Pyrogenic Organic Matter Research Articles

Carbon mineralizability determines interactive effects on mineralization of pyrogenic organic matter and soil organic carbon.

Soil organic carbon (SOC) is a critical and active pool in the global C cycle, and the addition of pyrogenic organic matter (PyOM) has been shown to change SOC cycling, increasing or decreasing mineralization rates (often referred to as priming). We adjusted the amount of easily mineralizable C in the soil, through 1-day and 6-month preincubations, and in PyOM made from maple wood at 350 °C, through extraction. We investigated the impact of these adjustments on C mineralization interactions, excluding pH and nutrient effects and minimizing physical effects. We found short-term increases (+20-30%) in SOC mineralization with PyOM additions in the soil preincubated for 6 months. Over the longer term, both the 6-month and 1-day preincubated soils experienced net ∼10% decreases in SOC mineralization with PyOM additions. Additionally, the duration of preincubation affected interactions, indicating that there may be no optimal preincubation time for SOC mineralization studies. We show conclusively that mineralizability of SOC in relation to PyOM-C is an important determinant of the effect of PyOM additions on SOC mineralization.

Carbon losses from pyrolysed and original wood in a forest soil under natural and increased N deposition

Abstract. Pyrogenic organic matter (PyOM) plays an important role as a stable carbon (C) sink in the soils of terrestrial ecosystems. However, uncertainties remain about in situ turnover rates of fire-derived PyOM in soil, the main processes leading to PyOM-C and nitrogen (N) losses from the soil, and the role of N availability on PyOM cycling in soils. We measured PyOM and native soil organic carbon losses from the soil as carbon dioxide and dissolved organic carbon (DOC) using additions of highly 13C-labelled PyOM (2.03 atom %) and its precursor pinewood during 1 year in a temperate forest soil. The field experiment was carried out under ambient and increased mineral N deposition (+60 kg N-NH4NO3 ha−1 year−1). The results showed that after 1 year: (1) 0.5% of PyOM-C and 22% of wood-C were mineralized as CO2, leading to an estimated turnover time of 191 and 4 years, respectively; (2) the quantity of PyOM and wood lost as dissolved organic carbon was negligible (0.0004 ± 0.0003% and 0.022 ± 0.007% of applied-C, respectively); and (3) N additions decreased cumulative PyOM mineralization by 43%, but did not affect cumulative wood mineralization and did not affect the loss of DOC from PyOM or wood. We conclude that mineralization to CO2 was the main process leading to PyOM losses during the first year of mineralization in a forest soil, and that N addition can decrease PyOM-C cycling, while added N showed no effect on wood C cycling.

Pyrogenic molecular markers: Linking PAH with BPCA analysis

Molecular characterization of pyrogenic organic matter (PyOM) is of great interest to understand the formation and behavior of these increasingly abundant materials in the environment. Two molecular marker methods have often been used to characterize and trace PyOM: polycyclic aromatic hydrocarbon (PAH) and benzenepolycarboxylic acid (BPCA) analysis. Since both methods target pyrogenic polycyclic compounds, we investigated the linkages between the two approaches using chars that were produced under controlled conditions. Rye and maize straws and their analogues charred at 300, 400 and 500°C, respectively, were thus analyzed with both methods. Moreover, we also measured BPCAs directly on the lipid extracts, on which PAHs were analyzed, and on the respective extraction residues, too. Both methods revealed important features of the chars, in particular the increasing degree of aromatic condensation with increasing highest heating temperature (HTT). The overlap between the two methods was identified in the lipid fraction, where the proportion of benzenetricarboxylic acids (B3CAs) correlated with PAH abundance. The results confirmed the validity and complementarity of the two molecular marker methods, which will likely continue to play a crucial role in PyOM research due to the recent developments of compound-specific PAH and BPCA stable carbon (δ13C) and radiocarbon (14C) isotope methods.

