Effects of throughfall and litterfall manipulation on concentrations of methylmercury and mercury in forest‐floor percolates

The forest floor was shown to be an effective sink of atmospherically deposited methylmercury (MeHg) but less for total mercury (Hg_total). We studied factors controlling the difference in dynamics of MeHg and Hg_total in the forest floor by doubling the throughfall input and manipulating aboveground litter inputs (litter removal and doubling litter addition) in the snow‐free period in a Norway spruce forest in NE Bavaria, Germany, for 14 weeks. The MeHg concentrations in the forest‐floor percolates were not affected by any of the manipulation and ranged between 0.03 (Oa horizon) and 0.11 (Oi horizon) ng Hg L^–1. The Hg_total concentrations were largest in the Oa horizon (24 ng Hg L^–1) and increased under double litterfall (statistically significant in the Oi horizon). Similarly, concentrations of dissolved organic C (DOC) increased after doubling of litterfall. The concentrations of Hg_total and DOC correlated significantly in forest‐floor percolates from all plots. However, we did not find any effect of DOC on MeHg concentrations. The difference in the coupling of Hg_total and MeHg to DOC might be one reason for the differences in the mobility of Hg species in forest floors with a lower mobility of MeHg not controlled by DOC.     

Bestimmung von Mikronährstoffen und Schwermetallen in Böden mit dem Verfahren der Elektro‐Ultrafiltration (EUF) durch Zugabe von DTPA

With the electro–ultrafiltration (EUF) technique, the plant availability of several plant nutrients in soils can be characterized. The basic principle of EUF is that an electric field is induced using platinum electrodes. Ions in the soil suspension move either to the cathode or to the anode and are filtrated through ultra‐membrane filters. In the standard EUF procedure, two extractions steps are used: 30 min at 20°C and 5 min at 80°C. However, the determination of micronutrients and heavy metals with the standard EUF procedure is not possible, because the solubility of these elements in water is low and most of the watersoluble elements are precipitated when passing the platinum electrodes. The addition of DTPA, a well known complexing agent, during a third EUF fraction (5 min at 80°C) enables extraction of micronutrients and heavy metals. Highest concentrations in the 33 soils of the study were found for iron, followed by zinc, manganese, lead, copper, and nickel. Lower concentrations were obtained for cobalt, chromium, cadmium, and molybdenum. For two soils, the EUF/DTPA procedure was compared to CaCl₂/DTPA and EDTA soil extraction methods, showing that higher or comparable amounts were found with CaCl₂/DTPA and much higher amounts with the EDTA method. These results reveal that the EUF/DTPA technique in principle can be used for the determination of plant‐available micronutrients and heavy metals. However, in a next step the relationship between EUF/DTPA‐extractable elements and their availability for plants needs to be quantified.