Mercury retention in soil in association with organic matter decomposition and sulfur cycle

Soil is the largest sink for mercury (Hg), and it is important to understand mechanisms that retain or mobilize this element. Hg binds to soil organic matter (SOM), and is particularly associated with organic sulfur (S). It is expected, therefore, that Hg retention and mobilization is strongly controlled by C and S cycles. When carbon is converted to CO2 by decomposition, at least some Hg that was bound to the lost C is retained in the soil. Therefore, the Hg/C ratio increases as SOM ages. Some Hg is lost with dissolved organic matter (DOM). We report a study that examines the importance of SOM decomposition, S content, land use, and soil type on retention of Hg indicated by changes in Hg/C ratios. Soil samples were collected from Bartlett experimental forest, NH with three age groups - 120 yrs (OG), 40 yrs (MG) and 15 yrs (YG) after clear cutting, and two soil types - spodosol and inceptisol. A multiple regression was conducted with Hg/C against four explanatory variables, δ13C, S/C, age group and soil type. All four variables significantly contribute to the explanation of the Hg/C variance with an overall R2 = 0.89 (p<0.0001). As expected, the partial coefficients for δ13C and S/C ratio are both positive. These two variables explain 83% of the total variance. The soil type explains 3% of the total variance. The inceptisol soil type had a higher mean Hg/C ratio (2.8 ug/g) than spodosol soils (1.3 ug/g). This difference could result from greater leaching potential of spodosol soil as indicated by Kd values. Age also explains 3% of the total variance. The mean Hg/C ratio is highest in OG (2.6 ug/g) followed by YG (1.5 ug/g) and MG (1.2 ug/g). This may result from vegetation type, hill slope, or other variables not explicitly controlled in the multiple regression analysis. We conclude that SOM decomposition and S/C ratio are the most important factors for Hg retention in the soil, while the soil type and growth age have limited but significant influences.