Variation of Hg concentration and accumulation in the soil of maritime pine plantations along a coast-inland transect in SW Europe

Climatic conditions have been shown as a major driver of the fate of Hg in forest ecosystems at a global scale, but less is known about climatic effects at shorter scales. This study assesses whether the concentration and pools of Hg in soils collected from seventeen Pinus pinaster stands describing a coastal-inland transect in SW Europe vary along a regional climatic gradient. In each stand, samples of the organic subhorizons (OL, OF + OH) and the mineral soil (up to 40 cm) were collected and some general physico-chemical properties and total Hg (THg) were analyzed. Total Hg was significantly higher in the OF + OH than in the OL subhorizons (98 and 38 mu g kg(-1), respectively), favored by a greater organic matter humification in the former. In the mineral soil, mean THg values decreased with depth, ranging from 96 mu g kg(-1) in the 0-5 cm layers to 54 mu g kg(-1) in the deepest layers (30-40 cm), respectively. The average Hg pool (PHg) was 0.30 mg m(-2) in the organic horizons (92% accumulated in the OF + OH subhorizons), and 27.4 mg m(-2) in the mineral soil. Changes in climatic factors, mainly precipitation, along the coast-inland transect resulted in a remarkable variation of THg in the OL subhorizons, consistent with their role as the first receiver of atmospheric Hg inputs. The high precipitation rate and the occurrence of fogs in coastal areas characterized by the oceanic influence would explain the higher THg found in the uppermost soil layers of pine stands located close to the coastline. The regional climate is key to the fate of mercury in forest ecosystems by influencing the plant growth and subsequent atmospheric Hg uptake, the atmospheric Hg transference to the soil surface (wet and dry deposition and litterfall) and the dynamics that determine net Hg accumulation in the forest floor.

Automated summary

This study investigates the variation of mercury concentration and pools in soils collected from Pinus pinaster stands along a coastal-inland transect in SW Europe. The results show that total mercury concentration is higher in the organic subhorizons compared to the mineral soil, and it decreases with depth. The climatic factors, particularly precipitation, along the transect have a significant influence on the variation of total mercury concentration in the organic subhorizons.