Modeling mercury dynamics in terrestrial ecosystems in response to climate change

Background/Question/Methods   Soil has long been considered the largest mercury pool with the main input from the atmosphere though wet and dry deposition. The recent research indicates, however, that the significant input to the soil mercury pool could come from vegetation through litter fall and follow the carbon pathway in the ecosystem, since, like carbon, net ecosystem mercury exchange depends on air temperature, irradiance, and soil water content. Recent studies report the positive linear relationship between mercury and carbon in the ecosystem pools, thus, mercury pools and fluxes can be estimated indirectly through carbon pools and fluxes.The purpose of this study is to scale the results of estimation of the mercury carbon ratios to the ecosystems' level and assess the magnitude of contribution of the litter fall to the soil mercury pool using NCAR's Community Land Model. The model was used to explore mercury dynamics in the ecosystems in the following scenarios: 1) The carbon and mercury cycles are fully coupled, the only source of mercury is the litter fall and roots; 2) The carbon and mercury cycles are not fully coupled: when respiration occurs, mercury stays in the soil, the sources of mercury in soil are litter fall and roots.

Results/Conclusions   The initial soil mercury pools for the first scenario were 0, the model was run for 55 years, the resulting soil pool in the final year for the Sierra Nevada forest was 0.35 g/ha with the mean annual increase of 0.006 g/ha. Hence, the current soil mercury content (taking as a start deposition year 1850) is 1.55 g/ha. This number is nearly 45 times smaller from what was reported for that region: 70 g/ha from our previous study. Given that the carbon and mercury cycles are fully coupled (once carbon is released back into the atmosphere mercury is released with it), the mean annual wet deposition results in nearly 0.27 g/ha, which is too high (according to NADP Annual Data Summary, mean annual wet deposition of mercury for Sierra Nevada region is 0.04 g/ha). The reason for such discrepancy in data is most likely the hypothesis that mercury and carbon cycle are not fully coupled: while carbon is respired to the atmosphere, mercury stays in soil. The second scenario of the model is currently being examined.