Limited response of boreal forest litterfall mercury deposition to declines in atmospheric mercury concentrations (1987–2000)

Atmospheric mercury (Hg) uptake by vegetation and subsequent deposition via litterfall constitutes a major pathway in the global Hg cycle. However, the temporal dynamics of litterfall Hg deposition and its environmental controls remain poorly understood. Here, we present a detailed assessment of Hg concentrations and deposition fluxes for individual litter components in a Swedish boreal forest from 1987 to 2000. Atmospheric Hg concentrations declined 41 % during this period. Correspondingly, Hg concentrations in Scots pine and Norway spruce needles decreased significantly (∼22 % and ∼26 %, respectively). However, the total litterfall Hg deposition flux remained stable at 11.7 ± 1.8 μg m^-2 yr^-1, showing no clear temporal trend. Foliar litter (needles) contributed 44 % of total Hg deposition, while non-foliar litter (twigs, residual material, and cones) accounted for the remaining 56 % (32 %, 22 %, and 2 %, respectively). The importance of this non-foliar component in modulating litterfall Hg deposition is often overlooked. Litterfall Hg deposition was 1.7 times higher in the non-growing season than in the growing season, primarily due to greater litterfall biomass. The weak response of litterfall Hg deposition to declining atmospheric Hg concentrations highlights the importance of biological factors (e.g., litterfall compositions and productivity) in regulating Hg inputs to boreal forest. Our findings also underscore the need for long-term assessments of Hg deposition dynamics via different litterfall components for assessing the effectiveness of the Minamata Convention on Mercury.