Abstract
Abstract. Biomass burning produces black carbon (BC), effectively transferring a fraction of the biomass C from an actively cycling pool to a passive C pool, which may be stored in the soil. Yet the timescales and mechanisms for incorporation of BC into the soil profile are not well understood. The High Park fire (HPF), which occurred in northwestern Colorado in the summer of 2012, provided an opportunity to study the effects of both fire severity and geomorphology on properties of carbon (C), nitrogen (N) and BC in the Cache La Poudre River drainage. We sampled montane ponderosa pine forest floor (litter plus O-horizon) and soils at 0–5 and 5–15 cm depth 4 months post-fire in order to examine the effects of slope and burn severity on %C, C stocks, %N and BC. We used the benzene polycarboxylic acid (BPCA) method for quantifying BC. With regard to slope, we found that steeper slopes had higher C : N than shallow slopes but that there was no difference in BPCA-C content or stocks. BC content was greatest in the forest floor at burned sites (19 g BPCA-C kg−1 C), while BC stocks were greatest in the 5–15 cm subsurface soils (23 g BPCA-C m−2). At the time of sampling, unburned and burned soils had equivalent BC content, indicating none of the BC deposited on the land surface post-fire had been incorporated into either the 0–5 or 5–15 cm soil layers. The ratio of B6CA : total BPCAs, an index of the degree of aromatic C condensation, suggested that BC in the 5–15 cm soil layer may have been formed at higher temperatures or experienced selective degradation relative to the forest floor and 0–5 cm soils. Total BC soil stocks were relatively low compared to other fire-prone grassland and boreal forest systems, indicating most of the BC produced in this system is likely lost, either through erosion events, degradation or translocation to deeper soils. Future work examining mechanisms for BC losses from forest soils will be required for understanding the role BC plays in the global carbon cycle.
Highlights
While pyrogenic or black carbon (BC) is recognized as a ubiquitous soil carbon (C) fraction, it is one of the least understood components of the terrestrial C cycle
The aims of this work were to (1) determine the C and BC stocks, and the proportion of C that was BC, in ponderosa pine forest floor and soils following the High Park fire (HPF); (2) examine the effects of burn severity and landscape slope on soil C, N and proportion of BC; and (3) use the distribution of individual benzene polycarboxylic acid (BPCA) to understand the degree of condensation of BC through the soil profile
This should be taken into consideration when comparing BPCA estimates with BC distribution values in systems that have been assessed with different methods (Brodowski et al, 2005; Glaser et al, 1998; Ziolkowski et al, 2011)
Summary
While pyrogenic or black carbon (BC) is recognized as a ubiquitous soil carbon (C) fraction, it is one of the least understood components of the terrestrial C cycle. Black C is utilized by soil microbes, but at a slow rate (Santos et al, 2012); it generally resides in the soil for a long time (from centuries to millennia; Singh et al, 2012), acting as a long-term C sink, with a potential negative feedback on climate warming. Persistent BC particles in soils are composed of a refractory, aromatic core and a reactive, oxidized patina (Keiluweit et al, 2010; Lehmann et al, 2005) characterized by carbonyl and Published by Copernicus Publications on behalf of the European Geosciences Union
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