Abstract
Abstract. This study presents data on mercury (Hg) concentrations, stochiometric relations to carbon (C) and nitrogen (N), and Hg pool sizes in four Sierra Nevada forest sites of similar exposure and precipitation regimes, and hence similar atmospheric deposition, to evaluate how ecosystem parameters control Hg retention in ecosystems. In all four sites, the largest amounts of Hg reside in soils which account for 94–98% of ecosystem pools. Hg concentrations and Hg/C ratios increase in the following order: Green Needles/Leaves<Dry Needles/Leaves<Oi litter<Oe litter<Oa litter. Stochiometric relations show negative correlations between Hg and C (r2=0.58) and N and C (r2=0.64) in decomposing litter, but a positive correlation between litter Hg and N (r2=0.70). These inverse relations may reflect preferential retention of N and Hg over C during decomposition, or may be due to older age of decomposed litter layers which are exposed to longer-term atmospheric Hg deposition in the field. The results indicate that litter Hg levels depend on decomposition stage and may not follow generally observed positive relationships between Hg and organic C. Mineral soil layers show strong positive correlations of Hg to C across all sites and soil horizons (r2=0.83), but Hg concentrations are even more closely related to N with a similar slope to that observed in litter (r2=0.92). Soil N levels alone explain over 90% of Hg pool sizes across the four Sierra Nevada forest sites. This suggests that soil organic N and C groups provide sorption sites for Hg to retain atmospheric deposition. However, the patterns could be due to indirect relationships where high soil N and C levels reflect high ecosystem productivity which leads to corresponding high atmospheric Hg deposition inputs via leaf litterfall and plant senescence. Our results also show that two of the sites previously affected by prescribed burning and wildfires show significant depletion of above-ground Hg pools but that belowground Hg pools remain unaffected. We conclude that sequestration of Hg in remote Sierra Nevada forest sites is strongly co-determined by ecosystem parameters with C and N pools being excellent determinants for the pool sizes of Hg.
Highlights
Significant quantities of mercury (Hg) primarily derived from the atmosphere are associated with vegetation in terrestrial ecosystems, and large pools of atmospheric Hg deposition is retained in surface litter and soil pools (Mierle, 1990; Grigal, 2003; Munthe et al, 1998)
This study aims to compare Hg concentrations and pools sequestered in major ecosystem compartments and their relationships to the respective carbon (C) and nitrogen (N) contents in four remote forest sites in the Sierra Nevada mountain range of the Western United States, as part of a larger project aimed at quantifying Hg pools and fluxes across US forest ecosystems (EPA Science To Achieve Results (STAR) #RD833378010)
Results from our study show that most Hg in these forest ecosystems is retained in the soil and that soil pools are strongly influenced by the presence of soil C and N
Summary
Significant quantities of mercury (Hg) primarily derived from the atmosphere are associated with vegetation in terrestrial ecosystems, and large pools of atmospheric Hg deposition is retained in surface litter and soil pools (Mierle, 1990; Grigal, 2003; Munthe et al, 1998). Hg pools in terrestrial ecosystems greatly exceed annual fluxes to and from these systems (Mason et al, 1994; Grigal et al, 2000; Schwesig and Matzner, 2000; Lindberg, 1996; Munthe et al, 1998). Retention and sequestration of Hg in vegetation and surface soils is attributed to a strong affinity of Hg to organic matter (OM; Meili, 1991; Yin et al, 1996; Schuster, 1991). Obrist et al.: Mercury pools in remote Sierra Nevada forests to harmful levels even in relatively remote areas such as the Western United States (Peterson et al, 2007)
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