Forest Fires in the Critical Zone: The Impact of Wildfires on Solute Fluxes, Stream Flow, Hydrologic Flowpath Development, and Biogeochemical Processes

Abstract

The critical zone (CZ) is the section of the Earth where most of the life takes place, which extends from the top of the canopy to the bottom of the groundwater. Abiotic and biotic processes that occur in soils and fractured regolith and bedrock drive the biological and geochemical signatures of the CZ. Much of the CZ literature assesses these processes under steady-state conditions (i.e., assuming undisturbed conditions) but anthropogenic and natural disturbances alter mass and energy transfers in the CZ. This study takes place in the Jemez River Basin Critical Zone Observatory (JRB-CZO), located in the Valles Caldera National Preserve (VCNP), northern New Mexico, where a stand-replacing wildfire in May-June of 2013 affected much of the JRB-CZO. The JRB-CZO comprises three headwater catchments—La Jara, Upper Jaramillo, and History Grove each subject to different fire intensities with La Jara and History Grove were most affected by the fire. The severely burned catchments exhibited greater solute fluxes than the one that experienced low burn severity (i.e., Upper Jaramillo). Despite these differences, towards the end of the post-fire period these effluxes decreased, and similar magnitudes were observed in the three watersheds, indicating a return to pre-fire conditions. Overall, there is an increase in solute concentration in the surface waters at the JRB-CZO after the fire. The initial pulse of solute released to the streams was attributed to ash dissolution during the first monsoon storms immediately after the fire but remained elevated throughout the post-fire period. Moreover, as biological activities recover after burning, nutrient ion export (e.g., NO3-, Cl- and SO42-) steadily decreased towards the end of the post fire period, but remained above pre-fire levels, particularly for NO3- and SO42-. Surface water concentrations of polyvalent cations (e.g., Al and Fe) decreased significantly after the fire. These reductions may be related to changes in organic matter composition after the fire and the presence of pyrogenic carbon may not favor organo-metal complexation and transport. Next, we coupled concentration-discharge (C-Q) relationships and mixing models to shed light on the underlying mechanisms that drive the surface water chemical responses before and after fire. The post-fire mixing model results presented herein suggest that there is a greater influence of soil water and precipitation (i.e., snowmelt and rainfall) inputs to the streams relative to pre-fire contributions. Moreover, post-burn C-Q relations showed strong flushing trends further suggesting enhanced solute mobilization from lateral flows through soils to streams. These results suggests that after fire there is a strong geochemical fingerprint of near surface flows to streams. Finally, we investigated how the fire disturbance propagated into the CZ continuum. Using the extensive pre- and post-fire isotopic data across the JRB-CZO we provide compelling evidence that these disturbances in fact distribute through the CZ. Relative more enriched waters were observed in the post-fire δ2H and δ18O relations, and consistent enrichment lines were shown in all the waters at the JRB-CZO. Moreover, deuterium excess (d-excess) calculations showed that evaporation enrichment were more evident during dry periods and relative warm temperatures after the fire. This evidence allowed us to posit that any physical and biogeochemical change that terrestrial systems experience after a fire may also propagate to the vadose zone and deep subsurface at relatively fast rates. Increased fire frequencies and intensities may lead to long-lasting impacts of disturbances in forested systems and can have major ecological and functional implications.