Pyrogenic carbon from tropical savanna burning: production and stable isotope composition

G Saiz,J G Wynn,P N Nelson,M I Bird,C M Wurster,I Goodrick

doi:10.5194/bg-12-1849-2015

G Saiz, J G Wynn + Show 4 more

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https://doi.org/10.5194/bg-12-1849-2015

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Abstract

Abstract. Widespread burning of mixed tree–grass ecosystems represents the major natural locus of pyrogenic carbon (PyC) production. PyC is a significant, pervasive and yet poorly understood "slow-cycling" form of carbon present in the atmosphere, hydrosphere, soils and sediments. We conducted 16 experimental burns on a rainfall transect through northern Australian savannas with C4 grasses ranging from 35 to 99% of total biomass. Residues from each fire were partitioned into PyC and further into recalcitrant (HyPyC) components, with each of these fluxes also partitioned into proximal components (>125 μm), likely to remain close to the site of burning, and distal components (<125 μm), likely to be transported from the site of burning. The median (range) PyC production across all burns was 16.0 (11.5) % of total carbon exposed (TCE), with HyPyC accounting for 2.5 (4.9) % of TCE. Both PyC and HyPyC were dominantly partitioned into the proximal flux. Production of HyPyC was strongly related to fire residence time, with shorter duration fires resulting in higher HyPyC yields. The carbon isotope (δ13C) compositions of PyC and HyPyC were generally lower by 1–3‰ relative to the original biomass, with marked depletion up to 7‰ for grasslands dominated by C4 biomass. δ13C values of CO2 produced by combustion were computed by mass balance and ranged from ~0.4 to 1.3‰. The depletion of 13C in PyC and HyPyC relative to the original biomass has significant implications for the interpretation of δ13C values of savanna soil organic carbon and of ancient PyC preserved in the geologic record, as well as for global 13C isotopic disequilibria calculations.

Highlights

Pyrogenic carbon (PyC) describes carbon (C) in a continuum of thermally altered materials produced by incomplete biomass combustion and ranging from partly charred organic matter to condensed polyaromatic compounds (Hammes et al, 2007; Masiello, 2004)
Because PyC represents a continuum of materials of various degradabilities, we focus on the quantification of the refractory PyC component (HyPyC, polyaromatic ring number > 7; see methods for full definition), likely to have a residence time in the environment on at least the centennial scale
The production of PyC and HyPyC was quantified in 16 experimental fires conducted along a transect of sites in northern Australian savannas

Summary

Introduction

Pyrogenic carbon (PyC) describes carbon (C) in a continuum of thermally altered materials produced by incomplete biomass combustion and ranging from partly charred organic matter to condensed polyaromatic compounds (Hammes et al, 2007; Masiello, 2004). Components of the PyC continuum have been referred to by a variety of other terms, including soot, char and black carbon (Bird and Gröcke, 1997; Seiler and Crutzen, 1980). This range of names reflects the compositional complexity of PyC and the wide array of analytical methods employed for its quantification (Hammes et al, 2007; Ascough et al, 2009). Calculations of the size of a global atmospheric C sink to PyC are complicated because some components of PyC appear to be susceptible to degradation on comparatively short timescales (Bird et al, 1999; Zimmermann et al, 2012), while some are resistant to degradation, remain-

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