Litterfall Hg deposition to an oak forest soil from southwestern Europe

Abstract

Litterfall constitutes one of the main vectors for mercury (Hg) transfer to forested ecosystems, so we studied the deposition of Hg through senescent vegetation (oak leaves, twigs and miscellaneous) in a deciduous forest plot of Southwest Europe dominated by Quercus robur in 2015 and 2016. Total Hg concentrations increased in the following order: bole wood (1.4 μg kg−1) < bark (8.3 μg kg−1) < twigs (12.2 μg kg−1) < miscellaneous (36.0 μg kg−1) < oak leaves (39.3 μg kg−1) < mineral soil (42.4 μg kg−1) < Oi horizons (48.7 μg kg−1) < Oe + Oa horizons (71.6 μg kg−1). Mercury accumulation rates in oak leaves during the growing season were 0.15–0.18 μg kg−1 day−1. Mercury deposition fluxes were 26 and 21 μg m−2 yr−1 for 2015 and 2016, respectively, with oak leaves being the fraction that contributed the most. Mercury determination in litterfall sorted biomass fractions lead to a more accurate estimation of the total annual Hg deposition fluxes through litterfall. Higher Hg content was obtained for organic horizons (average of 60.2 μg kg−1) than for mineral soil (mean of 42.4 μg kg−1), but the soil Hg pool was higher in the latter. The results confirmed the necessity of taking into account the Hg pool in the deeper mineral soil layers as they accumulate substantial quantities of Hg associated to organic C and Al compounds, preventing its mobilization to other compartments of the terrestrial ecosystems.