- Email: [email protected]
Black carbon assessment using benzene polycarboxylic acids: Limitations for organic-rich matrices
Organic Geochemistry 94 (2016) 47–51
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Black carbon assessment using benzene polycarboxylic acids: Limitations for organic-rich matrices Arne Kappenberg ⇑, Melanie Bläsing, Eva Lehndorff, Wulf Amelung Institute of Crop Science and Resource Conservation – Soil Science and Soil Ecology, University of Bonn, Nussallee 13, 53115 Bonn, Germany
a r t i c l e
i n f o
Article history: Received 5 November 2015 Received in revised form 22 December 2015 Accepted 21 January 2016 Available online 28 January 2016 Keywords: Black carbon Benzene polycarboxylic acids Analytical constraints
a b s t r a c t For the assessment of black carbon (BC), its oxidation to benzene polycarboxylic acids (BPCAs) is an established method. However, doubts about biological precursors remain and not all published data were obtained at low carbon concentration. We hypothesised that a considerable proportion of BC may be produced during sample treatment in the presence of a high amount of organic carbon (OC). We therefore tested whether and to which degree (i) BC-free material from stems of Zea mays L. (maize straw) and leaves of Capsicum annuum L. (bell pepper), as well as (ii) cyclic and non-cyclic carbon forms (chlorophyllin, ellagic acid and b-carotene) afford BPCAs when method protocols are overloaded with a sample above the recommended amount of 5 mg OC. The results showed that small amounts (< 2 g/kg OC) of BPCAs with three and four carboxyl groups may be formed even at low sample weight (< 5 mg OC), thereby falsely representing biological BC production. When this threshold was exceeded, all BPCA forms were detected. The artificial BPCA production yield in g OC increased with increasing amount of OC (R2 P 0.81), adding up to 8.7 g/kg OC (19.7 g BC/kg OC) artificial production. We therefore strongly recommend that a threshold of 5 mg OC sample concentration be maintained in future studies and that future BC assessments be restricted to BPCAs with five and six carboxyl groups. This constrains the application of the BPCA method for organic rich samples and for samples expected to contain a relatively low amount of BC. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Black carbon (BC) is formed by incomplete combustion of biomass and fossil fuel and can be found ubiquitously in soils, sediments and water, and as an aerosol in the atmosphere (e.g. Goldberg, 1985; Crutzen and Andreae, 1990; Jaffé et al., 2013). It is persistent in the environment (Goldberg, 1985; Glaser et al., 2000; Brodowski et al., 2007) and may thus contribute to a global carbon sink (Czimczik and Masiello, 2007; Rodionov et al., 2010; Santín et al., 2015). Different methods have been suggested for its detection in soil, all having limitations (Hammes et al., 2007; Roth et al., 2012). Among them, converting BC to benzene polycarboxylic acids (BPCAs) as a proxy for pyrogenic organic carbon (OC) input and as specific markers for combustion processes has gained increasing attraction because it is the only routine method that provides information on BC composition, by indicating the degree of aromatic condensation (Glaser et al., 1998; Brodowski et al., 2005) and hence the temperature of BC formation in the environment (Schneider et al., 2010, 2013; Wolf et al., 2013). Total BPCA ⇑ Corresponding author. Tel.: +49 228 732194; fax: +49 228 732782. E-mail address: [email protected] (A. Kappenberg). http://dx.doi.org/10.1016/j.orggeochem.2016.01.009 0146-6380/Ó 2016 Elsevier Ltd. All rights reserved.
