Deposition in boreal forests in relation to type, size, number and placement of collectors.

Open precipitation and throughfall was collected at a Norway spruce stand in Finland using funnel-type collectors and at a black spruce stand in Canada using trough-type collectors. The presence or absence of a rim on the funnel, funnel diameter (9, 14 and 20 cm) and length of sampling period (1, 2 and 4 weeks) on monthly values were evaluated at the Norway spruce stand, and the number of collectors required for defined levels of accuracy and precision of throughfall loads to be reached and the influence of the spatial arrangement of collectors on solute concentrations was studied at both stands. The presence of a rim had no significant effect on open precipitation and throughfall amounts, but did on throughfall DOC, Ca(2+), Mg(2+), K(+), Na(+) and Cl(-) ion loads. Deposition loads increased with decreasing funnel diameter; for open precipitation, this was due to increased catch efficiency while for throughfall the increase was attributed to canopy interaction and leaching of litter trapped in the collectors. Calculated monthly H(+) loads decreased and those for all other constituents increased with collection period length. Using 15 collectors at the Norway spruce stand would allow throughfall loads to be determined to within 20% of the true mean weekly value with a confidence level of 95% for most solute, but not for NH(4) (+)-N, NO(3) (-)-N, Mg(2+) and SO(4) (2-)-S. Using 15 trough collectors, the same confidence level at the more heterogeneous black spruce stand would only be achieved for H(+), Cl(-), DOC and SO(4) (2-)-S loads. In both stands, using either random or systematic placements of throughfall collectors gave similar results.

Enrichment of Cu, Ni, Zn, Pb and As in an ombrotrophic peat bog near a Cu-Ni smelter in southwest Finland.

The accumulation of selected trace elements (Cu, Ni, Zn, Pb, As) in the surface peat layer of an ombrotrophic bog 2.4 km from a Cu-Ni smelter at Harjavalta, Finland was studied using a peat core. A reference core was taken from an ombrotrophic bog at a background site, Hietajärvi, in eastern Finland. Element concentrations were analysed from 1-cm slices and enrichment factors (EF) were calculated. The enrichment factors of both Cu and Ni in the Harjavalta peat bog are extremely high compared to the Hietajärvi site. However, only the 6-cm surface peat Pb values are higher in Harjavalta compared to Hietajärvi. Precipitation was collected during 1992-1996, in the vicinity of the Harjavalta smelter, in order to estimate the current atmospheric deposition load. Comparison between the precipitation and peat data reveals that at Harjavalta the surface peat is relatively much more polluted than the current precipitation. The variation in EF of the Harjavalta peat core with respect to depth shows two patterns: Cu and Pb are similar, as are Ni, Zn and As. The vertical gradient in Harjavalta Cu EF suggests that Cu supplied to the peat by atmospheric deposition is very well preserved by the bog.

Heavy metal budgets for two headwater forested catchments in background areas of Finland.

Mean annual (1994-1996) budgets for Cd, Cu, Ni, Pb and Zn at two background, forested catchments, VK and HJ, in Finland are presented. Budgets for plots (VK3, HJ1 and HJ4) included throughfall (TF), litterfall (LF) and soil leaching fluxes, and for catchments terrestrial retention and leaching and lake sedimentation fluxes. Total deposition (TD) loads were relatively low (Cd < 0.1, Cu < 2, Ni < 1, Pb < 3 and Zn < 5 mg m-2 year-1) and that even in these areas almost half of the TD was in the form of dry deposition. Retention of TD within catchments was > or = 77% for all metals, except for Ni at VK (54%). For Cu and Pb, the retention was 94-97%. Most of the retention (74-97%) took place in the terrestrial part of the catchment, lake sedimentation accounting for the remainder. Plot-scale soil leaching fluxes at 40 cm of Cd, Cu (VK3) and Ni (VK3) were greater (> or = 100%) than TD inputs. Most of the catchment retention must therefore have taken place either deeper in the soil or in the lowland peatland areas. The humus layer was particularly effective in retaining Cu and Cd (65-81% and 51-78% of total inputs to the forest floor (TF + LF)). The retention of Pb by the humus layer was less than expected (26-54% of TF + LF). Litterfall was a particularly important internal flux for Zn.

The importance of leaching from litter collected in litterfall traps.

Litterfall (LF) is usually collected by means of open traps. However, this litter will be subject to leaching by the throughfall which passes through the accumulated litter in the traps. The nutrients lost during this leaching are not taken into account in the calculation of LF nutrient fluxes. We report the results from a 2-month (August-September) study carried out in 8 northern coniferous forest stands to assess the possible importance of leaching from litter in LF traps. Compared to throughfall (TF), the litterfall leachate (LFLgross), which includes a throughfall component, had significantly (p < 0.05) higher concentrations and fluxes of Ca, Mg, Na and S. The average net LFL (i.e., LFLnet = LFLgross - TF) fluxes were 21 (Ca), 7 (Mg), 57 (K), 10 (Na), 10 (N), and 19 (S) mg m-2 mo-1. LFLnet accounted for 42%, 37% and 50% of the LFLgross flux, and for 91%, 51% and 49% of the total litterfall flux (i.e., LF + LFLnet) of Na, S and K, respectively. For Ca, Mg and N, the LFLnet flux accounted for 64%, 58% and 29% of the LFLgross flux, and for < 14% of the total litterfall flux. Compared to TF, LF was the dominant return pathway for Ca, Mg and N to the forest floor regardless of whether LFLnet was included or not. However, for K and S, taking LFLnet into account determined whether TF or LF was the dominant pathway. TF remained the dominant pathway for Na even when LFLnet was included.