Physicochemical changes in pyrogenic organic matter (biochar) after 15 months of field aging

Abstract. Predicting the effects of pyrogenic organic matter (OM) addition (either natural or intentional as in the case of biochar amendment) on soil chemistry and crop yields has been hampered by a lack of understanding of how pyrogenic OM evolves in the environment over time. This work compared the physicochemical characteristics of newly made and 15-month-field-aged biochars and biochar–soil mixtures. After aging, biochars made by pyrolysis of wood and grass at 250, 400 and 650 °C exhibited 5-fold increases in cation exchange capacity (CEC), on average; appearance of anion exchange capacity (AEC); and significant decreases in pH, ash content and nanopore surface area. Cross polarization 13C nuclear magnetic resonance (NMR) analyses indicated relative increases in O-containing functional groups, including substituted aryl, carboxyl and carbonyl C, and losses of O-alkyl groups. Similar chemical trends were observed for soil–biochar mixtures, suggesting the same biochar aging processes occurred in the soil environment. However, there was evidence for a role of soil OM–microbe–biochar interaction during aging. Field aging of soil with biochar resulted in large increases in C and N content (up to 124 and 143%, respectively) and exchange capacity (up to 43%) beyond that calculated by the weighted addition of the properties of biochar and soil aged separately. These beneficial interactive effects varied with soil and biochar type. Scanning electronic microscopy (SEM) images of biochar particles aged with soil showed colonization by microbes and widespread OM coatings. Thus, sorption of both microbially produced and soil OM are likely processes that enhanced biochar aging. Thus, biochar's full beneficial effects on soil properties likely increase over time, and proper assignment of C sequestration credits to biochar users will require consideration of soil–biochar interactions.

Nano-structural and chemical characterization of charred organic matter in a fire-affected Arenosol

The objective of this study was to characterize the formation and properties of black carbon (BC) domains within pyrogenic organic material (PyOM) of an Arenosol derived from a fire-affected site in Portugal (Algarve, Faro). Both the particulate organic matter (POM) and organo-mineral particle size fractions were investigated by means of 13C CPMAS NMR (composition of the soil organic matter, SOM) and 129Xe NMR of adsorbed Xe atoms (nano-structure of PyOM). Fire-induced alteration of the soil mineral phase was revealed by the X-ray diffraction and chemical extraction methods. An enhanced contribution of aromatic species combined with depletion in carbohydrates and alkyl functionalities (13C NMR) was observed in the surface soil horizon (Ah), suggesting that the dehydration of carbohydrates and thermal breakdown of n-alkanes was mainly responsible for the formation of BC species. As a measure of a charring degree of PyOM, we suggested to use the relative intensity of the down-field 129Xe NMR signal (130–160ppm) characterizing micropores (<2nm) that were formed by the fused aromatic units of BC. The revealed microporous BC regions and the low-dense domains with larger pores (supposedly formed by the alkyl functional groups) exist within the same PyOM particle, as this was supported by the calculated mean Xe displacement distance between these two regions. The ascertained micropore heterogeneity within the BC domains is supposedly caused by the varying size of aromatic clusters that approach each other at a varying distance. This nano-scale heterogeneity may have important implications for the specific adsorption behavior and persistence of BC in a soil.In the organo-mineral soil fractions, the BC species mainly occurred in a highly dispersed (mineral-associated) form. Supposedly, the association “PyOM–soil minerals” did not necessarily require the aromatic ring opening, as we detected rather high content of aromatic 13C NMR moieties in these fractions. An increased content of the short-range order Fe- and Al oxyhydroxides and mixed layer silicates (supposedly fire-induced) appeared to promote the interactions “PyOM–soil minerals” via an enhanced extent of hydroxylated mineral surfaces.