content has been suggested as a proxy for BC content, employing a minimum conversion factor of 2.27 for the loss of C during oxidation (Glaser et al., 1998). This factor assumes that all BPCAs detected stem from BC. Intriguingly, BPCA yield correlated repeatedly and linearly with total organic matter (OM) content of soils and sediments (e.g. Gustafsson and Gschwend, 1998; Glaser and Amelung, 2003; Cornelissen et al., 2005). One explanation for co-occurrence of BC and soil OM (SOM) is that the production of BC is highest at sites with a high amount of biomass production (Glaser and Amelung, 2003; Czimczik and Masiello, 2007; Lehndorff et al., 2015). Another explanation could be either a method artefact (minimised from alteration of the method by Brodowski et al., 2005) or production of BPCAs from biological substances. The latter theory was substantiated by Glaser and Knorr (2008), who found that up to 25% of the isolated BC fraction from soils contained a 13C label added as non-pyrogenic C during lab and field studies. There are, however, no studies that could validate a biological BC-forming mechanism. In theory, Aspergillus niger, for instance, is able to produce mellitic acid (B6CA) biologically (Brodowski et al., 2005). Some of the B6CA could occur in a free form. Such a free form exists in soil
48
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51
(Möller et al., 2000; Haumaier, 2010), but should be removed during the sample preparation steps in the routine methodology (Brodowski et al., 2005). Besides, the total quantity of A. niger is likely not sufficient to explain the occurrence of B6CA (Brodowski et al., 2005). Moreover, we are not aware of any study that found other microbial strains able to condense OM to BC-like macromolecules. We therefore felt that neither the reason for some close correlations between BC and OC, nor for the high 13C labelling extent in BPCAs in the work of Glaser and Knorr (2008) was resolved satisfactorily. We concluded that potential biological precursors and the original method should be revisited a third time to better elucidate the risk of non-pyrogenic BPCA formation. Biological substances close to aromatic carbon structures are cyclic carbon forms, e.g. natural pigments like chlorophyll and carotene. Chlorophylls occur in plants, algae and cyanobacteria and may therefore contribute significantly to organic-rich soils and sediments. Large amounts of chlorophyll have been detected particularly in lake sediments (Möller and Scharf, 1986), paralleling elevated content of BC (Lehndorff et al., 2015). Nevertheless, Ziolkowski et al. (2011) found that aromatic carbon forms with fewer than three benzene rings did not afford BPCAs, making it unlikely that cyclic carbon forms produced in nature (e.g. carotene, chlorophyll derivatives) were a source for the production of BPCAs. Also, Brodowski et al. (2005) tested different plants and biological sources, such as the pigment aspergillin, for production of BPCAs. They found only a marginal amount of BPCAs (< 1% OC), but recommended that samples expected to contain > 200 g BC/kg OC and/or 100 g/kg OC, should be oxidised using a maximum of only 5 mg OC. This limit could be easily exceeded when samples contain an inherently high amount of OC, e.g. organic-rich lake sediments or peat and vegetation. Besides, it remained uncertain how degradation products of, e.g. carotene- and chlorophyll-related structures would behave under conditions of the exceeded concentration limit. The present study aimed at (i) identifying whether and which BPCAs could be produced artificially from non-pyrogenic OM from samples that exceeded the 5 mg OC boundary for the BPCA method, as suggested by Brodowski et al. (2005) and (ii) reelucidating BPCA formation from potential biological precursors under the high temperature and pressure conditions of the BPCA method. In addition, we aimed at (iii) evaluating the influence of such a high OC load on using the distribution of BPCAs as a proxy for BC combustion temperature.
2. Materials and methods 2.1. Sample description First, to test possible BPCA production from biological material we used BC-free material from stems of Zea mays L. and from leaves of Capsicum annuum L. The Z. mays L. stems were peeled and the leaves from C. annuum L. were grown in a high purity greenhouse in order to exclude the risk of contamination with atmospheric BC. The fresh material was dried at 40 °C and milled. Second, we re-tested production of BPCAs from potential biogenic precursor material using the cyclic carbon form, ellagic acid (2,3, 7,8-tetrahydroxy-chromeno[5,4,3-cde]chromene-5,10-dione), the synthetic, cyclic compound chlorophyllin and the unsaturated long chain hydrocarbon b-carotene. We chose these substances due to their structure and their abundance in nature, i.e. chlorophyllin, a tetrapyrrole, was tested due to the high abundance of chlorophylls in organic rich sediments (e.g. lake sediments). Chlorophyllin is a synthetic product but it is structurally comparable to natural chlorophyll forms. b-Carotene, a plant pigment, is a tetraterpene with two ionone rings connected by a chain of nine double bonded
