Mercury loss from soils following conversion from forest to pasture in Rondônia, Western Amazon, Brazil

Abstract

This work reports on the effect of land use change on Hg distribution in Amazon soils. It provides a comparison among Hg concentrations and distribution along soil profiles under different land use categories; primary tropical forest, slashed forest prior to burning, a 1-year silviculture plot planted after 4 years of forest removal and a 5-year-old pasture plot. Mercury concentrations were highest in deeper (60–80 cm) layers in all four plots. Forest soils showed the highest Hg concentrations, ranging from 128 ng g−1 at the soil surface to 150 ng g−1 at 60–80 cm of depth. Lower concentrations were found in pasture soils, ranging from 69 ng g−1 at the topsoil to 135 ng g−1 at 60–80 cm of depth. Slashed and silviculture soils showed intermediate concentrations. Differences among plots of different soil-use categories decreased with soil depth, being non-significant below 60 cm of depth. Mercury burdens were only statistically significantly different between pasture and forest soils at the topsoil, due to the large variability of concentrations. Consequently, estimated Hg losses were only significant between these two land use categories, and only for the surface layers. Estimated Hg loss due to forest conversion to pasture ranged from 8.5 mg m−2 to 18.5 mg m−2, for the first 20 cm of the soil profile. Mercury loss was comparable to loss rates estimated for other Amazon sites and seems to be directly related to Hg concentrations present in soils.