A meta‐analysis on pyrogenic organic matter induced priming effect

AbstractPyrogenic organic matter (PyOM) is considered an important soil carbon (C) sink. However, there are evidences that its addition to soil may induce a priming effect (PE) thus influencing its C abatement potential. The direction, the size and the mechanisms responsible for PyOM induced PE are far from being understood. We collected approximately 650 data points from 18 studies to analyse the characteristics of the PE induced by PyOM. The database was divided between the PE induced on the native soil organic matter and on fresh organic matter. Most of the studies were short‐term incubation therefore the projections of findings on the long term may be critical. Our findings indicate that over 1 year PyOM induces an average positive PE of 0.3 mg C g−1 soil on native soil organic matter and a PE of approximately the same size but opposite direction on fresh organic matter. We studied the correlation of PE with several properties of soil, of the added PyOM, and time after PyOM addition. We found that PyOM primes positively the native soil organic matter in the first 20 days while negative PE appears in a later stage. Negative PE was correlated with the soil C content. PyOM characterized by a low C content induced a higher positive PE on native soil organic carbon. No correlation was found between the factors record in our database and the PE induced on the fresh organic matter. We reviewed the mechanisms proposed in literature to explain PE and discussed them based on findings from our meta‐analysis. We believe that the presence of a labile fraction in PyOM may trigger the activity of soil microorganisms on the short term and therefore induce a positive PE, while on the long term PyOM may induce a negative PE by promoting physical protection mechanisms.

Transformation and stabilization of pyrogenic organic matter in a temperate forest field experiment.

Pyrogenic organic matter (PyOM) decomposes on centennial timescale in soils, but the processes regulating its decay are poorly understood. We conducted one of the first studies of PyOM and wood decomposition in a temperate forest using isotopically labeled organic substrate, and quantified microbial incorporation and physico-chemical transformations of PyOM in situ. Stable-isotope (¹³C and ¹⁵N) enriched PyOM and its precursor wood were added to the soil at 2 cm depth at ambient (N0) and increased (N+) levels of nitrogen fertilization. The carbon (C) and nitrogen (N) of added PyOM or wood were tracked through soil to 15 cm depth, in physically separated soil density fractions and in benzene polycarboxylic acids (BPCA) molecular markers. After 10 months in situ, more PyOM-derived C (>99% of initial 13C-PyOM) and N (90% of initial ¹⁵N-PyOM) was recovered than wood derived C (48% of 13C-wood) and N(89% under N0 and 48% under N+). PyOM-C and wood-C migrated at the rate of 126 mm yr ⁻¹ with 3-4% of PyOMC and 4-8% of wood-C recovered below the application depth. Most PyOM C was recovered in the free light fraction(fLF) (74%), with 20% in aggregate-occluded and 6% in mineral associated fractions – fractions that typically have much slower turnover times. In contrast, wood C was recovered mainly in occluded (33%) or dense fraction (27%).PyOM addition induced loss of native C from soil (priming effect), particularly in fLF (13%). The total BPCA-C content did not change but after 10 months the degree of aromatic condensation of PyOM decreased, as determined by relative contribution of benzene hexa-carboxylic acid (B6CA) to the total BPCA C. Soil microbial biomass assimilated 6-10% of C from the wood, while PyOM contributions was negligible (0.14–0.18%). The addition of N had no effect on the dynamics of PyOM while limited effect on wood.

Pyrogenic carbon additions to soil counteract positive priming of soil carbon mineralization by plants

Important due to both its role in fire-affected ecosystems, and also its proposed intentional production and application for carbon (C) management, pyrogenic organic matter (PyOM) is thought to contain very stable forms of C. However, the mechanisms behind its interactions with non-PyOM soil organic C (SOC) remain speculative, with studies often showing short-term positive and then long-term negative “priming effects” on SOC decomposition after PyOM applications. Furthermore, studies of these interactions to date have been limited to systems that do not include plants. This study describes results from a 12-week greenhouse experiment where PyOM-SOC priming effects with and without plants were investigated using stable isotope partitioning. In addition, we investigated the optimal δ13C proxies for sources of SOC, PyOM, and plant-derived CO2 emissions. The two-factorial experiment included the presence or absence of corn plants and of 13C-labelled PyOM. In order to control for pH and nutrient addition effects from PyOM, its pH was adjusted to that of the soil and optimal nutrient and water conditions were provided to the plants. The δ13C of PyOM sub-components were significantly different. Significant losses of 0.4% of the applied PyOM-C occurred in the first week. We find evidence for a “negative priming” effect of PyOM on SOC in the system (SOC losses are 48% lower with PyOM present), which occurred primarily during the first week, indicating it may be due to transient effects driven by easily mineralizable PyOM. Additionally, while the presence of corn plants resulted in significantly increased SOC losses (“positive priming”), PyOM additions counteract this effect, almost completely eliminating net C losses either by decreasing SOC decomposition or increasing corn C additions to soil. This highlights the importance of including plants in studies of PyOM-SOC interactions.