carbons. Ellagic acid, a polyphenol with two aromatic rings, occurs in plants. All these compounds have structures, which are not obvious direct precursors for BPCA formation, unlike high molecular weight polycyclic aromatic carbon compounds, for instance (Glaser et al., 1998). However, all these compounds might well be degraded in HNO3 to potential precursor materials for aromatic re-condensation (e.g. Smith and Baran, 2015), which could then be the source of BPCAs. 2.2. BPCA method All samples were processed as outlined by Glaser et al. (1998) and Brodowski et al. (2005), at an OC weight < 5 mg, as recommended by Brodowski et al. (2005), as well as at elevated amount. Briefly, to remove polyvalent cations, samples were treated with 10 ml 4 M CF3CO2H. The residue was collected on glass fibre filters (GF 6, Hahnemühle FineArt GmbH, Dassel, Germany), and oxidised to BPCAs with hot HNO3 (8 h, 170 °C) within a high pressure digestion apparatus. After cleanup via a cation exchange column (Dowex 50WX8, 200–400 mesh, Fluka, Steinheim, Germany), the BPCAs were silylated and separated and quantified using gas chromatography (GC; Hewlett Packard 6890; Hewlett Packard GmbH, Waldbronn, Germany) equipped with a HP-5 column (30 m 0.32 mm i.d. 0.25 lm film thickness; Macherey-Nagel, Düren, Germany) and flame ionisation detection (FID). Injection was with a split ratio of 1:50 and 1:10 respectively when OC < 31 mg and < 5 mg OC were oxidised (Table 1 and Fig. 1). The detection limit for BPCAs was 7 ng injection amount (Brodowski et al., 2005). Precision from duplicate analysis was < 5% and recovery of the internal standard (citric acid) was always > 75%. 3. Results and discussion To identify whether and which BPCAs could be produced artificially from non-pyrogenic OM from samples that exceeded the 5 mg OC boundary for the BPCA method of Brodowski et al. (2005), we used plant material as a precursor, subjected the BCfree stems of Z. mays L. and leaves of C. annuum L. to OC and BPCA analysis. OC concentration was 430 g/kg in Z. mays L. and 385 g/kg in C. annuum L., respectively (dry wt). To explore potential condensation reactions we extended the range of weight to 0.6–215 mg OC for the stems of Z. mays L. and to 0.4–192 mg OC for the leaves of C. annuum L. (Fig. 1). For the C. annuum L. leaves, BPCA detection started at sample weight exceeding 5 mg OC. For the stems of Z. mays L., however, we detected B3CA and B4CA even at sample weight < 5 mg OC. We therefore concluded that, at excess OC amount in the reaction tubes, B3CA and B4CA may be artificially formed, i.e. they should not be considered in BC evaluation. We did not detected B5CA and B6CA at a sample weight < 5 mg OC. As long as the use of BPCA analysis as a proxy for combustion temperature relies solely on these BPCAs (ratio B5CA to B6CA; Wolf et al., 2013), the reconstruction of burning conditions from the BPCA pattern should not be affected. The total yield of BPCAs relative to OC increased with sample weight and hence with OC weight (R2 0.81, p < 0.001 for Z. mays L. and R2 0.91, p < 0.001 for C. annuum L., respectively). The amount of BPCAs increased systematically when the concentration of OC exceeded 10 mg OC in 2 ml HNO3. The maximum amount detected was ca. 8.7 mg BPCAs/g OC (Fig. 1). Using a conversion factor of 2.27, this accounted for up to 19.7 mg BC/g OC, i.e. a BC contribution of 2% to OC. Soil usually contains 10–30% aromatic carbon in the SOM (Kögel-Knabner and Amelung, 2014), i.e. in most cases artificial BPCA formation would remain insignificant even if the recommended threshold value for BC oxidation of 5 mg OC (Brodowski et al., 2005) were exceeded. Yet, in some fire-protected
49
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51 Table 1 BPCA production from chlorophyllin, ellagic acid and b-carotene (BDL, below detection limit).a
a b c d e
Sample (weight)
B3CAb (mg/g OC)
B4CAc (mg/g OC)
B5CAd (mg/g OC)
B6CAe (mg/g OC)
Sum (mg/g OC)
Chlorophyllin (2.95 mg OC) Chlorophyllin (6.35 mg OC) Ellagic acid (3.01 mg OC) Ellagic acid (6.41 mg OC) b-Carotene (4.25 mg OC) b-Carotene (9.37 mg OC)
BDL 1.05 ± 0.17 BDL BDL BDL 0.35 ± 0.04
BDL 11.15 ± 1.87 BDL BDL BDL 8.89 ± 0.69
BDL 7.40 ± 0.6 BDL BDL BDL 3.69 ± 0.09
BDL BDL BDL BDL BDL BDL
BDL 19.6 ± 2.61 BDL BDL BDL 12.92 ± 0.75
Substances were injected for GC with a split ratio of 1:10 and 1:50 (1:10 < 5 mg C, 1:50 > 5 mg; n, 4; BDL, below detection limit). B3CA, R hemimellitic, trimellitic, trimesic acids. B4CA, R pyromellitic, mellophanic, prehnitic acids. B5CA, benzene pentacarboxylic acid. B6CA, mellitic acid.
