Tracking the Pulse of Pyrogenic Carbon and Lithogenic Solutes through Surface Soil of a Western Forest Headwater Catchment Immediately Following Wildfire

Abstract

This study tracked the catchment-scale solid and aqueous phase soil chemistry during the immediate aftermath of the Thompson Ridge Fire of 2013 (TRF2013), a moderate to severe wildfire that impacted portions of the Jemez River Basin Critical Zone Observatory (JRB-CZO) in the greater the Valles Caldera National Preserve (VCNP). The study area was a single zero- order basin (ZOB) - a headwater catchment (ca. 0.15 km2; 2986 -3099 m asl) with well constrained seasonal water inputs that prior to the fire supported mixed conifer forest vegetation at higher elevations and herbaceous and wetland vegetation at lower elevations. Soil samples were collected at six depth intervals to 40 cm from 22 locations at key landscape positions spanning a range of slope and elevation. Initial sampling commenced 50 days post-fire (2013), with two additional dry season sampling in post-fire years (2014 and 2015). A suite of chemical analyses were conducted on bulk soil (e.g., total organic carbon, total nitrogen, pyrogenic carbon) and aqueous extract filtrates (e.g., pH, EC, anions, cations, water extractable organic carbon, total dissolved nitrogen). Archived ZOB soil samples and some pre-fire aqueous chemical analysis provided site-specific unburned controls.Due to the unpredictability of wildfire, many post-fire studies rely exclusively on proximal unburned controls, and typically involve collection of representative samples from several catchments or landscape positions; these may lack precise site-specific spatial information such as pre-fire vegetation or burn severity (Abney et al., 2017; Boot et al., 2015; Galanter et al., 2018). Further, construction of post-fire soil properties chronosequences typically utilize a broad temporal resolution that can miss changes to surface soil attributes that occur in the immediate post-fire period (Elliott & Parker, 2001). Variation in ash quality has been demonstrated for distinct vegetation types at the microscale or from controlled burns of different severity but opportunities to capture of initial ash deposition patterns within a single catchment are rare. Here we had an opportunity due to a relatively fast post-fire sampling 50 days after the disturbance. Most measured chemical attributes (e.g., anions, cations, carbon, pyrogenic carbon) were strongly correlated to pre-fire above ground biomass carbon (AGB-C); some more volatile attributes were distinctly sensitive to burn severity measured by difference normalized burn ratio (dNBR; e.g., aqueous measures of carbon and nitrogen). The results of this research provided important spatial clarity of patterns of depositions following wildfire before significant geomorphologic and climate/weather effects could redistribute fresh pyrogenic material. Deep 9 pre-fire soil profiles to depth of refusal found presumably relic PyC qualitatively indistinguishable from fresh deposits in pyrogenic aromaticity and degree of condensation measures, a strong indication that burial and long-term stability of PyC is typical for ZOB soils. Studies of the fate of PyC in aquatic systems has highlighted dissolution as a primary transport mechanism by way of rivers and streams. However, relic and contemporary PyC in the ZOB lacked definitive spatial relationships to a conventional aqueous aromaticity measure- specific ultraviolet absorbance (SUVA254). It may be that in headwater positions, and in locations where burial is effective in physical occlusion, there lacked sufficient contributions of dissolved PyC necessary to exceed the signal of non-pyrogenic sources of aromaticity. It is presumed that higher order catchment soils or streams that receive greater total water fluxes may be better suited to delivery of aged or fresh PyC to soil and surface waters. There was little evidence of significant physical redistribution within the two years post-fire. With a compelling time zero of the initial lithogenic solute pulse, and minimal erosion during the remainder of the study period, we were able to track annual progress of pyrogenic solutes through 40 cm soil profiles across three ZOB transects. Contour plots of total solute concentrations (tracked as an ionic strength (IS) pulse) were complimented with moment analysis to track the ‘plume’. Collocated charge balance diagrams identified the primary spatial and temporal changes to ionic contributions. The post-fire solute dynamics of the ZOB may have been driven in part by deeper critical zone structure (e.g., clay rich zones acting as impermeable layers) measured in previous CZO research, in addition to clear influences of water flux imposed by varied topography, landscape position, landform shape, and time since fire.