Ryegrass-derived pyrogenic organic matter changes organic carbon and nitrogen mineralization in a temperate forest soil

Pyrogenic organic matter (PyOM) is considered as a technique to improve soil fertility and store carbon (C) in soil. However, little is known regarding soil organic C and nitrogen (N) mineralization in PyOM-amended soils. To investigate the relationship between the C and N mineralization rates and the possible consequences in terms of C storage and N availability, we incubated ryegrass-derived PyOM (pyrolyzed at 450 °C) enriched in 13C (4.33 atom %) in a forest Cambisol for 158 days with and without mineral N addition. We determined PyOM and native soil organic C mineralization, NH4+ and NO3− contents in the soil, gross N mineralization, phenol-oxidase and protease activities, and microbial biomass throughout the incubation experiment and the incorporation of PyOM in microbial biomass at the end of the experiment (158 days). We determined that 4.3% of the initial PyOM-C was mineralized after 158 days. Moreover, PyOM induced a strongly positive priming effect within the first 18 days; a negative priming effect was observed from Days 18 to 158. The initial increase in organic matter mineralization corresponded to a higher gross N mineralization and NH4+ content in the PyOM-treated soil than in the untreated soil. Ammonium was rapidly transformed into nitrate and stored in this form until the end of the experiment. We conclude that the presence of PyOM affected the mineralization pattern of native soil organic matter mineralization and increased mineral N content, while N addition did not influence PyOM or soil organic matter mineralization.

Predicting pyrogenic organic matter mineralization from its initial properties and implications for carbon management

Predicting pyrogenic carbon (PyC) or biochar stability from its precursor properties is critical for evaluating and managing terrestrial C stocks. Transmission mode Fourier transform infrared spectroscopy (FTIR) spectroscopy was compared with proximate analysis data and H/C and O/C for predicting C mineralization. PyC produced at 7 different temperatures from 6 different feedstocks, in addition to the original feedstock materials, was incubated for 3yr at 30°C in a sand matrix. A C debt or credit ratio was calculated by comparing the C remaining in the incubated PyC sample (accounting for the measured C lost during initial PyC production) to the C remaining in the incubated original feedstock. A value>1 indicates that more C remains in the PyC than in the original feedstock (credit), while a value<1 indicates a debt. After 3yr, PyC produced at 300°C lost significantly more C than higher temperature PyC material, but significant differences in C loss between PyC produced at temperatures⩾350°C were not detectable. The best predictor of C loss was a multiple linear regression model using the fractional FTIR signals at 816, 1048, 1374, 1424, 1460, 1590, 1700 and 2925cm−1 as parameters (R2 0.80, p<0.0001). After 3yr, the C debt or credit ratio reached values significantly>1 for all corn PyC samples and some bull, dairy and poultry PyC samples, resulting in net C credit, while all pine and oak PyC samples remained in debt. This C debt or credit ratio reveals that, depending on the timeline of interest, producing relatively low temperature PyC with less initial C loss can result in greater C savings than producing higher temperature PyC, even though the C remaining after exposure to higher pyrolysis temperatures is more stable.

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.