Fig. 1. Amounts of BPCAs from (a) Zea mays L. and (b) Capsicum annuum L. detected using GC-FID with a split ratio of 1:10 and 1:50 (B3CA, R hemimellitic, trimellitic, trimesic acids; B4CA, R pyromellitic, mellophanic, prehnitic acids; B5CA, benzene pentacarboxylic acid; B6CA, mellitic acid); n.d., not detected; BQL, below quantifiable limit (for B5CA, 0.5 g C/kg C for B6CA 0.17 g C/kg C; split 1:50).
environments, the specific contribution of BC to the OM may be as low as 4% (Rodionov et al., 2010; Eckmeier et al., 2013; KögelKnabner and Amelung, 2014). In this case the risk would be significant by way of artificially produced BPCAs being falsely attributed to the presence of BC. In organic-rich material the proportion of BC in OC may be even smaller, e.g. ranging around 2% in varved lake sediments (Lehndorff et al., 2015), and likely decreasing further
for BC deposited on vegetation surfaces. The exclusion of B3CA and B4CA from BC content estimation may be used to avoid over estimation (Lehndorff et al., 2015); however, the smaller the ratio of BC to OC in organic-rich environmental samples, the greater the risk of artificial BC production. Notably, the magnitude of potential artificial BPCA formation is even greater than the amount of BC that has been potentially
50
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51
ascribed to biological sources (Glaser and Knorr, 2008). Indeed Glaser and Knorr (2008) had to use a sample weight of > 5 mg OC in order to obtain a reliable quantification of BPCAs. The authors used 750 mg soil with an OC content of 12.95–47.6 g/kg, corresponding to 9.7–35.7 mg OC during HNO3 digestion. They detected a BC content of 0.25–0.64 g/kg soil (samples with low amounts of OC also contained low amounts of BC, R2 0.85), corresponding to 12.6–28 mg BC/g OC. Of this BC, 25% contained the isotope signal, i.e. 3.2–7 mg BC/OC were not attributed to a pyrogenic origin. At comparable sample weights, though with totally different samples (pure plant material, no soil present), we found that artificial BPCA production may reach 3.2–19.7 mg BC/g OC (Fig. 1). This extent of potential artificial BPCA formation in plant litter could easily add up to the portion of BC that has been potentially ascribed to biological sources in mineral soil by Glaser and Knorr (2008). At least part of the BC formerly assigned to biological sources may thus be due to carbonisation reactions. Therefore, it is of the utmost importance not to exceed 5 mg OC of sample weight in future BC studies, or at least to test the role of soil minerals on the additional BPCA formation. The results give support to the hypothesis that there are specific precursors or precursor degradation products favouring BPCA formation under the given experimental conditions. Consequently, we tested next whether artificial BPCA production needed biological precursors that were similar to BC or not. We used cyclic carbon forms, ellagic acid, the unsaturated long chain hydrocarbon, bcarotene, and a derivative of chlorophyll (chlorophyllin) for testing BPCA production. OC content was 572 g/kg for ellagic acid and 891 g/kg for b-carotene. The tests showed that BPCAs were not produced in detectable amount from chlorophyllin, ellagic acid or b-carotene, provided that OC weight during hot HNO3 treatment did not exceed 5 mg (Table 1). Our data thus agree with the findings of Ziolkowski et al. (2011), who showed that aromatic structures were fully transformed during sample treatment if they consisted of fewer than three aromatic rings. When sample weight exceeded 5 mg OC, we were able to detect BPCAs with 3, 4, 5 and 6 carboxyl groups (B3CA, B4CA, B5CA; Table 1) from chlorophyllin and b-carotene. Surprisingly, ellagic acid did not produce BPCAs. However, the structures with less similarity to BC, b-carotene and chlorophyllin, did, suggesting that these compounds were likely transformed to new precursors of BC during hot HNO3 treatment. The data in Table 1 suggest that, at given experimental conditions, the samples produced < 20 mg BPCAs/g OC, corresponding to a contribution of < 2% BPCAs (4.5% BC) relative to OC treated. There is certainly no soil with OM consisting solely of b-carotene or chlorophyll-type structures. Nevertheless, since interference from other compounds not tested here appears likely, we strongly recommend maintaining this threshold of 5 mg OC sample weight (treated with 2 ml HNO3). A potential explanation for the artificial BC production during sample treatment is hydrothermal carbonisation (HTC). HTC is an exothermic process, which converts biomass at temperatures from ca. 180–250 °C and self-generated pressure to form HTC–char (duration < 12 h; Libra et al., 2011; Poerschmann et al., 2015). With the exception of the HNO3 addition, the HTC experimental conditions have some similarity with the conditions for oxidation of BC from soil, sediment and vegetation to BPCAs (2 ml 65% NHO3, 170 °C, 8 h in a high pressure digestion apparatus). Apparently, it cannot be excluded, a priori, that at least some part of OM at a sample concentration > 5 mg OC per 2 ml HNO3 could be transferred to HTC char during our sample preparation step, which would then be oxidised to BPCAs. As each soil and sediment sample also contain vegetation residues, there is a risk that the reactions described are relevant for all environmental samples, even if the specific role of soil minerals on HTC char formation has not been explicitly tested.
