Long‑term monitoring of mercury in Vermont’s forest soils: Trends over 20 years in near–surface horizons
Mercury is a soil pollutant of widespread concern, usually derived from airborne deposition. A long–term forest soil monitoring program began in 2002 at five sites (elevation 590–1140 m) in Vermont, USA. Total mercury (THg) and soil organic carbon (SOC) were assessed in the uppermost humified soil layer, either an Oa or A horizon. Sampling occurred every 5 years from ten subplots at each site. Tree species ranged from deciduous, mixed deciduous/conifer, to all conifer. After five samplings, the site means for THg ranged from 167 to 447 µg kg –1 and increased linearly with elevation ( R 2 = 0.78). One site had a significant temporal increase in THg of 3.9 µg kg –1 yr –1 ( p = 0.02). Overall SOC site means ranged from 133 to 434 g kg –1 . One site showed a significant temporal increase in SOC ( p = 0.01). The mean ratio of THg:SOC was similar at four of the sites (1318–1518 µg kg –1 ) but much lower (648 µg kg –1 ) at the site with the highest SOC and the temporal increase in THg. Within individual subplots, THg increased up to a maximum SOC concentration of ~ 320 g kg –1 and usually decreased above that threshold. Although both wet and dry mercury deposition in the northeast USA have declined, no evidence of declining soil THg concentration was found, likely due to strong retention by SOC. Continued monitoring is essential considering ongoing changes in deposition and future changes in the source–sink balance of mercury.