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

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.