Stability of elemental carbon in a savanna soil

Abstract

Abstract. We have investigated the stability of oxidation-resistant elemental carbon (OREC) in a sandy savanna soil at the Matopos fire trial site, Zimbabwe. The protection of some soil plots from fire for the last 50 years at this site has enabled a comparison of OREC abundances between those plots which have been protected from fire and plots which have continued to be burnt. The total 0-5 cm OREC inventory of the soil protected from fire is estimated to be 2.0-k-_0.5 mg cm-2; approximately half the "natural" OREC inventory at the study site of 3.8+0.5 mg cm '2 (the mean for plots burnt every 1-5 years). The associated half-life for natural OREC loss from the 0-5 cm interval of the protected plots is calculated to be <100 years, with the half-life for large carbonized particles (>2000 gm) in the soil being considerably <50 years. These results suggest that at least in well-aerated tropical soil environments, charcoal and OREC can be can be significantly degraded on decadal to centennia• timesca,es. OREC abundance and carbon-isotope data suggest that OREC in coarse particles is progressively degraded into fmer particle sizes, with a concomitant increase in resistance to oxidafive degradation of OREC in the fmer particle sizes due to the progressive loss of more readily degraded OREC. It remains unclear whether the OREC that is degraded is oxidized completely to CO2 and subsequently emitted from the soil, reduced to a sufficiently small particle size to be illuviated to deeper parts of the soil profile, solubilized and lost from the profile as dissolved organic carbon or transmuted into a chemical form which is susceptible to attack by the acid-dichromate reagent. The conclusion that a significant proportion of OREC can undergo natural degradation in well-aerated environments on decadal/centennial timescales suggests that only a fraction of the total production of OREC from biomass burning and fossil fuel combustion is likely to be sequestered in the slow-cycling "geological" carbon reservoir.

How relevant is recalcitrance for the stabilization of organic matter in soils?

“…While individual charcoal particles found in soils have 14 C ages of up to several thousand years (Gavin, 2003;Sanborn et al, 2006), other studies have determined mean residence times of black C of only a few decades (Bird et al, 1999). These differences may largely be caused by different charring temperatures since Baldock and Smernick (2002) showed that the degradability of charred wood decreased with increasing heating temperature.…”

Biochar from different residues on soil properties and common bean production

“…This is probably because biochar also undergoes biodegradation, although it is considered stable in the soil system. According to Bird et al (1999), the time required for biodegradation of soil-charred particles is related to their granulometry. These authors estimated that the half-life of particles smaller than 2 mm is lower than 50 years and that of particles larger than 2 mm is lower than 100 years.…”

Technical Greenhouse-Gas Mitigation Potentials of Biochar Soil Incorporation in Germany

“…Beyond feedstock type and production conditions, a wide range of additional factors, such as soil type, climate, and other environmental conditions, might strongly influence biochar carbon stability in soils (e.g., Mašek et al 2013;Schmidt et al 2011;Bird et al 1999). For this reason, great uncertainties remain concerning the precise fraction of biochar carbon that remains stable in the long-term (e.g., Mašek et al 2013;Shackley et al 2011).…”

“…62 While Lehmann et al (2008) also arrived at MRTs of 718-9,259 years for biochar from vegetation fires in Australian soils, however, the MRT of naturally occurring biochar in Zimbabwean savannah soils ranged only from decades to centuries (Bird et al 1999) and that in Kenyan soils was even less with 8.3 years (Nguyen et al 2008). 63 These mixed results suggest that biochar stability in soil depends on a variety of factors and cannot be determined conclusively, not least due to methodological differences and shortcomings in quantifying biochar and/or its stable fraction in soils (e.g., Preston and Schmidt 2006;Hammes et al 2007).…”