Remediation of heavy metal-contaminated forest soil using recycled organic matter and native woody plants.

The main aim of this study was to determine how the application of a mulch cover (a mixture of household biocompost and woodchips) onto heavy metal-polluted forest soil affects (i) long-term survival and growth of planted dwarf shrubs and tree seedlings and (ii) natural revegetation. Native woody plants (Pinus sylvestris, Betula pubescens, Empetrum nigrum, and Arctostaphylos uva-ursi) were planted in mulch pockets on mulch-covered and uncovered plots in summer 1996 in a highly polluted Scots pine stand in southwest Finland. Spreading a mulch layer on the soil surface was essential for the recolonization of natural vegetation and increased dwarf shrub survival, partly through protection against drought. Despite initial mortality, transplant establishment was relatively successful during the following 10 yr. Tree species had higher survival rates, but the dwarf shrubs covered a larger area of the soil surface during the experiment. Especially E. nigrum and P. sylvestris proved to be suitable for revegetating heavy metal-polluted and degraded forests. Natural recolonization of pioneer species (e.g., Epilobium angustifolium, Taraxacum coll., and grasses) and tree seedlings (P. sylvestris, Betula sp., and Salix sp.) was strongly enhanced on the mulched plots, whereas there was no natural vegetation on the untreated plots. These results indicate that a heavy metal-polluted site can be ecologically remediated without having to remove the soil. Household compost and woodchips are low-cost mulching materials that are suitable for restoring heavy metal-polluted soil.

Decomposition of coniferous forest litter along a heavy metal pollution gradient, south-west Finland.

The decomposition of Scots pine (Pinus sylvestris) fine root and needle litter was examined along a heavy metal pollution gradient in the vicinity of Harjavalta Metals Smelter complex at Harjavalta, south-west Finland. The study area was found to exhibit a defined gradient of copper and nickel soil contamination along a 8 km long transect starting at 0.5 km from the point emission source with the highest levels found at this site. The background site is located 200 km from the point source. The majority of the heavy metals are confined to the soil organic layer. Litter decomposition and litter quality were compared between litter types after 12, 18, and 30 months incubation at each of four sites. The analyses clearly showed differences in accumulated mass loss, C:N ratio, and nutrient composition that were related to the site of incubation. Needle and fine root litter that was incubated at 0.5 km had a lower accumulated mass loss, 28.1 and 40.9%, respectively, over time when compared to litter incubated at the background site (37.9 and 50.9%, respectively). Concentrations of exchangeable cations in the litter incubated at 0.5 km were considerably less. The heavy metal contamination may provide an explanation for the amount of mass lost and the litter quality through impacts on the soil microbiota.

Fine root biomass and production in Scots pine stands in relation to stand age.

We determined fine root biomass and production of 15-, 35- and 100-year-old Scots pine (Pinus sylvestris L.) stands during three growing seasons. Fine roots were sampled by the soil core method. Mean (+/- SE) annual fine root biomass of Scots pine in the 15-, 35- and 100-year-old stands was 220 +/- 25, 357 +/- 21 and 259 +/- 26 g m(-2), respectively. Fine root biomass of the understory vegetation was 159 +/- 54 g m(-2), 244 +/- 30 and 408 +/- 81 g m(-2), and fine root necromass was 500 +/- 112, 1,047 +/- 452 and 1,895 +/- 607 g m(-2) in the sapling, pole stage and mature stands, respectively. Both understory and Scots pine fine root production increased with stand age. Mean annual Scots pine fine root production was 165 +/- 131, 775 +/- 339 and 860 +/- 348 g m(-2) year(-1) in the sapling, pole stage and mature stand, respectively. The respective mean annual production of all fine roots (Scots pine and understory) was 181 +/- 129, 1,039 +/- 497 and 1,360 +/- 869 g m(-2) year(-1). The Scots pine and understory fine root biomass, necromass and production varied in relation to stand age, although the variation was not statistically significant.

Interactions between precipitation and Scots pine canopies along a heavy-metal pollution gradient.

Bulk precipitation and stand throughfall were collected during 1992-96 at distances of 0.5, 4 and 8 km from the Harjavalta Cu-Ni smelter, southwestern Finland. The amounts of heavy metals (Cu, Ni, Zn, Fe) and mineral nutrients in bulk precipitation and throughfall were highest at 0.5 km. Although the canopy coverage was low at 0.5 km, the amounts of heavy metals intercepted by the canopy were extremely high. The proportion of foliar leaching relative to the wash-off of dry deposition from the needle surfaces decreased on moving towards the smelter for all elements, except for K. The high rate of K leaching from the needle tissues close to the smelter demonstrated that the K throughfall flux has been greatly altered by the heavy pollution load.

Nutrient retranslocation in the foliage of Pinus sylvestris L. growing along a heavy metal pollution gradient.

Retranslocation of N, P, K, Ca, Mg, Mn, Fe, Zn, Cu and Ni within the foliage of Scots pine (Pinus sylvestris L.) was studied during autumnal needle senescence along a heavy-metal pollution gradient in western Finland. The stands were located at distances of 0.5 (Har 0.5), 4 (Har 4) and 8 km (Har 8) to the southeast of a copper-nickel smelter at Harjavalta. A background study stand was located at Hämeenkangas (Häm), 60 km northeast of the smelter. During needle senescence, the mobile nutrients N, P and K decreased by 67-88% and needle dry mass decreased by 31-43%. Retranslocation of P and K was less efficient in the stand nearest the smelter (Har 0.5) than in the other stands. During needle senescence at stand Har 0.5, heavy metal content either decreased slightly (Zn, Ni) or increased (Fe, Cu), whereas in the background study stand, retranslocation efficiency of Cu was 75%. Net throughfall (the amount of a nutrient in throughfall minus the amount in open precipitation) was higher in stand Har 0.5 than in stand Har 8 for all of the elements measured, except N and P which were intercepted by the canopy. In stand Har 8, deposition of S, Cu and Ni was intercepted by the canopy, and net throughfall of Fe and Zn was very low. In contrast, S and heavy metal contents of net throughfall in stand Har 0.5 were greater than the decrease in these elements in the needles indicating that accumulation on needle surfaces and subsequent wash-off accounted for a major part of the fluxes. Furthermore, the Cu content in net throughfall during the senescence period was higher than the original Cu content of the needles in July, providing additional evidence that much of the needle Cu content was comprised of dry deposits of Cu on the needle surface.

Nutrient retranslocation within the foliage of Pinus sylvestris.

During the period 1983-1987, retranslocation of N, P, K, Ca, Mg, Mn, Zn, Fe, B and Al within the foliage of Scots pine (Pinus sylvestris L.) was studied in sapling, pole-stage and mature stands in eastern Finland. Needle concentrations of the mobile nutrients N, P, and K varied seasonally because of retranslocation. In unfertilized plots, needle contents of N, P and K decreased 62-92% during senescence and needle dry weight decreased 19-51%. In some years, needle contents of Ca, Mn, Zn, Fe, B and Al increased during senescence. Retranslocation was more efficient from needles with greater dry weight. In the fertilized, pole-stage stand, needle dry weight did not decrease at senescence four years after fertilization and nutrient content decreased less than in the other years. In the mature stand, there were no clear differences in needle dry weight and nutrient changes during senescence between years or between unfertilized and fertilized plots.