Pyrogenic Organic Carbon Research Articles

Mapping soil organic carbon fractions for Australia, their stocks, and uncertainty

Abstract. Soil organic carbon (SOC) is the largest terrestrial carbon pool. SOC is composed of a continuous set of compounds with different chemical compositions, origins, and susceptibilities to decomposition that are commonly separated into pools characterised by different responses to anthropogenic and environmental disturbance. Here we map the contribution of three SOC fractions to the total SOC content of Australia's soils. The three SOC fractions, mineral-associated organic carbon (MAOC), particulate organic carbon (POC), and pyrogenic organic carbon (PyOC), represent SOC composition with distinct turnover rates, chemistry, and pathway formation. Data for MAOC, POC, and PyOC were obtained with near- and mid-infrared spectral models calibrated with measured SOC fractions. We transformed the data using an isometric-log-ratio (ilr) transformation to account for the closed compositional nature of SOC fractions. The resulting back-transformed ilr components were mapped across Australia. SOC fraction stocks for 0–30 cm were derived with maps of total organic carbon concentration, bulk density, coarse fragments, and soil thickness. Mapping was done by a quantile regression forest fitted with the ilr-transformed data and a large set of environmental variables as predictors. The resulting maps along with the quantified uncertainty show the unique spatial pattern of SOC fractions in Australia. MAOC dominated the total SOC with an average of 59 % ± 17 %, whereas 28 % ± 17 % was PyOC and 13 % ± 11 % was POC. The allocation of total organic carbon (TOC) to the MAOC fractions increased with depth. SOC vulnerability (i.e. POC/[MAOC+PyOC]) was greater in areas with Mediterranean and temperate climates. TOC and the distribution among fractions were the most influential variables in SOC fraction uncertainty. Further, the diversity of climatic and pedological conditions suggests that different mechanisms will control SOC stabilisation and dynamics across the continent, as shown by the model covariates' importance metric. We estimated the total SOC stocks (0–30 cm) to be 13 Pg MAOC, 2 Pg POC, and 5 Pg PyOC, which is consistent with previous estimates. The maps of SOC fractions and their stocks can be used for modelling SOC dynamics and forecasting changes in SOC stocks as a response to land use change, management, and climate change.

Dissolved pyrogenic carbon leaching in soil: Effects of soil depth and pyrolysis temperature

Pyrogenic organic carbon (PyC) is found in soils as a heterogeneous mixture of thermally altered plant residues that range in their susceptibility to losses via mineralization, leaching, and erosion. Leaching of PyC as DOC (DPyC) within the soil profile is likely influenced by the chemical composition of solid PyC and the dominant soil processes and properties at a particular depth. Here we report the results of a 2-year laboratory decomposition and leaching study designed to investigate the interactive effects of pyrolysis temperature levels (no pyrolysis, 300 °C, and 450 °C) and soil depth (surface soil, 0–10 cm depth; subsurface soil, 50–70 cm depth) on losses of 13C-labeled jack pine (Pinus banksiana) wood, wood pyrolyzed at 300 °C (PyC300) and wood pyrolyzed at 450 °C (PyC450) as dissolved organic carbon (DOC). Losses of wood and PyC in the 13C- DOC pool were measured in leachates drained from soils once a month over the course of 1 year, and at one single leaching event after 2 years. We found that pyrolysis temperature levels interacted with time and soil depth to affect losses of wood and PyC as DOC throughout the 1-year incubation and leaching study, with greater DOC losses from wood in surface soil (0.73 ± 0.076% of added C) than in subsurface soil (0.40 ± 0.063% of added C) averaged across sampling time. Monthly DOC losses from PyC were not affected by soil depth. Cumulative data indicated a small contribution of wood (2.86 ± 0.07%), PyC300 (0.40 ± 0.04%), and PyC450 (0.16 ± 0.01%) to total DOC leached from soils. DOC losses from wood and PyC300 (as a proportion of added C) were greater in surface soil than in subsurface soil, whereas DOC losses from PyC450 were unaffected by differences between surface and subsurface soil. Losses of DOC were greater from PyC300 than from PyC450 in surface soil, with no significant differences in DOC losses between PyC300 and PyC450 in subsurface soil. One single leaching event at the end of the 2-year decomposition study resulted in higher DOC losses from wood in subsurface soil than in surface soil likely due to desorption, and no differences in DOC losses between PyC300 and PyC450 regardless of soil depth. Our results suggest strong interactions between the initial physicochemical composition of organic C inputs and soil properties (soil depth as a proxy) that control the mobility and transport of PyC and should be better represented in Earth system models.

