Anthropogenic impacts on global storage and emissions of mercury from terrestrial soils: Insights from a new global model

We develop a mechanistic global model of soil mercury storage and emissions that ties the lifetime of mercury in soils to the lifetime of the organic carbon pools it is associated with. We explore the implications of considering terrestrial mercury cycling in the framework of soil carbon cycling and suggest possible avenues of future research to test our assumptions and constrain this type of model. In our simulation, input of mercury to soil is by atmospheric deposition, in part through leaf uptake and subsequent litter fall, and is moderated by surface photoreduction and revolatilization. Once bound to organic carbon, mercury is transferred along a succession of short-lived to long-lived carbon pools and is ultimately reemitted by respiration of these pools. We examine the legacy of anthropogenic influence on global mercury storage and emissions and estimate that storage of mercury in organic soils has increased by similar to 20% since preindustrial times, while soil emissions have increased by a factor of 3 (2900 Mg yr(-1) versus 1000 Mg yr(-1)). At steady state, mercury accumulates in the most recalcitrant soil carbon pools and has an overall lifetime against respiration of 630 years. However, the impact of anthropogenic emissions since preindustrial times has been concentrated in more labile pools, so that the mean lifetime of present-day anthropogenic mercury in all pools is similar to 80 years. Our analysis suggests that reductions in anthropogenic emissions would lead to immediate and large reductions in secondary soil mercury emissions.