Modification of biomarkers in pyrogenic organic matter during the initial phase of charcoal biodegradation in soils

The biochemical stability of typical biomarkers from the free lipid extract of different plant chars during biotic degradation was examined using pyrogenic organic matter (PyOM) derived from rye grass (Lolium perenne; Gr) and pine wood (Pinus sylvestris; P) heated at 350°C for 1 (1M) and 4min (4M) under oxic conditions. The composition of the n-alkanes and n-fatty acids (FA) as well as the contents of the molecular markers levoglucosan and vanillin were determined and related to charring intensity and stability to microbial reworking. Our results confirmed that charring of plant residues results in typical thermal breakdown processes of n-alkanes and FA. Prolonged charring reduced the average chain length of n-alkanes by up to four carbons and shifted the characteristic odd/even predominance of fresh plants towards a balanced odd/even distribution. The unsaturated FA fraction was more prone to heat degradation than the saturated counterparts. Especially, linoleic acid (C18:2) and α-linolenic acid (C18:3) were depleted in the severely charred grass, whereas oleic acid (C18:1) was still present. Levoglucosan was detectable in all PyOMs, but the pine wood charred for 1min (P1M) contained the largest amount. Progressive heating resulted in a strong depletion of levoglucosan for both plant materials. The pine char showed a relative accumulation of vanillin, supporting that some lignin-derived structures can survive charring. Microbial decomposition of the chars within a soil matrix for seven weeks demonstrates that during initial PyOM biodegradation, n-alkanes and FA series are rapidly modified by microbial reworking and biosynthesis. After the incubation, the pine chars were enriched in octadecane (C18) and homologues in the range from C22 to C26. The n-alkanes and FA of the grass chars were more severely affected than those of the pine chars. Levoglucosan was efficiently decomposed, indicating that care has to be taken if these compounds are used as char tracer in soils, since their instability against biodegradation may hamper the identification of charcoal residues.

Elucidating the chemical structure of pyrogenic organic matter by combining magnetic resonance, mid-infrared spectroscopy and mass spectrometry

Fire-derived organic matter (pyrogenic organic matter, or PyOM), despite its apparent long term stability in the environment, has recently been reported to degrade faster than previously thought. Current studies have suggested that the composition and structure of PyOM can provide new insights on the mechanisms by which C and N from pyrolyzed biomaterials are stabilized in soils. To better understand the chemical structure of PyOM produced under typical fire conditions in temperate forests, samples of dual-enriched (13C/15N) Pinus ponderosa wood and the charred material produced at 450°C were analyzed by solid state nuclear magnetic resonance (ssNMR), electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy, and both isotopic and elemental composition (C, H, O, and N). Notably, the use of high magnetic field strengths in combination with isotopic enrichment augmented the NMR detection sensitivity, and thus improved the quality of molecular information as compared with previously reported studies of pyrogenic materials. The key molecular groups of pine wood and the corresponding PyOM materials were determined using magic-angle spinning (MAS) 13C, 15N, and 1H NMR. Together with DRIFT and EPR measurements, ssNMR revealed the formation of a free radical-containing disordered blend of nitrogenous aromatics and heat resistant aliphatics in the PyOM due to incomplete combustion of the precursor wood. 13C ssNMR and DRIFT analyses showed the removal of oxygenated aliphatics due to pyrolysis of the precursor wood and the dominant contribution of multiply-bonded and oxygenated aromatic structures in the resulting PyOM. However, the 18O isotopic analyses indicated selective retention of ligneous moieties during charring at 450°C. 15N ssNMR studies implied that the nitrogenous species in PyOM corresponded to thermally altered rather than heat resistant domains of the pine wood precursor. Our molecular characterization suggests that biomaterials pyrolyzed near 450°C may degrade in soils faster than those pyrolyzed at higher temperatures and may not represent a stable C sink in terrigenous ecosystems.

