. Sustainable phytostabilization projects require the metals to be stabilized in the soil and should thus aim at minimizing metal dispersion via both above- and below-ground pathways. As there is a clear tree species effect on metal uptake, on litter decomposition, on biogeochemical processes in the soil profile and on the water balance, the distribution and fluxes of metals in the forest-soil system will be tree species specific as well. Selecting appropriate tree species is thus crucial for achieving successful phytostabilization. The main objectives of this thesis were to assess the feasibility of phytostabilization by afforesting diffusely Cd and Zn contaminated sandy soils, and to determine the tree species effects on Cd and Zn cycling on these soils. Our study was carried out in ‘Waaltjesbos’, a young post-agricultural forest on a metal contaminated site on sandy soil. At the moment of our study, the trees were 10-15 years old. The six selected tree species were silver birch (Betula pendula), oak (Quercus robur and Q. petraea), black locust (Robinia pseudoacacia), aspen (Populus tremula), Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii). We showed that aspen translocates high amounts of Cd and Zn into its foliage, generating a significant accumulation of total Cd and Zn concentrations in the topsoil (0-5 cm), and this after only 10 years of tree growth. Moreover, accumulated metals in the leaves may pose a long-term risk to primary consumers and enter the food web. Given these risks of above-ground metal dispersion, aspen should be avoided when afforesting Cd and Zn contaminated sites. We also showed that Cd and Zn leaching at 50 cm depth was elevated under black locust, Scots pine and Douglas fir, compared to lower seepage fluxes under silver birch, oak and aspen. This pattern was significantly correlated with leaching of anions and base cations, and to a lesser extent to soil solution pH, at least at this young stage of forest development at a post-agricultural site. Due to former (agricultural) liming and fertilization processes, the base cation status of the soil and the solution pH were still relatively high. However, we expect both parameters to decrease with forest age, especially for species with slowly decomposing litter, implying that the effect of pH on Cd and Zn leaching will become relatively more important in the long term. In this respect, we recommend not to plant tree species that generate high anion, base cation or H+ fluxes when afforesting Cd and Zn contaminated lands, because these species pose a serious risk for metal leaching to deeper soil layers and potential contamination of the groundwater. In general, coniferous as well as N-fixing species should thus be avoided. Finally, contrary to what is often argued in literature about phytostabilization, we found that Cd and Zn leaching losses at our study site showed no resemblance with the downward soil water fluxes. This might imply that the impact of the biogeochemical processes in the soil (solution) on Cd and Zn leaching (complexation with anions, competition for sorption sites with base cations and protons) was relatively more important than the effect of evapotranspiration. Summarized, taking into account all the aforementioned recommendations, it seemed that silver birch and oak may be planted on Cd and Zn contaminated sandy soils. Aspen, black locust, Scots pine and Douglas fir, on the other hand, cause risks for above-ground or below-ground metal dispersion and should therefore be avoided. These conclusions were drawn from our study in a young forest (10 – 15 years). Consequently, it is unsure whether they can be extended to the long term. Further research in older forests is thus essential

Impact of mechanized harvesting on compaction of sandy and clayey forest soils : results of a meta-analysis

Nowadays, harvest operations are predominantly performed fully mechanized using heavy tractors or forestry machines. The resulting soil compaction may negatively affect the soil ecosystem. We wanted to draw general conclusions concerning the impact of mechanized harvesting on forest soil bulk density and the influencing factors. Therefore, we combined the data of several studies using a meta-analysis approach. The impact decreased from the surface towards deeper soil layers. At 0-10 cm depth, the impact on clayey soils was highest although not significantly different from the impact on sandy soils. Higher initial bulk densities, i.e., on already compacted forest soils, generally led to smaller extra increases of bulk density after machine traffic. For sandy soils, the impact was also significantly smaller when machines were lighter. No significant relationship was observed between the compaction degree and traffic intensity. We observed clear compaction on both clayey and sandy soils, especially in case of low initial soil compaction degrees and heavy machines. The compacted initial state of many forest soils, the long recovery period, and the generally high impact of the first passes that is frequently mentioned in literature all count in favour of designated skid trails and an adjustment of the machine type to the job

Soil inorganic N leaching in edges of different forest types subject to high N deposition loads

We report on soil leaching of dissolved inorganic nitrogen (DIN) along transects across exposed edges of four coniferous and four deciduous forest stands. In a 64-m edge zone, DIN leaching below the main rooting zone was enhanced relative to the interior (at 128 m from the edge) by 21 and 14 kg N ha-1 y-1 in the coniferous and deciduous forest stands, respectively. However, the patterns of DIN leaching did not univocally reflect those of DIN throughfall deposition. DIN leaching in the first 20 m of the edges was lower than at 32–64 m from the edge (17 vs. 36 kg N ha-1 y-1 and 15 vs. 24 kg N ha-1 y-1 in the coniferous and deciduous forests, respectively). Nitrogen stocks in the mineral topsoil (0–30 cm) were, on average, 943 kg N ha-1 higher at the outer edges than in the interior, indicating that N retention in the soil is probably one of the processes involved in the relatively low DIN leaching in the outer edges. We suggest that a complex of edge effects on biogeochemical processes occurs at the forest edges as a result of the interaction between microclimate, tree dynamics (growth and litterfall), and atmospheric deposition of N and base cations