Mercury accumulation in leaves of different plant types – the significance of tissue age and specific leaf area

Abstract

Abstract. Mercury, Hg, is one of the most problematic metals from an environmental perspective. To assess the problems caused by Hg in the environment it is crucial to understand the processes of Hg biogeochemistry, but the exchange of Hg between the atmosphere and vegetation is not sufficiently well characterised. We explored the mercury concentration, [Hg], in foliage from a diverse set of plant types, locations and sampling periods to study whether there is a continuous accumulation of Hg in leaves/needles over time. Measurements of [Hg] were made in deciduous and conifer trees in Gothenburg, Sweden (Botanical Garden and city area) as well as of evergreen trees in Rwanda. In addition, data for wheat from an ozone experiment conducted at Östad, Sweden, were included. Conifer data were quantitatively compared with literature data. In every case where older foliage was directly compared with younger, [Hg] was higher in older tissue. Covering the range of current year up to four-year old needles, there was no sign of Hg saturation in conifer needles with age. Thus, over time scales of approximately one month to several years, the Hg uptake in foliage from the atmosphere always dominated over Hg evasion. Rwandan broadleaved trees had generally older leaves due to lack of seasonal abscission and higher [Hg] than Swedish broadleaved trees. The significance of atmospheric Hg uptake in plants was shown in a wheat experiment where charcoal filtrated air lead to significantly lower leaf [Hg]. To search for general patterns, the accumulation rates of Hg in the diverse set of tree species in the Gothenburg area were related to the specific leaf area (SLA). Leaf area based [Hg] was strongly negatively and non-linearly correlated with SLA, while mass-based [Hg] had a somewhat weaker positive relationship with SLA (both relationships with p < 0.001). An elaborated understanding of the relationship behind [Hg] and SLA would support large-scale modelling of Hg uptake by vegetation and Hg circulation in general.

Comment on bg-2021-239

“…We therefore suggest that differences in daily foliar Hg uptake rates between the sampling years 2015 and 2017 are negligible. In agreement with previous studies (Ollerova et al, 2010;Hutnik et al, 2014;Navrátil et al, 2019;Wohlgemuth et al, 2020;Pleijel et al, 2021), we found a clear trend of Hg concentrations in differently aged needles with older needles exhibiting higher Hg concentrations (Fig. 7), demonstrating that Hg accumulation continues in older needles.…”

“…Average foliar Hg concentrations (mean ± sd) differed between tree species groups (see Table S1 for definition of tree species groups). Ash leaves exhibited highest Hg concentrations (32.2 ± 5.7 ng Hg g -1 d.w.; n = 10), followed by beech leaves (25.5 ± Increasing foliar Hg concentrations with progressing sampling date are in line with previous observations demonstrating that at individual sites Hg concentrations increased linearly over the growing season (Rea et al, 2002;Laacouri et al, 2013;Wohlgemuth et al, 2020;Pleijel et al, 2021). To make Hg levels in foliage sampled at different times comparable, we calculated daily foliar Hg uptake rates by normalizing foliar Hg concentrations with the life period of samples.…”

Physiological and climate controls on foliar mercury uptake by European tree species

“…We therefore suggest that differences in daily foliar Hg uptake rates between the sampling years 2015 and 2017 are negligible. In agreement with previous studies (Ollerova et al, 2010;Hutnik et al, 2014;Navrátil et al, 2019;Wohlgemuth et al, 2020;Pleijel et al, 2021), we found a clear trend of Hg concentrations in differently aged needles with older needles exhibiting higher Hg concentrations (Fig. 7), demonstrating that Hg accumulation continues in older needles.…”

“…Ash leaves exhibited highest Hg concentrations (32.2 ± 5.7 ng Hg g -1 d.w.; n = 10), followed by beech leaves (25. Increasing foliar Hg concentrations with progressing sampling date are in line with previous observations demonstrating that at individual sites Hg concentrations increased linearly over the growing season (Rea et al, 2002;Laacouri et al, 2013;Wohlgemuth et al, 2020;Pleijel et al, 2021). To make Hg levels in foliage sampled at different times comparable, we calculated daily foliar Hg uptake rates by normalizing foliar Hg concentrations with the life period of samples.…”

Seasonal variation of mercury concentration of ancient olive groves of Lebanon

“…Hg in foliage originates predominantly from the air, and it can be delivered to the soil through the litter (Tomiyaso et al 2003). Adding to that, the atmospheric Hg uptake in foliage dominates over Hg release (Pleijel et al, 2021). Since normally the main source of Hg into the foliage is atmospheric and minimally through the soil, this explains the higher Hg concentration in the foliage, while in the stems the main uptake is from the soil and it seems to be minimal (Tomiyasu et al, 2005) explaining the much lower level of Hg concentration in the stems than that in the foliage.…”