The Fate of Rare Earth Elements in Post-Fire Soils in the Pocono Mountains, Pennsylvania (USA)

The wood ash contribution to soils represents a unique and important part of soil organic carbon following fires.  Wood ash imparts chemical and physical changes to the soil, evident in elements other than carbon.  Our case studies are from recent wildfires and experimental burns in mixed hardwood forests in the Pocono Mountains of Pennsylvania, USA.  In these studies, we identified increases in most of the major elements and some minor elements in soils following forest fires, analyzed with ICP-MS. Elements such as Mn, Mg, Na, Ca, Na, K, Cu, and Ba, derive from an infusion of biomass ash, with variable contribution depending on, for instance, tree species. In the case of Ba and Cu, their presence is distinctly different from any mineral parent material contribution to the soil, and therefore unique signatures of fire contribution. Signature post-fire elements persist in some cases over one year following the fire, and are found in both topsoil horizons and into illuvial soil horizons.

In the course of these investigations, we also found a curious depletion of all rare earth elements (REEs) and certain trace elements from the soil following forest fires, and in adjacent stream and wetland sediments. The post-fire difference in REE concentration was statistically significant (p < 0.10, N=51) in all but Eu and U, with light REEs La, Ce and Pr showing the most significant decreases. Among other trace elements, Sc (which behaves similarly to REEs), V, Cr, Ga, and Rb also exhibited statistically significant decreases (though other elements Cu and Sr increase along with the ash input). The reasons for the depletions are unclear. Other authors report that REE dynamics in soils are poorly understood, but may be associated with phosphates, carbonates, and silicates in the soil. These are relatively enriched via post-fire biomass ash, yet the associated REEs are missing. It is unlikely that the elements would have preferentially translocated through and below the soil profile. Erosion is ruled out, otherwise the ash-associated major and trace elements would also be depleted. Two possible causes for post-fire REE loss are 1) volatilization from the soil during the fire, and 2) rapid uptake by post-fire succession plants, notably ferns, which are known to bioaccumulate REEs. Further research is warranted, following the ongoing post-fire vegetation recovery, and the dynamics of REEs within the soil profile.