The distribution and uptake dynamics of mercury in leaves of common deciduous tree species in Minnesota, USA

Abstract

The origin of mercury in foliage has been found to be mostly atmospheric. In this study, we developed and tested an extraction technique to study mercury distribution within leaves of various temperate deciduous tree species. Knowing the exact location of mercury within the leaf structure will allow us to evaluate its potential to leach, to volatilize, or to accumulate. Additionally, we monitored mercury concentration in foliage during the entire foliage growing season to understand the intra-season variations and to compare the accumulation rates of different species. We also studied the inter-annual accumulation of Hg in foliage by comparing end of season foliage Hg concentrations of two different seasons. Mercury levels in foliage were reported in leaf area- and mass- normalized concentrations/ uptake rates. Additionally, mercury levels on the surface, in the cuticle and in the inner tissues of tamarack (Larix laricina L.) needles were monitored during an entire growing season to understand the dynamics of mercury in these three compartments. The results of this study showed that the extraction technique developed and used for separating Hg associated with the three compartments of the leaf is reliable and that leaf mercury can be separated into three fractions each associated with the surface (2%-4%), the cuticle (2%-6%), and the inner tissue of the leaf (90%-96%). While the surface and the cuticle seemed to act as transitory compartments, leaf inner tissue behaved as a permanent storage site, and Hg concentration increased steadily in this compartment. The results of the season long leaf mercury monitoring confirmed a strong and positive correlation between leaf mercury levels and the age of leaves (r2 > 0.81). Tamarack (43.13 ±1.7ng g-1 – 2981.12 ±116.3 ng m-2) and elm (41.01 ±6.8 ng g-1 –1519.44 ±175.4ng m-2) showed the highest end of season foliage mercury concentrations, while gingko (19.03 ±2.8 ng g-1 – 1421 ±180.5 ng m-2) and oak (25.17 ±5.5 ng g-1 – 1144 ±250.1 ng m-2) showed the lowest. The difference between the coniferous species and the broadleaf species was more pronounced when surface normalized concentrations were used for the comparison. Lastly, foliage accumulation rate of mercury was found to be the highest in mid-growing season (15.76 ng m-2 day-1) when photosynthesis is known to be at its peak.