Postdoctoral Fellowship: EAR-PF: Influence of Land Cover on the Isotopic Signature of Mercury in Surface- and Subsoils of the Conterminous United States

People have released large amounts of mercury through things like mining, using fossils fuels, and burning waste. Mercury is a problem because it is toxic to humans and other animals. Once in the environment, mercury may find its way into soils, where it can be released or stored for a long time.

How much mercury gets into the soil and how much stays there depends on things like what kind of landcover is present (for example, forests, croplands, or deserts), but the exact effect of landcover is not very well known for most landcover types. For this reason, it is important to better understand how soils of different landcover types take-up mercury.

This study will test the mercury signature of soils from 100 sites across the United States, focusing on different landcover types. This information will help show how mercury came to be in these soils and how much mercury is moving through them. Knowing how mercury moves through soils will help control mercury pollution in the United States and clarify the long-term impacts of mercury release.

Mercury is introduced to soils through several input pathways and may be sequestered there or released to the atmosphere and downstream aquatic ecosystems. The transport and fate of mercury in soils, therefore, depends on the relative contribution of various input pathways and the degree to which soil mercury is processed through the soil profile. Due to the strong association of mercury with organic matter, efforts to elucidate soil Hg cycling have largely focused on regions of high local emissions (i.e., developed systems) or areas with more abundant precipitation and elevated levels of organic carbon (i.e., forests, wetlands).

In contrast, other land cover types (e.g., shrublands, grasslands, barren) have received relatively little attention with regards to soil mercury. This bias has important implications, as much of our understanding of soil Hg and its biogeochemical cycle is based on observations from land cover types which together constitute less than 40% of the conterminous United States.

This project proposes sampling archived soils representative of the landcover distribution of the United States from a national-scale soil survey for analysis of mercury stable isotopes. This study aims to quantify 1) the relative importance of different mercury input pathways to soils by land cover type; and 2) the difference between isotopic signature of surface and subsoils across land cover types.

Together, these findings will clarify the relative importance of deposition and mineralogical processes on mercury cycling through soils. The results from the isotopic analysis will be compared to simulations from biogeochemical mercury models to determine how well non-forest soil processes are currently represented.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.