Articles | Volume 12, issue 6
Research article
20 Mar 2015
Research article |  | 20 Mar 2015

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

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

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.

Short summary
Around half of all pyrogenic carbon (charcoal+soot) derived from wildfires comes from semi-annual burning of tropical savannas. This pyrogenic carbon is significant because it is a component of global aerosols capable of modulating the greenhouse effect and is resistant to degradation. We use controlled field burns in northern Australian savannas to determine how much pyrogenic carbon is formed, how much of this is recalcitrant and how it is partitioned between ground residues and airborne soot.
Final-revised paper