This review highlights the ubiquity of black carbon (BC) produced by incomplete combustion of plant material and fossil fuels in peats, soils, and lacustrine and marine sediments. We examine various definitions and analytical approaches and seek to provide a common language. BC represents a continuum from partly charred material to graphite and soot particles, with no general agreement on clear-cut boundaries. Formation of BC can occur in two fundamentally different ways. Volatiles recondense to highly graphitized soot-BC, whereas the solid residues form char-BC. Both forms of BC are relatively inert and are distributed globally by water and wind via fluvial and atmospheric transport. We summarize, chronologically, the ubiquity of BC in soils and sediments since Devonian times, differentiating between BC from vegetation fires and from fossil fuel combustion. BC has important implications for various biological, geochemical and environmental processes. As examples, BC may represent a significant sink in the global carbon cycle, affect the Earth's radiative heat balance, be a useful tracer for Earth's fire history, build up a significant fraction of carbon buried in soils and sediments, and carry organic pollutants. On land, BC seems to be abundant in dark-colored soils, affected by frequent vegetation burning and fossil fuel combustion, thus probably contributing to the highly stable aromatic components of soil organic matter. We discuss challenges for future research. Despite the great importance of BC, only limited progress has been made in calibrating analytical techniques. Progress in the quantification of BC is likely to come from systematic intercomparison using BCs from different sources and in different natural matrices. BC identification could benefit from isotopic and spectroscopic techniques applied at the bulk and molecular levels. The key to estimating BC stocks in soils and sediments is an understanding of the processes involved in BC degradation on a molecular level. A promising approach would be the combination of short-term laboratory experiments and long-term field trials.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GB001208

Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges

https://digital.csic.es/bitstream/10261/49123/3/Effect_of_fire.pdf

The effect of fire on soil organic matter--a review.

Oxides, Hydroxides, and Aluminosilicates

Direct molecular evidence for the degradation and mobility of black carbon in soils from ultrahigh-resolution mass spectral analysis of dissolved organic matter from a fire-impacted forest soil

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±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 2000 μm) 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 centennial timescales. OREC abundance and carbon-isotope data suggest that OREC in coarse particles is progressively degraded into finer particle sizes, with a concomitant increase in resistance to oxidative degradation of OREC in the finer 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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GB900067

Stability of elemental carbon in a savanna soil

Dissolution of allophane by acid oxalate solution

Japan is a typical volcanic country Volcanic ash soils which develop in deposits of volcanic ash are widely distributed in Japan Studies have shown that an important feature of humus in the A horizons of volcanic ash soils is the predominance of Type A humic acids with a high degree of humification (1, 2) The formation of Type A humic acids may proceed with the mediation of soil enzymes and inorganic components, since they are capable of accelerating the conversion of phenolic compounds to humic polymers with a high degree of darkening (3–5) As another possible mode, the authors suggested that the charring process of plants might contribute to the formation of Type A humic acids, because the abundant accumulation of humus in volcanic ash soils may be attributed to the presence of a dense grass vegetation and to the burning of these materials (6) Furthermore, carbonaceous materials such as charcoal and cinder are considered to be one of the possible sources of soil humus (7) In the present study, to 

Humus Composition of Charred Plant Residues

The physico-chemical and spectroscopic properties of humic acids from the charred residues of Susuki (Eulalia, Miscanthus sinensis A.) and from the A horizons of volcanic ash soils were compared. The humic acid (M-HA) of charred Susuki residues belonged to Type A. The elementary composition (C: 61.0%. H: 3.1%. N: 1.5%.0: 34.4%), δlog K (0.57), and RF (97) values of M-HA, except for N content, fell Within the ranges of those of the Type A humic acids in the A horizons of volcanic ash soils. The UV, visible, and IR absorption spectra of M-HA were similar to those of the Type A soil humic acids. Like the Type A soil humic acids, the X-ray diffraction pattern of the M-HA revealed the presence of two peaks at d=3.5 and 2.1A, corresponding to the (002)- and (10)-bands of graphite, respectively. The Fourier transformation method of (002)-band enabled to show that the M-HA mainly consisted of stacked fractions of two and three layers, as in the case of the Type A soil humic acids.

Comparison of humic acids from charred residues of Susuki (Eulalia, Miscanthus sinensis A.) and from the A horizons of volcanic ash soils.

The catalytic agents which are responsible for the oxidative coupling activity in citrate buffer extracts of paddy and upland soils and the changes in the oxidative coupling activity of soil under different incubation conditions were investigated. The oxidative coupling activity was determined using 2,6-dimethoxyphenol (2,6-DMP) as a substrate. The oxidative coupling activity of 5 paddy soil extracts ranged from 0.2 to 6.2 units, while that of 5 upland soil extracts from 0.1 to 5.5 units. The oxidative coupling activity was highly correlated with the amounts of Mn in the soil extracts (r = 0.818, significance at 1% level). Synthetic Mn(IV) oxide catalyzed the oxidative coupling of 2,6-DMP, but synthetic Fe(III), Al, and Si oxides and Mn(ll) salt had no effect on the coupling. Furthermore, when Mn oxides were selectively removed from the soil, the subsequent extract was inactive. Although the oxidative coupling enzymes contain Cu or Fe as active factor, the metals had no effect on the coupling of 2,6-DMP. ...

https://www.tandfonline.com/doi/pdf/10.1080/00380768.1989.10434793

Toshio Higashi

Papers

Dissolution of allophane by acid oxalate solution

Humus Composition of Charred Plant Residues

Comparison of humic acids from charred residues of Susuki (Eulalia, Miscanthus sinensis A.) and from the A horizons of volcanic ash soils.

Oxidative coupling activity in citrate buffer extracts of soils