Biological degradation of pyrogenic organic matter in temperate forest soils

Pyrogenic organic matter (PyOM), derived from the incomplete combustion of plant biomass and fossil fuels, has been considered one of the most stable pools of soil organic matter (SOM) and a potentially important terrestrial sink for atmospheric CO2. Recent evidence suggests that PyOM may degrade faster in soil than previously thought, and can affect native SOM turnover rates. We conducted a six-month laboratory incubation study to better understand the processes controlling the degradation of PyOM in soils using dual-enriched (13C/15N) PyOM and its precursor wood (Pinus ponderosa). We examined the effects of soil type and inorganic N addition on PyOM and wood C and N mineralization rates, microbial C utilization patterns, and native SOM turnover rates. PyOM charred at 450 °C or its precursor pine wood was incubated in two temperate forest subsoils with contrasting short range order (SRO) clay mineralogy (granite versus andesite parent material). Duplicates of experimental treatments with and without PyOM added were sterilized and abiotic C mineralization was quantified. In a second incubation, PyOM or wood was incubated in granitic soil with and without added NH4NO3 (20 kg N ha−1). The fate of 13C/15N-enriched PyOM and wood was followed as soil-respired 13CO2 and total extractable inorganic 15N. The uptake of 13C from PyOM and wood by soil microbial community groups was quantified using 13C-phospholipids fatty acids (PLFA). We found that (1) The mean residence time (MRT) of PyOM-C was on a centennial time scale (390–600 yr) in both soil types; (2) PyOM-C mineralization was mainly biologically mediated; (3) Fungi more actively utilized wood-C than PyOM-C, which was utilized by all bacteria groups, especially gram (+) bacteria in the andesite (AN) soil; (4) PyOM-N mineralization was 2 times greater in granite (GR) than in AN soils; (5) PyOM additions did not affect native soil C or N mineralization rates, microbial biomass, or PLFA-defined microbial community composition in either soil; (6) The addition of N to GR soil had no effect on the MRT of C from PyOM, wood, or native SOM. The centennial scale MRT for PyOM-C was 32 times slower than that for the precursor pine wood-C or native soil C, which is faster than the MRT used in ecosystem models. Our results show that PyOM-C is readily utilized by all heterotrophic microbial groups, and PyOM-C and -N may be more dynamic in soils than previously thought.

Alteration of quality and stability of organic matter in grassland soils of Southern Brazil highlands after ceasing biannual burning

The impact of ceasing biannual burning on the composition and the amount of soil organic matter (SOM) in grassland soils from Southern Brazil was studied by means of solid-state 13C and 15N NMR spectroscopy using soil duplicates of fields without 1, 2, 5 and 22years of burning. The examination of the duplicates demonstrated good reproducibility. Demineralization of the soil with hydrofluoric acid (HF) indicated that the fire frequency had no impact on the amount of HF-extractable SOM. Exclusion of fire resulted in a considerable decrease of the organic C stocks, whereas N and S stocks were only slightly affected. The content of pyrogenic organic C (PyOC) in the soils was determined via a combination of chemical oxidation with acid dichromate and solid-state 13C NMR spectroscopy of the oxidation resistant fraction. This analysis revealed that this decline was caused by the reduced input of decaying plant roots remaining in the soil after combustion of the aboveground vegetation during a fire. In spite of frequent burning for centuries, the PyOC concentrations were unexpectedly low in the top 5cm of the studied soils but increased clearly with soil depth, indicating that some charcoal constituents can be translocated with the soil solution. With respect to the stability of charcoal in the grassland soils, our study revealed that comparable to SOM, pyrogenic organic matter (PyOM) is composed of fractions with different stability against microbial degradation. Whereas the labile PyOM fraction showed C losses similar to that of O-alkyl C, the more stable PyOM was selectively preserved together with an alkyl C fraction. Based on the results of the present study, one can conclude that frequent burning of grassland can lead to an increased C-sequestration potential of a soil. Because the input of the additional C is mainly caused by a higher biomass production after the fire, the additional SOM contributes to the labile pool with short turn-over times rather than to the recalcitrant carbon fraction. After ceasing burning, the reduced litter input cannot compensate the fast consumption of this pool and a quick decrease of the C stocks is expected.

