Methylmercury and Total Mercury in Plant Litter Decomposing in Upland Forests and Flooded Landscapes

Abstract

The overall objectives of this study were to examine the effects of flooding on the decomposition and mercury (Hg) content of tissues from plants common to boreal upland forests at the Experimental Lakes Area in northwestern Ontario. We used litterbags to study changes in total Hg (THg), methyl Hg (MeHg), carbon (C), and nitrogen (N) in 12 different plant tissues (birch, alder, blueberry, and Labrador tea leaves, bunchberry plants, jack pine needles, Sphagnum spp., Polytrichum spp., and Pleurozium spp. bryophytes, lichen, and fresh and extensively decomposed wood) placed on unflooded boreal forest soils and in experimentally created reservoirs over an approximately 800 day period. Rates of decomposition (as indicated by differences in the percentage of C and N mass left in the tissues over time) were slower in plant tissues placed on unflooded soils compared to the same tissues that were inundated in reservoirs. Depending on tissue type and initial THg concentrations, decomposing litter on both unflooded and flooded soils was either a source or a sink for THg. Tissues where initial THg concentrations were greater than 30 ng g(-1) represented a source of THg to the surrounding environment, whereas tissues that had initial concentrations of less than 30 ng g(-1) gained THg mass. Initial rates of change in THg were more rapid in plant tissues placed in reservoirs compared to the same plant tissue placed on unflooded soils, but there were no differences in final THg masses after approximately 800 days. Plant tissues placed in reservoirs exhibited large increases in MeHg mass, whereas MeHg mass decreased in the same plants placed on unflooded soils. This is the first study examining THg and MeHg cycling in decomposing plants in upland boreal forests and reservoirs.

Comparison of total mercury and methylmercury cycling at five sites using the small watershed approach

“…4), suggesting common sources for the two species. At Sleepers we have inferred that near-stream or in-stream particulate organic matter, mobilized during storms (Hall and St. Louis, 2004), is the common source for THg and MeHg (Schuster et al, 2008). Mobilization of particulate organic matter is also a plausible mechanism at Panola, where in-stream sediment is high in organic matter, and at Icacos, where bed sediment and suspended sediment are low in organic matter but there is a high correlation of THg and POC (see below).…”

Fate of mercury in tree litter during decomposition

“…1), show that sampling replications and time of exposure (i.e., 18 months) yielded substantial and detectable mass losses during the laboratory incubation study. Rates of mass decreases were different among litter species, and aspen leaf litter showed the strongest dry mass decrease (31 % after 18 months) while pine litter showed only small losses (8 % coniferous litter (Kaneko and Salamanca, 1999;Hall and St. Louis, 2004), while others found the opposite (Demers et al, 2007). Some studies observed inconsistent trends or time-dependent trends, e.g., where initial differences between litter species disappeared during longer time periods (Moore et al, 1999;Sundarapandian and Swamy, 1999;Prescott et al, 2000).…”

“…As a result, field observations generally show strong Hg enhancement in surface litter and soil organic C fractions, greatly exceeding Hg concentrations of the original plant detritus inputs (Lindberg and Harris, 1974;Nater and Grigal, 1992;Hall and St. Louis, 2004;Demers et al, 2007;Tsui et al, 2008;Obrist et al, 2011). Continued sorption of atmospheric Hg to litter and soil organic C pools in the field, however, complicates the study of fate of Hg associated with litter and soil organic C, as newly sorbed Hg is difficult to separate from Hg originally associated with plant tissues.…”

Mercury concentrations and pools in four Sierra Nevada forest sites, and relationships to organic carbon and nitrogen

“…Similar, Friedli et al (2007) reported surface litter levels between 68 and 127 µg kg −1 in a boreal forest site in Canada and higher Hg contents in organic layers (100 to 250 µg kg −1 ), andÅkerblom et al (2008) reported higher Hg/C ratios in the lower half of forest mor layers as compared to the upper half in boreal forest sites. Hall and St. Louis (2004) reported total Hg mass increases in decomposing boreal forest leaves and needles during an 800 day in situ decomposition study using litter bags along with decreases in C and N. Åkerblom et al (2008) attributed this trend to a natural downward enrichment of Hg due to soil decomposition or other processes, and Grigal (2003) similarly concluded that a higher Hg density of organic matter in the mineral soil as compared to the forest floor implies that less Hg than C is lost during OM mineralization. Similar enrichment of Hg in plant detritus was first reported for aquatic ecosystems by Lindberg and Harriss (1974).…”