4. Conclusions We tested the performance of plant material and defined structures (ellagic acid, b-carotene and the synthetic chlorophyll derivative chlorophyllin) on potential BPCA formation at elevated sample concentration. We provide strong evidence that BPCAs – especially B3CA and B4CA – may not originate only from BC, but may be produced from OM during sample treatment and measurement when sample weight exceeds 5 mg OC. The use of BPCAs as markers for BC quality and as a proxy for combustion temperature is thus only reliable when OC weight during treatment is maintained at < 5 mg. Our data imply that care has to be taken when applying the BPCA method to organic rich matrices in the environment or to vegetation surfaces (e.g. for biomonitoring purposes), especially when a low amount of BC is assumed. Acknowledgments The authors acknowledge the Deutsche Forschungsgemeinschaft, Germany (German Research Council; DFG), Collaborative Research Centre 806 ‘‘Our Way to Europe: Culture-Environment Interaction and Human Mobility in the Late Quaternary”, and the Deutsche Bundesstiftung Umwelt, Germany (German Federal Environmental Foundation; DBU) for financial support. We also thank two anonymous reviewers for constructive comments. Associate Editor—I. Kögel-Knabner References Brodowski, S., Rodionov, A., Haumaier, L., Glaser, B., Amelung, W., 2005. Revised black carbon assessment using benzene polycarboxylic acids. Organic Geochemistry 36, 1299–1310. Brodowski, S., Amelung, W., Haumaier, L., Zech, W., 2007. Black carbon contribution to stable humus in German arable soils. Geoderma 139, 220–228. Cornelissen, G., Gustafsson, O., Bucheli, T.D., Jonker, M.T.O., Koelmans, A.A., Van Noort, P.C.M., 2005. Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation. Environmental Science and Technology 39, 6881–6895. Crutzen, P.J., Andreae, M.O., 1990. Biomass burning in the tropics – impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669–1678. Czimczik, C.I., Masiello, C.A., 2007. Controls on black carbon storage in soils. Global Biogeochemistry Cycles 21. Eckmeier, E., Mavris, C., Krebs, R., Pichler, B., Egli, M., 2013. Black carbon contributes to organic matter in young soils in the Morteratsch proglacial area (Switzerland). Biogeosciences 10, 1265–1274. Glaser, B., Amelung, W., 2003. Pyrogenic carbon in native grassland soils along a climosequence in North America. Global Biogeochemistry Cycles 17. Glaser, B., Knorr, K.-H., 2008. Isotopic evidence for condensed aromatics from nonpyrogenic sources in soils – implications for current methods for quantifying soil black carbon. Rapid Communications in Mass Spectrometry 22, 935–942. Glaser, B., Haumaier, L., Guggenberger, G., Zech, W., 1998. Black carbon in soils: the use of benzene carboxylic acids as specific markers. Organic Geochemistry 29, 811–819. Glaser, B., Balashov, E., Haumaier, L., Guggenberger, G., Zech, W., 2000. Black carbon in density fractions of anthropogenic soils of the Brazilian amazon region. Organic Geochemistry 31, 669–678. Goldberg, E.D., 1985. Black Carbon in the Environment: Properties and Distribution. Wiley, New York. Gustafsson, Ö., Gschwend, P.M., 1998. The flux of black carbon to surface sediments on the New England continental shelf. Geochimica et Cosmochimica Acta 62, 465–472. Hammes, K., Schmidt, M.W.I., Smernik, R.J., Currie, L.A., Ball, W.P., Nguyen, T.H., Louchouarn, P., Houel, S., Gustafsson, Ö., Elmquist, M., Cornelissen, G., Skjemstad, J.O., Masiello, C.A., Song, J., Peng, P., Mitra, S., Dunn, J.C., Hatcher, P.G., Hockaday, W.C., Smith, D.M., Hartkopf- Fröder, C., Böhmer, A., Lüer, B., Huebert, B.J., Amelung, W., Brodowski, S., Huang, L., Zhang, W., Gschwend, P.M., Flores-Cervantes, D.X., Largeau, C., Rouzaud, J.-N., Rumpel, C., Guggenberger, G., Kaiser, K., Rodionov, A., Gonzalez-Vila, F.J., Gonzalez-Perez, J.A., de la Rosa, J.M., Manning, D.A.C., López-Capél, E., Ding, L., 2007. Comparison of black carbon quantification methods using reference materials from soil, water, sediment and the atmosphere, and implications for the global carbon cycle. Global Biogeochemical Cycles 21, 2–18. Haumaier, L., 2010. Benzene polycarboxylic acids – a ubiquitous class of compounds in soils. Journal of Plant Nutrition and Soil Science 173, 727–736.
