Pyrogenic Organic Matter Research Articles

Degradation of grass-derived pyrogenic organic material, transport of the residues within a soil column and distribution in soil organic matter fractions during a 28month microcosm experiment

The microbial recalcitrance of char accumulated after vegetation fires and its transport within a soil column were studied in microcosms using 13C- and 15N-enriched pyrogenic organic material (PyOM). The PyOM from rye grass (Lolium perenne L.) was produced by charring at 350 °C under oxic conditions for 1 and 4 min to examine the impact of the charring degree. After 28 months, 13C recovery decreased to values between 62% and 65%, confirming that this material can be attacked by microorganisms and that the degradation occurs rapidly after accumulation of PyOM at the soil surface. The respective 15N recovery followed the same trend but tended to be higher (between 67% and 80%). Most of the added PyOM isotopic labels were recovered in the particulate organic matter (POM) fraction, containing between 84% and 65% of the added 13C and 15N after the first 2 months, being reduced by half at the end of the experiment. After 1 month, up to 13.8% of the 13C label and 12.4% of the 15N label were detected in the POM-free mineral fractions. This fast association of PyOM with the mineral phase indicates that physical soil properties have to be considered for the elucidation of PyOM stability. Addition of fresh unlabelled grass material as co-substrate resulted in comparable trends as for the pure PyOM but the total recovery of the isotopic labels clearly increased with respect to the amount of mineral-associated PyOM. Between 73% and 82% of the mineral-associated PyOM occurred in the clay separates (<2 μm) for which the highest values were obtained for the experiment with the more intensively charred PyOM and co-substrate addition. In summary, the study demonstrates the degradability of grass-derived PyOM. The addition of fresh plant material as an easily degradable co-substrate promoted the formation of partially decomposed PyOM and subsequently its association with the mineral phase, but did not increase the respective mineralisation rates. Detection of 13C and 15N content at different depths of the microcosm column demonstrated an additional loss of PyOM from top soil by way of mobilisation and transport to deeper horizons. All these processes have to be taken into account in order to obtain a more realistic view about the behaviour of PyOM in environmental systems and for estimation of the C and N sequestration potential.

“Black nitrogen” – an important fraction in determining the recalcitrance of charcoal

The chemical and thermal stability of organic nitrogen (N) in pyrogenic organic matter (PyOM) was examined by charring casein at 350 °C and 450 °C. The alteration was compared with that observed for char derived from lignin, cellulose, grass and wood. With respect to heating, casein showed a considerably higher stability than cellulose. Comparable proportions of carbon (C) and N were recovered, supporting the idea that black nitrogen (BN) represents an integral part of the char structure. Although some amides were still present, they lost importance with increasing temperature. Charring of grass revealed an enrichment in N compounds because of the low thermal stability of cellulose. The similarity in the resulting nuclear magnetic resonance (NMR) spectra to those of casein char confirmed that BN can play a major role in the chemical composition of plant char. Subjecting the chars to oxidation with acidic dichromate demonstrated that, in spite of their relatively high resistance to heat, the N-containing compounds of the chars were less recalcitrant than the components of the cellulose char. Thus, in soil, N-rich chars are likely to be underestimated on the basis of this method. On the other hand, for an ancient paddy soil whose N-containing char compounds were calculated to account for ca. 25% of the total organic C in the soil. This clearly underlines the pedogenic stability of BN and confirms that it has the potential to contribute significantly to the refractory soil organic matter pool.