Biodegradability of organic matter in fire-affected mineral soils of Southern Spain

Incorporated into the soil, naturally formed pyrogenic organic matter (PyOM) is considered as highly recalcitrant, but direct estimation of PyOM decomposition rates are scarce. With this aim in mind, we subjected organic matter (OM) of fire-affected and unaffected soils to biochemical degradation under laboratory conditions and monitored CO2 production over a period of seven months. The soils derived from the Sierra de Aznalcóllar, Southern Spain, and were sampled 4 weeks and 5 years after a severe fire. Virtual fractionation of the solid-state 13C nuclear magnetic resonance (NMR) spectra of the fire-affected soils into fire-unaffected soil organic matter (SOM) and PyOM yielded charcoal C contributions of 30 to 50% to the total organic C (Corg) of the sample. Fitting the respiration data with a double exponential decay model revealed a fast carbon flush during the first three weeks of the experiment. Solid-state 13C NMR spectroscopy evidenced the contribution of aromatic moieties of the PyOM to this initial carbon release and to the biosynthesis of new microbial biomass. Considering the loss of microbiologically easily available fresh litter by wildfires, this relatively labile PyOM fraction may contribute to a fast recovery of a fire-affected site. The input of PyOM resulted in an increase of the mean residence time (MRT) of the slow OM pool of the soil by a factor of 3–4 to approximately 40 years. Assuming that under field conditions, the microbial activity corresponds to approximately 10% of the value observed under optimal laboratory conditions, MRTs of 500–600 years were estimated for the slow PyOM pool. The fact that these times are only 5–6 times longer than those calculated for fire-unaffected SOM rises doubts about the presumed big influence of PyOM as an additional C-sink in soils. On the other hand, although being small the difference in turnover rates is evident and has some major implication with respect to long-term alteration of the chemical composition of OM in fire-affected soils. In case of a reduced input of fresh litter, the preferential degradation of fire-unaffected SOM yields in a selective preservation of PyOM. To what extent this can alter soil properties, has still to be elucidated. In cultivated soils rarely affected by fire or with low charcoal input after burning of harvest, the impact of PyOM accumulation may be of minor importance. On the other hand, for soils regularly amended with high amounts of biochar or subjected to frequent natural or prescribed burnings, it may be an important factor.

Stable isotopic analysis of pyrogenic organic matter in soils by liquid chromatography–isotope‐ratio mass spectrometry of benzene polycarboxylic acids

Pyrogenic organic matter (PyOM), the incomplete combustion product of organic materials, is considered stable in soils and represents a potentially important terrestrial sink for atmospheric carbon dioxide. One well-established method of measuring PyOM in the environment is as benzene polycarboxylic acids (BPCAs), a compound-specific method, which allows both qualitative and quantitative estimation of PyOM. Until now, stable isotope measurement of PyOM carbon involved measurement of the trimethylsilyl (TMS) or methyl (Me) polycarboxylic acid derivatives by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). However, BPCA derivatives can contain as much as 150% derivative carbon, necessitating post-analysis correction for the accurate measurement of δ¹³C values, leading to increased measurement error. Here, we describe a method for δ¹³C isotope ratio measurement and quantification of BPCAs from soil-derived PyOM, based on ion-exchange chromatography (IEC-IRMS). The reproducibility of the δ¹³C measurement of individual BPCAs by IEC-IRMS was better than 0.35‰ (1σ). The δ¹³C-BPCA analysis of PyOM in soils, including at natural and artificially enriched ¹³C-abundance, produced accurate and precise δ¹³C measurements. Analysis of samples that differed in δ¹³C by as much as 900‰ revealed carryover of <1‰ between samples. The weighted sum of individual δ¹³C-BPCA measurements was correlated with previous isotopic measurements of whole PyOM, providing complementary information for bulk isotopic measurements. We discuss potential applications of δ¹³C-BPCA measurements, including the study of turnover rates of PyOM in soils and the partitioning of PyOM sources based on photosynthetic pathways.