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51 Jaffé, R., Ding, Y., Niggemann, J., Vähätalo, A.V., Stubbins, A., Spencer, R.G.M., Campbell, J., Dittmar, T., 2013. Global charcoal mobilization from soils via dissolution and riverine transport to the oceans. Science 340, 345–347. Kögel-Knabner, I., Amelung, W., 2014. Dynamics, chemistry, and preservation of organic matter in soils. In: Turekian, H.D.H.K. (Ed.), Treatise on Geochemistry, second ed. Elsevier, Oxford, pp. 157–215. Lehndorff, E., Wolf, M., Litt, T., Brauer, A., Amelung, W., 2015. 15,000 years of black carbon deposition – a post-glacial fire record from maar lake sediments (Germany). Quaternary Science Reviews 110, 15–22. Libra, J.A., Ro, K.S., Kammann, C., Funke, A., Berge, N.C., Neubauer, Y., Titirici, M.-M., Fühner, C., Bens, O., Kern, J., Emmerich, K.H., 2011. Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels 1, 89–124. Möller, W.A.A., Scharf, B.W., 1986. The content of chlorophyll in the sediment of the volcanic maar lakes in the Eifel region (Germany) as an indicator for eutrophication. Hydrobiologia 143, 327–329. Möller, A., Kaiser, K., Amelung, W., Niamskul, C., Udomsri, S., Puthawong, M., Haumaier, L., Zech, W., 2000. Forms of organic C and P extracted from tropical soils as assessed by liquid-state 13C- and 31P-NMR spectroscopy. Australian Journal of Soil Research 38, 1017–1035. Poerschmann, J., Weiner, B., Wedwitschka, H., Zehnsdorf, A., Koehler, R., Kopinke, F.D., 2015. Characterization of biochars and dissolved organic matter phases obtained upon hydrothermal carbonization of Elodea nuttallii. Bioresource Technology 189, 145–153.
51
Rodionov, A., Amelung, W., Peinemann, N., Haumaier, L., Zhang, X., Kleber, M., Glaser, B., Urusevskaya, I., Zech, W., 2010. Black carbon in grassland ecosystems of the world. Global Biogeochemical Cycles 24. Roth, P.J., Lehndorff, E., Brodowski, S., Bornemann, L., Sanchez-Garcia, L., Gustafsson, Ö., Amelung, W., 2012. Differentiation of black carbon pools in soils: method comparisons and perspectives. Organic Geochemistry 46, 66–75. Santín, C., Doerr, S.H., Kane, E.S., Masiello, C.A., Ohlson, M., de la Rosa, J.M., Preston, C.M., Dittmar, T., 2015. Towards a global assessment of pyrogenic carbon from vegetation fires. Global Change Biology. Schneider, M.P., Hilf, M., Vogt, U.F., Schmidt, M.W.I., 2010. The benzene polycarboxylic acid (BPCA) pattern of wood pyrolyzed between 200 °C and 1000 °C. Organic Geochemistry 41, 1082–1088. Schneider, M.P.W., Pyle, L.A., Clark, K.L., Hockaday, W.C., Masiello, C.A., Schmidt, M. W.I., 2013. Toward a ‘‘molecular thermometer” to estimate the charring temperature of wildland charcoals derived from different biomass sources. Environmental Science and Technology 47, 11490–11495. Smith, M.W., Baran, P.S., 2015. As simple as [2 + 2]. Science 349, 925–926. Wolf, M., Lehndorff, E., Wiesenberg, G.L.B., Stockhausen, M., Schwark, L., Amelung, W., 2013. Towards reconstruction of past fire regimes from geochemical analysis of charcoal. Organic Geochemistry 55, 11–21. Ziolkowski, L.A., Chamberlin, A.R., Greaves, J., Druffel, E.R.M., 2011. Quantification of black carbon in marine systems using the benzene polycarboxylic acid method: a mechanistic and yield study. Limnology Oceanography: Methods 9, 140.
