Behavior of mercury in soil profiles: Impact of increased precipitation, acidity, and fertilization on mercury methylation

The behaviour of Hg in mineral pine forest (Pinus sylvestris} soil profiles (0-16 cm) was studied using a lysimeter experimental design combined with the labeling technique with (HgCl2)-Hg-203. The labeled surface soils were exposed to four different treatments: (i) normal precipitation (600 mm a(-1)), (ii) four-fold precipitation, (iii) soil P-K fertilizing with normal precipitation, and (iv) the increased acidity in the rainwater with normal amount of precipitation. After the treatment period of 97 days the Hg-203 and methyl Hg-203 were measured. The distribution of Hg in different soil matrices was determined by extracting the soil with distilled water, 1 M NH4-acetate, 1 M HCl, and 0.1 M NaOH. The results showed that the organic surface layer, especially the living moss (Pleurozium schreberi), was the dominant scavenger of the added Hg-203. Methyl Hg-203 formation was most intensive in this layer, too. Increased precipitation mobilized a part of the Hg-203 from the moss layer to the deeper layers and leachate water. In the fertilized soil with increased pH, the accumulation of both total Hg-203 and methyl Hg-203 was recorded in the organic surface soil. A small increase in rainwater acidity had no clear effect on the behavior of Hg-203 in soil. Hg-203 was mainly associated in the organic fraction (NaOH-extractable) whether in the living moss or humus layer. The fertilized soil yielded the highest concentration of water extractable Hg-203. In view of the climate change with increasing precipitation, the use of fertilizers, in the watersheds should be minimal and well-controlled. This is, because soil fertilizing seems to increase the availability of Hg for methylation, and because the mobilization of both total Hg and methyl Hg from watershed soils to lakes, and probably to ground water, increases when precipitation increases.