Bioavailability of N released from N-rich pyrogenic organic matter: An incubation study

In Mediterranean ecosystems, natural wildfires and prescribed burning play an important role. In order to obtain a better understanding of the impact of charcoal on the nitrogen (N) cycling in soil, 15N-enriched pyrogenic organic material (PyOM) obtained from “ Lolium perenne” charred for 4 min at 350 °C was mixed with a typical Mediterranean agricultural soil and incubated for 72 days under controlled conditions. The main objectives were to analyze the availability of N from this material and to obtain more insight into the recalcitrance of PyOM in soils. Addition of artificially produced 15N-PyOM increased the biomass production and N retention. After 72 days of incubation time, 10% of the 15N added to the soil ( 15N add) was incorporated into new grass biomass. Solid-state 15N NMR spectroscopy revealed that at least some of this N derived from the degradation of pyrrole-type structures. Increase of the amide-N intensity in the solid-state 15N NMR spectrum of the incubated soils indicated further that some 15N from the PyOM was also incorporated into new microbial biomass. Our results confirmed a relatively low recalcitrance of N-rich PyOM. Since during its degradation, N is only slowly transferred into a plant-available form, it may contribute to the observed improvement of soil fertility by avoiding fast N losses due to leaching and volatilization as a slow N-release fertilizer.

Pyrogenic organic matter in soil: Its origin and occurrence, its chemistry and survival in soil environments

Since geological times, vegetation fires have shaped Earth and produced charcoal, which is accumulating in sediments and soils. Only recently the role of pyrogenic organic matter (PyOM) as an important constituent of slow turning soil organic matter (SOM) has received more attention. Whereas in some environments, considerable long residence times of millennia and more were determined for PyOM, newer field and laboratory studies determined turnover times that were in the range of humified fire-unaffected SOM. In order to obtain an explanation of those contradictory observations, a better understanding of the nature of PyOM and its stability in different soil environments is needed. Therefore, after emphasizing the importance of fire and charcoal production during history, the present knowledge about occurrence, chemistry of PyOM formation and its recalcitrance is reviewed in the present work. Summarizing this information, it is revealed that PyOM can be decomposed by physical, chemical and biological means, and that its resistance against degradation in soils largely depends upon its chemistry and the environment in which it is accumulating. Comparable to humification of fire-unaffected SOM, different mechanisms including O 2-deficiency, unfavorable conditions for microorganisms, or interaction with the mineral phase seem to be involved in PyOM preservation and stabilization. Depending on soil conditions, the efficiency of the single pathway varies, resulting in a PyOM pattern which is typical for a certain soil, and it may even represent a typical feature of it. This concept could explain the varying abundance and recalcitrance of PyOM in fire-affected soils, some of which are presented in more detail within this work. In turn, the respective PyOM pattern, which could have been generated over decades and millennia, determines typical soil properties. Thus, PyOM can be seen to be actively involved in the pedogenic process, leading to soil classes such as Terra preta soils in the Amazon region, and possibly also to Chernozems in Central Europe or other Black Soils all over the world.

Carbon and nitrogen degradation on molecular scale of grass-derived pyrogenic organic material during 28 months of incubation in soil

The present study focuses on the microbial recalcitrance of pyrogenic organic material (PyOM) on a molecular scale. We performed microcosm incubation experiments using 13C- and 15N-enriched grass-derived PyOM mixed with a sub soil material taken from a Haplic Cambisol. Solid-state 13C and 15N NMR studies were conducted to elucidate the humification processes at different stages of PyOM degradation. The chemical structure of the remaining PyOM after incubation was clearly different from the initial pyrogenic material. The proportion of O-containing functional groups was increased, whereas that of aryl C and of N-containing heterocyclic structures had decreased, probably due to mineralisation and conversion to other C and N groups. After 20 months of incubation the aryl C loss reached up to 40% of the initial amount and up to 29% of the remaining PyOM C was assigned to carboxyl/carbonyl C and O-aryl C. These reactions alter the chemical and physical properties of the char residue and make it more available for further microbial attack but also for adsorption processes. Our study presents direct evidence for the degradation of N-heterocyclic domains in charred plant remains adding new aspects to the understanding of the N cycling in fire-affected ecosystems.