Long-term bare-fallow soil fractions reveal thermo-chemical properties controlling soil organic carbon dynamics

Abstract. Evolution of organic carbon content in soils has the potential to be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding soil carbon dynamics is a challenge due to a wide range of residence times of soil organic matter and limited constraints on the mechanisms influencing its persistence. In particular, large uncertainties exist regarding the persistence of pyrogenic organic carbon in soils. In order to characterize organic matter with varying degrees of persistence and to distinguish pyrogenic organic carbon, we combined Rock-Eval analysis, a thermo-chemical method, with the benzene polycarboxylic acid molecular marker method and Raman spectroscopy to characterize samples from long-term bare-fallow experiments, progressively depleted in the most labile organic carbon over time. Considering the heterogeneity of soil samples, size fractions have been separated to distinguish pools of organic carbon with distinct properties. We observe that organic carbon dynamics is dependent on granulometry. A pool of organic carbon with intermediate residence times, from years to a few decades, representing ca. 65 % of the bulk soil organic carbon stock, is mainly associated with fine fractions (< 20 µm). With time under bare fallow, this organic carbon is progressively transferred towards finer fractions through the breakdown of organic matter. Coarse fractions (> 20 µm) are rich in centennially persistent organic carbon, representing ca. 20 % of the initial organic carbon stock, due to the chemical recalcitrance of organic matter in these fractions, dominated by pyrogenic organic carbon. A second pool of persistent organic carbon, representing ca. 15 % of the initial organic carbon stock, is associated with the clay fraction, indicating mechanisms of protection occurring at the submicron scale (< 2 µm). This persistent organic carbon only represents 30 % of the organic carbon initially present in the clay fraction. Persistent organic carbon exhibits heterogeneous chemical signatures depending on the considered pool but a consistent thermal signature demonstrating the relationship between thermal stability and biogeochemical stability of soil organic carbon. This gives the possibility of assessing the size of the persistent organic carbon pool in soils using thermal parameters. The persistence of pyrogenic organic carbon in the clay fraction is similar to the one of total organic carbon. The different persistence of coarse and fine pyrogenic organic carbon resides in the highly condensed nature of sand-sized pyrogenic carbon, which may result from burning temperatures over 700 ∘C. Pyrogenic organic carbon is not inert but exhibits a consistent increase in the level of condensation with time in all size fractions, showing the role of chemical quality in pyrogenic organic carbon persistence. Overall, this study helps improve the separation, evaluation and characterization of carbon pools with distinct residence times in soils and gives insight into the mechanistic origin of soil organic carbon dynamics.

Pyrolysis temperature and soil depth interactions determine PyC turnover and induced soil organic carbon priming

Pyrogenic organic carbon (PyC) is a complex, heterogeneous class of thermally altered organic substrates, but its dynamics and how its behavior changes with soil depth remain poorly understood. We conducted a laboratory incubation study to investigate the interactive effects of pyrolysis temperature and soil depth on the turnover of PyC compared to its precursor wood and native SOC (NSOC). We incubated dual-labeled (13C and 15N) jack pine pyrogenic organic matter produced at 300 °C (PyC300), 450 °C (PyC450), and their precursor pine wood in a fine-loamy, mixed-conifer forest soil for 745 days. A mixture of surface (0–10 cm) and subsurface (50–70 cm) forest soils, with and without labeled biomass were incubated in the dark at 55% soil water field capacity and 25 °C. Total 13C from PyC and wood mineralized as 13C-CO2 (as % of C added to soil) declined with an increase in pyrolysis temperature as follows: 54 ± 7.7% for wood, 3.1 ± 0.2% for PyC300, and 0.94 ± 0.08% for PyC450. After 2 years, soil depth interacted with pyrolysis temperature to affect C turnover, with total wood C losses significantly declining from 70.6% in surface soils to 37.5% in subsurface soil, while total losses of PyC300 and PyC450 were unaffected by differences between surface and subsurface soils. Wood induced negative priming (i.e., decreased mineralization rates) in surface soil at days 3 and 60, while PyC300 induced positive priming (i.e., increased mineralization rates) in subsurface soil at day 60. After 2 years, unlabeled NSOC losses increased from 9.2 ± 0.8% of NSOC in unamended treatments to 16.5 ± 2.6% of NSOC with PyC450 additions. Our results suggest that PyC pyrolyzed at a given temperature can mineralize at similar rates between soil depths, and high amounts of PyC450 in subsurface soils can stimulate NSOC losses. These findings indicate that soil depth imposes critical controls on PyC dynamics belowground.