Thermal alteration of organic matter during a shrubland fire: A field study

Vegetation fires profoundly alter the C cycle of terrestrial ecosystems, notably through the potential formation of highly stable pyrogenic structures. Fire-induced changes in the structure of organic matter (OM) have been studied mainly under controlled laboratory conditions. The objective of this work was to characterise changes in OM chemistry occurring in the litter layer of a scrub–oak ecosystem subjected to a prescribed fire. Maximum temperatures reached during the fire were monitored with thermo-sensitive paint. Litter samples collected before and after the fire were subjected to size fractionation, each size fraction being divided on the basis of visual observation into burnt and unburnt components, i.e. black and brown, respectively. All fractions were analysed for C and N contents and stable carbon isotopic composition. Shifts in the composition of >2 mm fraction components were evaluated using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). The effects of charring were evaluated through comparisons of black vs. brown post-fire litter in >370 °C plots. Comparison of bulk pre- vs. post-fire litter proved unreliable because of the complicating effect of fresh litter fall during the fire event. Charring significantly increased the litter C content by 115–142 mg g −1 and significantly decreased the δ 13C value by an average of 0.8‰. The NMR and DSC analyses indicated that O-alkyl compounds were preferentially lost vs. aryl and alkyl compounds. This suggests a preferential loss of cellulose components and a relative preservation of lignin and lipids. However, the charred litter samples had a low degree of condensation vs. a graphitic-like model. The findings suggest that leaf-derived charcoal produced during natural vegetation fires does not contribute much to the highly stable fraction of pyrogenic OM.

Depletion of soil organic carbon and nitrogen under Pinus taeda plantations in Southern Brazilian grasslands (Campos)

SummaryEstablishment of pine (Pinus spp.) plantations on grasslands could increase carbon (C) sequestration to counteract increased atmospheric carbon dioxide concentrations. In the grasslands of the southern Brazilian highland (Campos), large areas have been converted to Pinus plantations over the last 30 years. In order to assess the impact of this land‐use change on the amount and composition of soil organic matter (SOM), we investigated a grassland pasture site (G), and both an 8‐year‐old (P8) and a 30‐year‐old (P30) plantation with Pinus taeda. Soil samples down to 45 cm were analysed for texture, pH, soil organic carbon (SOC) and total nitrogen (Ntot) concentrations. Chemical composition of SOM was determined by using cross‐polarization magic angle spinning (CPMAS) 13C NMR spectroscopy. We analysed for stable C isotope (δ13C) and assessed the lignin composition by CuO oxidation. Additionally, contents of pyrogenic organic material (PyOM) were determined because the Campos is regularly burnt. Both pine plantations revealed relatively small SOC concentrations in the mineral soil of 72.6 mg g−1 (P8) and 56.8 mg g−1 (P30) and Ntot concentrations of 4.0 mg g−1 (P8) and 2.9 mg g−1 (P30) for the A horizon, while grassland showed significantly (P &lt; 0.01) larger contents of 100.2 mg g−1 for SOC and 5.9 mg g−1 for Ntot. Accumulation of litter layers suggests decreased input of organic material into the mineral soil under pine, which was confirmed by the δ13C values and lignin composition. Smaller contents of vanillyl‐ (V), syringyl‐ (S), and cinnamyl (C)‐phenols, smaller ratios of S/V and C/V, and smaller ratios of acidic to aldehydic forms of V and S phenols indicated a high degree of decomposition of residual grass‐derived SOM in the upper part of the mineral soil (0–10 cm) under pine plantations. This was confirmed by CPMAS 13C NMR spectroscopy, showing an increasing Alkyl C/O‐Alkyl C ratio at the same depth. No significant changes in the contents of PyOM could be detected, but all sites tended to show the greatest concentrations at deeper soil depths &gt; 15 cm, indicating a vertical relocation of PyOM. The results suggest that decomposition of residual SOM originating from grassland species contributes to the decrease of SOC and Ntot and to an acidification in the topsoil under pine plantations. We also suggest that slow litter decomposition and incorporation and the absence of fires at the plantations are additional reasons for the reduced amount of SOM. Depletion of SOM and the acidification of the topsoil may reduce the availability and supply of nutrients and diminish the C sequestration potential of the mineral soil.

