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

[摘要] Widespread burning of mixed tree–grass ecosystems represents the majornatural locus of pyrogenic carbon (PyC) production. PyC is a significant,pervasive and yet poorly understood "slow-cycling" form of carbon presentin the atmosphere, hydrosphere, soils and sediments. We conducted 16experimental burns on a rainfall transect through northern Australiansavannas with C₄ grasses ranging from 35 to 99% of total biomass.Residues from each fire were partitioned into PyC and further intorecalcitrant (HyPyC) components, with each of these fluxes also partitionedinto proximal components (>125 μm), likely to remain close to the site ofburning, and distal components (<125 μm), likely to be transported from thesite 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 theproximal flux. Production of HyPyC was strongly related to fire residencetime, with shorter duration fires resulting in higher HyPyC yields. Thecarbon isotope (δ¹³C) compositions of PyC and HyPyC were generallylower by 1–3‰ relative to the original biomass, with markeddepletion up to 7‰ for grasslands dominated by C₄ biomass.δ¹³C values of CO₂ produced by combustion were computed by massbalance and ranged from ~0.4 to 1.3‰. The depletion of¹³C in PyC and HyPyC relative to the original biomass has significantimplications for the interpretation of δ¹³C values of savanna soilorganic carbon and of ancient PyC preserved in the geologic record, as wellas for global ¹³C isotopic disequilibria calculations.

[学科分类] 地球化学与岩石