Massive carbon addition to an organic-rich Andosol increased the subsoil but not the topsoil carbon stock

Abstract. Andosols are among the most carbon-rich soils, with an average of 254 Mg ha−1 organic carbon (OC) in the upper 100 cm. A current theory proposes an upper limit for OC stocks independent of increasing carbon input, because of finite binding capacities of the soil mineral phase. We tested the possible limits in OC stocks for Andosols with already large OC concentrations and stocks (212 g kg−1 in the first horizon, 301 Mg ha−1 in the upper 100 cm). The soils received large inputs of 1800 Mg OC ha−1 as sawdust within a time period of 20 years. Adjacent soils without sawdust application served as controls. We determined total OC stocks as well as the storage forms of organic matter (OM) of five horizons down to 100 cm depth. Storage forms considered were pyrogenic carbon, OM of < 1.6 g cm−3 density and with little to no interaction with the mineral phase, and strongly mineral-bonded OM forming particles of densities between 1.6 and 2.0 g cm−3 or > 2.0 g cm−3. The two fractions > 1.6 g cm−3 were also analysed for aluminium-organic matter complexes (Al–OM complexes) and imogolite-type phases using ammonium-oxalate–oxalic-acid extraction and X-ray diffraction (XRD). Pyrogenic organic carbon represented only up to 5 wt % of OC, and thus contributed little to soil OM. In the two topsoil horizons, the fraction between 1.6 and 2.0 g cm−3 had 65–86 wt % of bulk soil OC and was dominated by Al–OM complexes. In deeper horizons, the fraction > 2.0 g cm−3 contained 80–97 wt % of the bulk soil's total OC and was characterized by a mixture of Al–OM complexes and imogolite-type phases, with proportions of imogolite-type phases increasing with depth. In response to the sawdust application, only the OC stock at 25–50 cm depth increased significantly (α=0.05, 1-β=0.8). The increase was entirely due to increased OC in the two fractions > 1.6 g cm−3. However, there was no significant increase in the total OC stocks within the upper 100 cm. The results suggest that long-term large OC inputs cannot be taken up by the obviously OC-saturated topsoil but induce downward migration and gradually increasing storage of OC in subsurface soil layers. The small additional OC accumulation despite the extremely large OC input over 20 years, however, shows that long time periods of high input are needed to promote the downward movement and deep soil storage of OC.

Pyrogenic Carbon Lacks Long-Term Persistence in Temperate Arable Soils

Pyrogenic organic carbon (PyOC) derived from incomplete burning of biomass is considered the most persistent fraction of soil organic carbon (SOC), being expected to remain in soil for centuries. However, PyOC persistence has seldom been evaluated under field conditions. Based on a unique set of soils from five European long-term bare fallows (LTBF), i.e., vegetation-free field experiments, we provide the first direct comparison between PyOC and SOC persistence in temperate arable soils. We found that soil PyOC contents decreased more rapidly than expected from current concepts, the mean residence time (MRT) of native PyOC being just 1.6 times longer than that of SOC. At the oldest experimental site, 55% of the initial PyOC remained after 80 years of bare fallow. Our results suggest that while the potential for long-term C storage exists, the persistence of PyOC in soil may currently be overestimated.

Trial by Fire: On the Terminology and Methods Used in Pyrogenic Organic Carbon Research

Our understanding of the cycling of fire-derived, i.e. pyrogenic organic matter (pyOM), as well as the goals of the community of researchers who study it, may be inhibited by the many terms and methods currently used in its quantification and characterization. Terms currently used for pyOM have evolved by convention, but are often poorly defined. Further, each of the different methods now used to quantify solid and dissolved pyrogenic carbon (pyC) comes with its own biases and artifacts. That is, each detects only a fraction of the total pyrogenic products produced by fire, while, at the same time, include some fraction of non-pyrogenic OM. This may be evident in the commonly observed correlations between pyC and total organic C reported for both soils and dissolved OM in many different systems. We suggest that our research area can be placed on a stronger footing by: 1) agreement upon a common set of terms tied to the method used for detection (e.g. of the form pyCmethod), 2) implementation of another ‘ring trial’ study with a wider set of natural soil and water samples that cross-compare more recently developed methods, and 3) further investigation of the processes which preserve/degrade/transport pyOM in the environment.

