Chemical Weathering Indices for Fire-Impacted Soils

In previous research, we noted the presence of major, minor, and trace elements in soils following forest fires.  In case studies from wildfires and experimental burns from the Rocky Mountain conifer forest (Colorado, USA), Appalachian mixed hardwood forests (Pennsylvania and New Jersey, USA), and sandy pine barrens (New Jersey, USA), we identified increases in most of the major elements and some minor elements in soils following forest fires, identified with ICP-MS analysis. Elements such as Mn, Mg, Na, Ca, Na, K, Cu, and Ba derived from an infusion of biomass ash. Some elements present were distinctly different from any mineral parent material contribution to the soil, and were therefore unique signatures of fire contribution. Signature post-fire elements persisted in some cases more than one year following the fire, and were found in both topsoil horizons and into illuvial soil horizons.

Beyond detecting the presence of post-fire signature elements, we investigate here a quantification of the soil chemical impact by means of widely use weathering indices. Fires do contribute to mechanical weathering of rock, though the impact on chemical weathering is less studied. Chemical weathering is usually assumed to be a long-term process, weathering indices summarize these processes. In contrast, the application of weathering indices to fire impacts reflect instantaneous events in pedogenic time spans, with abrupt geochemical changes due to additions of biomass ash. Nevertheless, there is benefit in the ability to compare values, so that fire imposed geochemical changes can be assessed in context.

In general, our data show that fire-impacted soils exhibit weathering indices trending toward “less weathered”. The weathering ratios correlate closely with elements that are abundant in biomass ash; many of these elements would normally be preferentially depleted by weathering over time. Weathering ratio values may correlate with burn temperature, based on results from laboratory simulations. The fire-impacted soil chemistry has bearing on modern and paleoenvironmental biogeochemical assessments. First, the calculated weathering index values have potential as indicators of the degree of impact from modern fires. Second, where weathering indices are used as paleoenvironmental proxies in sedimentary records, there exists a potential to misinterpret values for times where fires might have been prevalent (such as during drought periods), if the fire chemical signature persists. Ongoing research comparing modern soils and paleosols will further explore these potential discrepancies.