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Black carbon assessment using benzene polycarboxylic acids: Limitations for organic-rich matrices Arne Kappenberg ⇑, Melanie Bläsing, Eva Lehndorff, Wulf Amelung Institute of Crop Science and Resource Conservation – Soil Science and Soil Ecology, University of Bonn, Nussallee 13, 53115 Bonn, Germany
a r t i c l e
i n f o
Article history: Received 5 November 2015 Received in revised form 22 December 2015 Accepted 21 January 2016 Available online 28 January 2016 Keywords: Black carbon Benzene polycarboxylic acids Analytical constraints
a b s t r a c t For the assessment of black carbon (BC), its oxidation to benzene polycarboxylic acids (BPCAs) is an established method. However, doubts about biological precursors remain and not all published data were obtained at low carbon concentration. We hypothesised that a considerable proportion of BC may be produced during sample treatment in the presence of a high amount of organic carbon (OC). We therefore tested whether and to which degree (i) BC-free material from stems of Zea mays L. (maize straw) and leaves of Capsicum annuum L. (bell pepper), as well as (ii) cyclic and non-cyclic carbon forms (chlorophyllin, ellagic acid and b-carotene) afford BPCAs when method protocols are overloaded with a sample above the recommended amount of 5 mg OC. The results showed that small amounts (< 2 g/kg OC) of BPCAs with three and four carboxyl groups may be formed even at low sample weight (< 5 mg OC), thereby falsely representing biological BC production. When this threshold was exceeded, all BPCA forms were detected. The artificial BPCA production yield in g OC increased with increasing amount of OC (R2 P 0.81), adding up to 8.7 g/kg OC (19.7 g BC/kg OC) artificial production. We therefore strongly recommend that a threshold of 5 mg OC sample concentration be maintained in future studies and that future BC assessments be restricted to BPCAs with five and six carboxyl groups. This constrains the application of the BPCA method for organic rich samples and for samples expected to contain a relatively low amount of BC. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Black carbon (BC) is formed by incomplete combustion of biomass and fossil fuel and can be found ubiquitously in soils, sediments and water, and as an aerosol in the atmosphere (e.g. Goldberg, 1985; Crutzen and Andreae, 1990; Jaffé et al., 2013). It is persistent in the environment (Goldberg, 1985; Glaser et al., 2000; Brodowski et al., 2007) and may thus contribute to a global carbon sink (Czimczik and Masiello, 2007; Rodionov et al., 2010; Santín et al., 2015). Different methods have been suggested for its detection in soil, all having limitations (Hammes et al., 2007; Roth et al., 2012). Among them, converting BC to benzene polycarboxylic acids (BPCAs) as a proxy for pyrogenic organic carbon (OC) input and as specific markers for combustion processes has gained increasing attraction because it is the only routine method that provides information on BC composition, by indicating the degree of aromatic condensation (Glaser et al., 1998; Brodowski et al., 2005) and hence the temperature of BC formation in the environment (Schneider et al., 2010, 2013; Wolf et al., 2013). Total BPCA ⇑ Corresponding author. Tel.: +49 228 732194; fax: +49 228 732782. E-mail address: [email protected] (A. Kappenberg). http://dx.doi.org/10.1016/j.orggeochem.2016.01.009 0146-6380/Ó 2016 Elsevier Ltd. All rights reserved.
content has been suggested as a proxy for BC content, employing a minimum conversion factor of 2.27 for the loss of C during oxidation (Glaser et al., 1998). This factor assumes that all BPCAs detected stem from BC. Intriguingly, BPCA yield correlated repeatedly and linearly with total organic matter (OM) content of soils and sediments (e.g. Gustafsson and Gschwend, 1998; Glaser and Amelung, 2003; Cornelissen et al., 2005). One explanation for co-occurrence of BC and soil OM (SOM) is that the production of BC is highest at sites with a high amount of biomass production (Glaser and Amelung, 2003; Czimczik and Masiello, 2007; Lehndorff et al., 2015). Another explanation could be either a method artefact (minimised from alteration of the method by Brodowski et al., 2005) or production of BPCAs from biological substances. The latter theory was substantiated by Glaser and Knorr (2008), who found that up to 25% of the isolated BC fraction from soils contained a 13C label added as non-pyrogenic C during lab and field studies. There are, however, no studies that could validate a biological BC-forming mechanism. In theory, Aspergillus niger, for instance, is able to produce mellitic acid (B6CA) biologically (Brodowski et al., 2005). Some of the B6CA could occur in a free form. Such a free form exists in soil
48
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51
(Möller et al., 2000; Haumaier, 2010), but should be removed during the sample preparation steps in the routine methodology (Brodowski et al., 2005). Besides, the total quantity of A. niger is likely not sufficient to explain the occurrence of B6CA (Brodowski et al., 2005). Moreover, we are not aware of any study that found other microbial strains able to condense OM to BC-like macromolecules. We therefore felt that neither the reason for some close correlations between BC and OC, nor for the high 13C labelling extent in BPCAs in the work of Glaser and Knorr (2008) was resolved satisfactorily. We concluded that potential biological precursors and the original method should be revisited a third time to better elucidate the risk of non-pyrogenic BPCA formation. Biological substances close to aromatic carbon structures are cyclic carbon forms, e.g. natural pigments like chlorophyll and carotene. Chlorophylls occur in plants, algae and cyanobacteria and may therefore contribute significantly to organic-rich soils and sediments. Large amounts of chlorophyll have been detected particularly in lake sediments (Möller and Scharf, 1986), paralleling elevated content of BC (Lehndorff et al., 2015). Nevertheless, Ziolkowski et al. (2011) found that aromatic carbon forms with fewer than three benzene rings did not afford BPCAs, making it unlikely that cyclic carbon forms produced in nature (e.g. carotene, chlorophyll derivatives) were a source for the production of BPCAs. Also, Brodowski et al. (2005) tested different plants and biological sources, such as the pigment aspergillin, for production of BPCAs. They found only a marginal amount of BPCAs (< 1% OC), but recommended that samples expected to contain > 200 g BC/kg OC and/or 100 g/kg OC, should be oxidised using a maximum of only 5 mg OC. This limit could be easily exceeded when samples contain an inherently high amount of OC, e.g. organic-rich lake sediments or peat and vegetation. Besides, it remained uncertain how degradation products of, e.g. carotene- and chlorophyll-related structures would behave under conditions of the exceeded concentration limit. The present study aimed at (i) identifying whether and which BPCAs could be produced artificially from non-pyrogenic OM from samples that exceeded the 5 mg OC boundary for the BPCA method, as suggested by Brodowski et al. (2005) and (ii) reelucidating BPCA formation from potential biological precursors under the high temperature and pressure conditions of the BPCA method. In addition, we aimed at (iii) evaluating the influence of such a high OC load on using the distribution of BPCAs as a proxy for BC combustion temperature.