13C-NMR, PAHs, WSOC and water repellence of fire-affected soils (Albic Podzols) in lichen pine forests, Russia

Soil organic matter (SOM) in boreal forest ecosystems can be strongly transformed by fires which, in turn, can affect soil physical and biological properties such as water repellency and nutrient cycling. The article contains the results of a study on SOM content in fire-affected lichen pine forests of the middle taiga subzone of the European part of Russia (Komi Republic). Soils which survived surface fires 2, 10, 16 years ago and unburned area were studied. We investigated changes in SOM composition using 13C NMR spectroscopy, and we measured water-soluble organic carbon (WSOC) content, polycyclic aromatic hydrocarbons (PAHs) concentrations, and contact angles of water droplets on soils. Forest fires increased pyrogenic organic carbon content and also largely transformed SOM composition. 13C NMR spectroscopy showed that the SOM composition of densimetric fractions in pyrogenic and unburned soil greatly differs. Fires increased the aromatic content in SOM composition of light fractions. The highest amount of aromatic carbon probably pyrogenic origin accumulated in fractions of free and occluded organic matter. According to 13C NMR spectroscopy, the aromatic component of organic matter increased in the pyrogenic horizons. The strongest PAHs concentrations occurred in the fractions of free particulate and occluded organic matter, in reference and fire-affected soils. The presence of PAHs in light densimetric fractions may serve as markers of past fire and fire intensity. In the initial years after fire, there was a decrease in the content of WSOC compounds; however, the WSOC content recovered with the recovery of ground vegetation. Our results showed that the greatest changes of contact angles were in upper pyrogenic soil horizons, whereas fires did not modify the contact angle value of lower mineral horizons.

Carbon and nitrogen molecular composition of soil organic matter fractions resistant to oxidation

The methods used to isolate and characterise pyrogenic organic carbon (PyC) from soils vary widely, and there is little agreement in the literature as to which method truly isolates the most chemically recalcitrant (inferred from oxidative resistance) and persistent (inferred from radiocarbon abundance) fraction of soil organic matter. In addition, the roles of fire, fuel type and soil morphology in the preservation of PyC are not yet defined. In an attempt to elucidate the importance of oxidative recalcitrance, fuel type and soil morphology to the persistence of soil organic matter, we examined two strongly contrasting soils using a variety of PyC isolation techniques coupled with quantifications of the molecular structure and mean residence time of the isolated organic materials. Surface and subsurface soil samples were examined from a Red Chromosol soil and a Black Vertosol soil. The δ13C values suggest that PyC in the Red Chromosol was sourced from eucalyptus, whereas PyC in the Black Vertosol was formed from grass. Soils were sieved at 53µm, treated with hydrofluoric acid to remove organics associated with mineral surfaces, then subjected to three common ‘PyC isolation’ treatments: chromic acid, photo-oxidation and chromic acid followed by photo-oxidation. Molecular structure of the organic residues remaining after each treatment was quantified by solid-state 13C cross polarisation magic angle spinning nuclear magnetic resonance and near edge X-ray absorption fine structure spectroscopy, and the mean residence time of the organic residues was estimated based on radiocarbon abundance. In all cases, treatment with chromic acid followed by photo-oxidation isolated the smallest proportion of organic matter (5–10% of <53µm C) which also had the longest mean residence time (estimated 600–3460 years). Additionally, molecular structure measurements indicated that this fraction was not composed solely of aromatic compounds, suggesting a non-homogenous source for the most oxidative-resistant fraction of soil organic matter.