Mineralisation and structural changes during the initial phase of microbial degradation of pyrogenic plant residues in soil

The microbial recalcitrance of char accumulated after vegetation fires was studied using pyrogenic organic material (PyOM) with increasing degrees of charring, produced from rye grass ( Lolium perenne) and pine wood ( Pinus sylvestris) at 350 °C under oxic conditions. Solid state 13C and 15N nuclear magnetic resonance (NMR) spectroscopy confirmed increasing aromaticity and the formation of heterocyclic N with prolonged charring. After mixing with a mineral soil, the PyOM was aerobically incubated for 48 days at 30 °C. To account for the input of fresh litter after a fire event, unburnt rye grass residue was added as a co-substrate. The grass-derived PyOM showed the greatest extent of C mineralisation. After 48 days incubation, up to 3.2% of the organic C (OC) was converted to CO 2. More severe thermal alteration resulted in a decrease in the total C mineralisation to 2.5% of OC. In the pine-derived PyOM, only 0.7% and 0.5% of the initial C were mineralised. The co-substrate additions did not enhance PyOM mineralisation during initial degradation. 13C NMR spectroscopic analysis indicated structural changes during microbial degradation of the PyOM. Concomitant with a decrease in O-alkyl/alkyl-C, carboxyl/carbonyl C content increased, pointing to oxidation. Only the strongly thermally altered pine PyOM showed a reduction in aromaticity. The small C losses during the experiment indicated conversion of aryl C into other C groups. As revealed by the increase in carboxyl/carbonyl C, this conversion must have included the opening and partial oxidation of aromatic ring structures. Our study demonstrates that plant PyOM can be microbially attacked and mineralised at rates comparable to those for soil organic matter (SOM), so its role as a highly refractory SOM constituent may need re-evaluation.

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.

A new conceptual model for the structural properties of char produced during vegetation fires

The elemental and 13C nuclear magnetic resonance (NMR) analysis of laboratory prepared char samples from the main plant biopolymers (cellulose, lignin, casein), as well as grass and beech sawdust, indicated a greater chemical heterogeneity of plant derived chars than generally assumed from common black carbon (BC) models. The data support a recent concept proposing char as a heterogeneous mixture of thermally altered biomacromolecules with N, O and likely also S substitutions as common features. In contrast to soot, graphitic polyaromatic domains play a minor role in such chars. The high resistance of casein against thermal treatment and the low atomic N/C values for grass chars demonstrate the quantitative importance of pyrogenic N. A more detailed consideration is needed if a more realistic view of the role of pyrogenic organic matter (PyOM) in soils and sediments is required.

Evolution of the source apportionment of the lipidic fraction from sediments along the Fensch River, France: A multimolecular approach

The Fensch River (FR) is one of the most contaminated rivers in France due to the population density and the concentration of industrial activities in this small watershed area. From upstream to downstream, the organic matter extracted from sediments has been analyzed by gas chromatography–mass spectrometry and molecules have been quantified and classified into natural, petrogenic, pyrogenic and sewage water (SW) markers. Upstream the river, anthropogenic molecules are already predominant and represent 87.1% of the molecules quantified. This proportion increases from upstream to downstream and rises to 96.8% at the confluence of the FR with the Moselle River. In the upper part of the FR the contamination is mainly due to human waste (coprostanol: 36.44 µg/g; 42.1% of anthropogenic markers). In the lower part, the contribution of SW markers decreases from 42.1 to 2.4% and the proportion of pyrogenic molecules increases from 29.6 to 59.6%. The major sources of pyrogenic organic matter have been determined by calculation of specific ratios on polycyclic aromatic hydrocarbons and by comparison with reported data. Coal tar, road runoff and atmospheric depositions of urban particles seem to be the major pyrogenic sources. Along the river, the proportion of petrogenic molecules remains constant and those molecules seem to be mainly inherited from road runoff, in the upper part of the FR. Industrial lubricants that occur in steel plant sludge are an additional source in the lower part of the river.