Physical and biogeochemical drivers of solute mobilization and flux through the critical zone after wildfire

Abstract

A nine-year time series of nutrient cation and anion concentration and efflux from three forested catchments in the Jemez River Basin Critical Zone Observatory (JRB-CZO) in northern New Mexico was used to quantify the pulse of chemical denudation resulting from varying levels stand-replacing wildfire intensity in May-June of 2013. The 3 years of pre-fire and 6 years of postfire data were also probed to shed light on the mechanisms underlying the pulsed release and the subsequent recovery. The initial large solute pulse released to the streams—K+, Ca2+, Mg2+, SO42-, Cl−, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and total dissolved nitrogen (TDN)—was caused by leaching of hillslope ash deposits during the first monsoon storms post-fire. Debris flow following the wildfire likely redistributed much of the ash-containing sediments along streams and valley bottoms. Sustained elevated solute concentrations observed in the surface waters throughout the post-fire period relative to pre-fire baselines is consistent with these soluble materials being periodically flushed from the soils during wet seasons, i.e., snowmelt and summer monsoons. As microbial mediated reactions and biotic uptake—due to plant regrowth—recover after fire, nutrient ion export (e.g., NO3-, Cl− and SO42-) steadily decreased toward 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. Our observations suggest that changes in organic matter composition after fire (e.g., increased humification index—HIX) and the presence of pyrogenic carbon may not favor organo-metal complexation and transport. Finally, differences in burn severity among the three watersheds presented in this study, provide insights of the relative impact of solute exports and resilience. The catchments that experienced high burn severity exhibited greater solute fluxes than the less severely burn. Moreover, despite these differences, toward the end of the post-fire period these surface waters presented low and similar solute effluxes, indicating system recovery. Nonetheless, magnitudes and rebounds were solute and process specific. The results of this study highlight the importance of surface and near surface physical and biogeochemical processes on the long-lasting geochemical denudation of forested catchments following wildfires of varying intensities.

Wildfire Impacts on Molecular Changes of Dissolved Organic Matter during Its Passage through Soil

“…These findings suggest that we captured the effects of wildfire in unburned areas and indicate that postfire rainfall events resulted in some hydrologic mixing on the hillslopes. The HIX exhibited a trend similar to that of C1, suggesting that C1 may influence soil DOM breakdown levels; the presence of burn-derived formulas correlated with increased HIX values (Figure d, h) . Conversely, only a few molecular formulas in the topsoil were positively correlated with the C2 fraction.…”

“…The HIX exhibited a trend similar to that of C1, suggesting that C1 may influence soil DOM breakdown levels; the presence of burn-derived formulas correlated with increased HIX values (Figure 3d, h). 67 Conversely, only a few molecular formulas in the topsoil were positively correlated with the C2 fraction. Interestingly, a significant portion (6.27%) of molecular formulas in subsoil DOM (at a depth of 30 cm with n = 2,342) exhibits a positive correlation with component C2 (n = 11, Spearman's r > 0.6, p < 0.05), suggesting that C2 primarily originates from deeper soil layers.…”

Persistent and lagged effects of fire on stream solutes linked to intermittent precipitation in arid lands

“…Overall, interactive effects of fire and precipitation influenced temporal patterns in transport of materials from burned, aridland watersheds to streams, but the duration and magnitude of these effects varied across climate regions and among watersheds, likely due to contrasts in surface and subsurface flowpaths, proximity to the fire, and catchment vegetation. Other studies in these same Mediterranean and monsoonal watersheds have examined temporal trends in stream chemistry using data specifically from storm events (Aguilera and Melack 2018b;Goodridge et al 2018) or following a single fire (Dahm et al 2015;Reale et al 2015;Sherson et al 2015;Sánchez et al 2023), and several quantify fire and precipitation as an intertwined effect (Reale et al 2015;Goodridge et al 2018;Sánchez et al 2023). Here, we examine these previous studies to illuminate patterns and mechanisms relevant to our findings, while providing novel insights into fire and precipitation interactions across distinct hydroclimates.…”

“…3). The volcanic lithology and low-relief valleys cause deep groundwater to dominate streamflow generation in these watersheds (White et al 2019), which likely contributed to combustion-liberated ions being stored on the landscape or leached into deeper flowpaths for years until being transported to streams during periods of elevated hillslope flows (Sánchez et al 2023).…”