Micropore clogging by leachable pyrogenic organic carbon: A new perspective on sorption irreversibility and kinetics of hydrophobic organic contaminants to black carbon

Black carbon (BC) plays a crucial role in sequestering hydrophobic organic contaminants in the environment. This study investigated key factors and mechanisms controlling nonideal sorption (e.g., sorption irreversibility and slow kinetics) of model hydrophobic organic contaminants (nitrobenzene, naphthalene, and atrazine) by rice-straw-derived BC. After removing the fraction of leachable pyrogenic organic carbon (LPyOC) (referring to composites of dissoluble non-condensed organic carbon and associated mineral components) with deionized water or 0.5 M NaOH, sorption of these sorbates to BC was enhanced. The sorption enhancement was positively correlated with sorbate molecular size in the order of atrazine > naphthalene > nitrobenzene. The removal of LPyOC also accelerated sorption kinetics and reduced sorption irreversibility. These observations were attributed to increased accessibility of BC micropores initially clogged by the LPyOC. Comparison of BC pore size distributions before and after atrazine sorption further suggested that the sorbate molecules preferred to access the micropores that were more open, and the micropore accessibility was enhanced by the removal of LPyOC. Consistently, the sorption of nitrobenzene and atrazine to template-synthesized mesoporous carbon (CMK3), a model sorbent with homogeneous pore structures, showed decreased kinetics, but increased irreversibility by impregnating sorbent pores with surface-grafted alkylamino groups and by subsequent loading of humic acid. These findings indicated an important and previously unrecognized role of LPyOC (i.e., micropore clogging) in the nonideal sorption of organic contaminants to BC.

Cupric Oxide (CuO) Oxidation Detects Pyrogenic Carbon in Burnt Organic Matter and Soils.

Wildfire greatly impacts the composition and quantity of organic carbon stocks within watersheds. Most methods used to measure the contributions of fire altered organic carbon–i.e. pyrogenic organic carbon (Py-OC) in natural samples are designed to quantify specific fractions such as black carbon or polyaromatic hydrocarbons. In contrast, the CuO oxidation procedure yields a variety of products derived from a variety of precursors, including both unaltered and thermally altered sources. Here, we test whether or not the benzene carboxylic acid and hydroxy benzoic acid (BCA) products obtained by CuO oxidation provide a robust indicator of Py-OC and compare them to non-Py-OC biomarkers of lignin. O and A horizons from microcosms were burned in the laboratory at varying levels of fire severity and subsequently incubated for 6 months. All soils were analyzed for total OC and N and were analyzed by CuO oxidation. All BCAs appeared to be preserved or created to some degree during burning while lignin phenols appeared to be altered or destroyed to varying extents dependent on fire severity. We found two specific CuO oxidation products, o-hydroxybenzoic acid (oBd) and 1,2,4-benzenetricarboxylic acid (BTC2) that responded strongly to burn severity and withstood degradation during post-burning microbial incubations. Interestingly, we found that benzene di- and tricarboxylic acids (BDC and BTC, respectively) were much more reactive than vanillyl phenols during the incubation as a possible result of physical protection of vanillyl phenols in the interior of char particles or CuO oxidation derived BCAs originating from biologically available classes of Py-OC. We found that the ability of these compounds to predict relative Py-OC content in burned samples improved when normalized by their respective BCA class (i.e. benzene monocarboxylic acids (BA) and BTC, respectively) and when BTC was normalized to total lignin yields (BTC:Lig). The major trends in BCAs imparted by burning persisted through a 6 month incubation suggesting that fire severity had first order control on BCA and lignin composition. Using original and published BCA data from soils, sediments, char, and interfering compounds we found that BTC:Lig and BTC2:BTC were able to distinguish Py-OC from compounds such as humic materials, tannins, etc. The BCAs released by the CuO oxidation procedure increase the functionality of this method in order to examine the relative contribution of Py-OC in geochemical samples.

Effects of fire on soil organic matter quality along an altitudinal sequence on Mt. Etna, Sicily

Abstract Soil organic matter (SOM) can directly be affected by wildfires, both in terms of quantity and quality. The combustion is often incomplete, leading to the formation of pyrogenic organic matter (PyOM), a highly heterogeneous material with prevailing aromatic nature. Focusing our attention on PyOM, we investigated an altitudinal soil sequence on Mount Etna, Italy, ranging from 500 m to 1800 m asl, where the fire frequency is assumed to have been higher at lower elevations due to a warmer and drier climate. We evaluated the effect of fire on the chemical and physical characteristics of SOM along the altitudinal sequence. At two sites at a similar altitude but having a different recent fire history, we also examined the effect of fire frequency on SOM. Chemical oxidation of SOM with acid dichromate was used to evaluate the contribution of pyrogenic organic carbon to total SOC. Furthermore, four SOM density fractions were separated and characterised for principal composition, spectroscopic properties by Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Nuclear Magnetic Resonance (NMR), and particle morphology by Scanning Electron Microscopy (SEM). The stocks of C and N as well as the chemical oxidation resistant carbon (COREC) increased with decreasing altitude, hence supporting the hypothesis of higher fire frequency at lower altitudes. The highest C concentrations were found in the density fraction 1.0–1.6 g cm − 3 . At the lower sites, a considerable amount of aromatics, and hence a significant part of COREC, was found in the density fraction 1.0–1.6 g cm − 3 except for (macro)charcoal that occurs in the lightest fraction ( − 3 ). The properties of this latter density fraction seemed to be well related to vegetation characteristics. The dichromate oxidation procedure, however, has methodological restrictions. This procedure resulted here to be poorly specific for PyOM in the investigated soils, since it isolated a fraction rich in lipids. The lack of specificity for PyOM is most likely due to the rather low content of aromatics in SOM, although fire frequency was relatively high.

