Rapid degradation of pyrogenic carbon

Abstract

Pyrogenic carbon (PC‐ charcoal, biochar or black carbon) represents a poorly understood component of the global carbon (C) cycle, but one that has considerable potential to mitigate climate change through provision of long‐term soil C sequestration. Mass balance calculations suggest global PC production and stocks are not in balance, indicating a major gap in our understanding of the processes by which PC is re‐mineralized. We collected PC samples derived from the same wood material and exposed to natural environmental conditions for 1 and 11 years. We subjected these materials to repeated laboratory incubation studies at temperatures of up to 60 °C, as ground surface temperatures above 30 °C and up to 60 °C occur regularly over a significant area of the tropics and sub‐tropics. Mineralization rates were not different for the two samples and followed an exponential Arrhenius function that suggest an average turnover time of 67 years for conditions typical of a tropical savannah environment. Microbial biomass as measured by chloroform fumigation and DNA extractions was the same for the two samples, but abiotic CO2 production was lower for the fresh PC sample than that for the aged sample. Nuclear magnetic resonance spectroscopy, hydrogen pyrolysis and scanning electron microscopy demonstrate that the measured CO2 production originates dominantly from polycyclic aromatic compounds rather than any minor labile components. Therefore, rapid, sub‐centennial rates of re‐mineralization of PC on the soil surface in tropical and sub‐tropical environments may represent a major and hitherto unidentified mechanism for balancing the PC production at the global scale.

Preferential Production and Transport of Grass-Derived Pyrogenic Carbon in NE-Australian Savanna Ecosystems

“…Sediment samples containing over 20 mg g −1 TOC showed relatively uniform PyC contributions to TOC (∼7%; Figure 4), broadly consistent with the global observations compiled by Reisser et al (2016) for soil samples collected from the 0-30 cm depth interval. Belowground OM inputs are directly subject to physicochemical protection offered by the soil matrix, but charcoal or PyC is overwhelmingly produced aboveground, and thus, is more prone to mineralization and/or transport from the site of production (Zimmermann et al, 2012;Bird et al, 2015). Rumpel et al (2006) worked along the slopes of slash and burn agricultural land and noted that PyC from topsoil horizons was not primarily bound to mineral phases, which made it susceptible to translocation downslope.…”

“…This particle size fraction has been shown to remain initially in the vicinity of the site of production (Clark, 1988;Saiz et al, 2015a), and was therefore classified as "proximal" PyC. Once produced, PyC remaining on the soil surface may subsequently (i) be re-combusted in future fires (Santín et al, 2013;Saiz et al, 2014) (ii) be re-mineralized by abiotic/biotic processes (Zimmerman, 2010;Zimmermann et al, 2012), (iii) accumulate in the SOC pool (Lehmann et al, 2008) and/or (iv) be re-mobilized by bioturbation, wind or water in either particulate (Rumpel et al, 2006;Major et al, 2010;Cotrufo et al, 2016) or dissolved form Dittmar et al (2012). Table 1 reveals that proximal PyC contained in small-size fractions on the soil surface is gradually lost after a fire, thereby increasing the relative contribution of the largest size fraction to the total carbon pool over time.…”

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

“…In order to better constrain the global PyC cycle, there is an obvious need for a deeper understanding of the factors controlling formation, translocation and mineralisation of PyC and its recalcitrant compounds, represented here by HyPyC (Bird et al, 2015;Conedera et al, 2009;Zimmermann et al, 2012). In this context, the partitioning of PyC between the proximal and distal fluxes may have a strong influence on preservation potential (Thevenon et al, 2010).…”

“…PyC that remains on the ground will potentially (i) be re-combusted in subsequent fire events (Saiz et al, 2014;Santín et al, 2013) (ii) be re-mineralised by biotic/abiotic processes (Saiz et al, 2014;Santín et al, 2013;Zimmerman, 2010;Zimmermann et al, 2012), (iii) be remobilised by bioturbation, wind or water in either particulate (Major et al, 2010;Rumpel et al, 2006) or dissolved form (Dittmar, 2008;Dittmar et al, 2012) and/or (iv) accumulate in the soil organic carbon (SOC) pool (Lehmann et al, 2008).…”

Biochar research activities and their relation to development and environmental quality. A meta-analysis

“…agricultural emissions of CH 4 and N 2 O (IPCC 2013). Although it is clear that biochar can reduce emissions of GHGs (Cayuela et al 2014;Borchard et al 2014c), stabilize labile organic matter (Borchard et al 2014a) and remain in agricultural soils for at least decades (Preston and Schmidt 2006;Borchard et al 2014b) contradictory findings (Wardle et al 2008;Borchard et al 2014c;Zimmermann et al 2012) elucidate needs to assess mechanisms controlling GHG emissions in more detail across varying environments and climatic zones (Schmidt et al 2011). …”