A simplified method for the quantification of pyrogenic organic matter in grassland soils via chemical oxidation

The elucidation of the impact of pyrogenic organic carbon (PyOC) on soil ecological functions and its role within geobiochemical cycles requires a quantification that is not restricted to a certain window of the Black Carbon continuum. In the present study the method of chemical oxidation with acid dichromate was modified to allow for the determination of the whole range of PyOC in soil systems frequently subjected to prescribed fires with a fairly homogenous intensity distribution. After confirming the reproducibility of the technique, top and subsoils of Leptosols and Umbrisols from a grassland ( Campo) landscape in the Planalto region, Southern Brazil were demineralized with hydrofluoric acid and subsequently the residues were chemically oxidized. Solid-state 13C NMR spectroscopy was applied to distinguish between chemical oxidation resistant elemental C (COREC) of pyrogenic organic carbon (PyOC) and lipid components surviving the harsh treatment because of their hydrophobic properties. Correlation of the aromatic COREC content of the samples with their aromatic C content resulted in a positive correlation with R 2 = 0.74 for the topsoils and R 2 = 0.53 for the subsoils. The correlation functions allowed the development of an equation that yields in a correction factor f, which is necessary to account for the C-losses of PyOC caused by the chemical oxidation and thus for the estimation of its original contribution to the soil. For the topsoils this correction factor was comparable to that determined for a model char derived from the Campo vegetation after charring for 4 min at 350 °C. Applying this factor to Campo plant-char/soil mixtures resulted in recoveries between 93 and 125%, indicating that it should be possible to elucidate the PyOC content of soils by the determination of COREC arom and subsequent multiplication with f. In soil systems with char input of fairly consistent chemical composition, as they occur in prescribed grassland fires, f may be directly obtained from model chars produced from the respective vegetation cover.

Diffraction by line screens : L. R. Wilberforce. ( Phil. Mag., Nov., 1925.)

The elucidation of the impact of pyrogenic organic carbon (PyOC) on soil ecological functions and its role within geobiochemical cycles requires a quantification that is not restricted to a certain window of the Black Carbon continuum. In the present study the method of chemical oxidation with acid dichromate was modified to allow for the determination of the whole range of PyOC in soil systems frequently subjected to prescribed fires with a fairly homogenous intensity distribution. After confirming the reproducibility of the technique, top and subsoils of Leptosols and Umbrisols from a grassland (Campo) landscape in the Planalto region, Southern Brazil were demineralized with hydrofluoric acid and subsequently the residues were chemically oxidized. Solid-state 13C NMR spectroscopy was applied to distinguish between chemical oxidation resistant elemental C (COREC) of pyrogenic organic carbon (PyOC) and lipid components surviving the harsh treatment because of their hydrophobic properties. Correlation of the aromatic COREC content of the samples with their aromatic C content resulted in a positive correlation with R2 = 0.74 for the topsoils and R2 = 0.53 for the subsoils. The correlation functions allowed the development of an equation that yields in a correction factor f, which is necessary to account for the C-losses of PyOC caused by the chemical oxidation and thus for the estimation of its original contribution to the soil. For the topsoils this correction factor was comparable to that determined for a model char derived from the Campo vegetation after charring for 4 min at 350 °C. Applying this factor to Campo plant-char/soil mixtures resulted in recoveries between 93 and 125%, indicating that it should be possible to elucidate the PyOC content of soils by the determination of CORECarom and subsequent multiplication with f. In soil systems with char input of fairly consistent chemical composition, as they occur in prescribed grassland fires, f may be directly obtained from model chars produced from the respective vegetation cover.