2. Materials and methods 2.1. Sample description First, to test possible BPCA production from biological material we used BC-free material from stems of Zea mays L. and from leaves of Capsicum annuum L. The Z. mays L. stems were peeled and the leaves from C. annuum L. were grown in a high purity greenhouse in order to exclude the risk of contamination with atmospheric BC. The fresh material was dried at 40 °C and milled. Second, we re-tested production of BPCAs from potential biogenic precursor material using the cyclic carbon form, ellagic acid (2,3, 7,8-tetrahydroxy-chromeno[5,4,3-cde]chromene-5,10-dione), the synthetic, cyclic compound chlorophyllin and the unsaturated long chain hydrocarbon b-carotene. We chose these substances due to their structure and their abundance in nature, i.e. chlorophyllin, a tetrapyrrole, was tested due to the high abundance of chlorophylls in organic rich sediments (e.g. lake sediments). Chlorophyllin is a synthetic product but it is structurally comparable to natural chlorophyll forms. b-Carotene, a plant pigment, is a tetraterpene with two ionone rings connected by a chain of nine double bonded
carbons. Ellagic acid, a polyphenol with two aromatic rings, occurs in plants. All these compounds have structures, which are not obvious direct precursors for BPCA formation, unlike high molecular weight polycyclic aromatic carbon compounds, for instance (Glaser et al., 1998). However, all these compounds might well be degraded in HNO3 to potential precursor materials for aromatic re-condensation (e.g. Smith and Baran, 2015), which could then be the source of BPCAs. 2.2. BPCA method All samples were processed as outlined by Glaser et al. (1998) and Brodowski et al. (2005), at an OC weight < 5 mg, as recommended by Brodowski et al. (2005), as well as at elevated amount. Briefly, to remove polyvalent cations, samples were treated with 10 ml 4 M CF3CO2H. The residue was collected on glass fibre filters (GF 6, Hahnemühle FineArt GmbH, Dassel, Germany), and oxidised to BPCAs with hot HNO3 (8 h, 170 °C) within a high pressure digestion apparatus. After cleanup via a cation exchange column (Dowex 50WX8, 200–400 mesh, Fluka, Steinheim, Germany), the BPCAs were silylated and separated and quantified using gas chromatography (GC; Hewlett Packard 6890; Hewlett Packard GmbH, Waldbronn, Germany) equipped with a HP-5 column (30 m 0.32 mm i.d. 0.25 lm film thickness; Macherey-Nagel, Düren, Germany) and flame ionisation detection (FID). Injection was with a split ratio of 1:50 and 1:10 respectively when OC < 31 mg and < 5 mg OC were oxidised (Table 1 and Fig. 1). The detection limit for BPCAs was 7 ng injection amount (Brodowski et al., 2005). Precision from duplicate analysis was < 5% and recovery of the internal standard (citric acid) was always > 75%. 3. Results and discussion To identify whether and which BPCAs could be produced artificially from non-pyrogenic OM from samples that exceeded the 5 mg OC boundary for the BPCA method of Brodowski et al. (2005), we used plant material as a precursor, subjected the BCfree stems of Z. mays L. and leaves of C. annuum L. to OC and BPCA analysis. OC concentration was 430 g/kg in Z. mays L. and 385 g/kg in C. annuum L., respectively (dry wt). To explore potential condensation reactions we extended the range of weight to 0.6–215 mg OC for the stems of Z. mays L. and to 0.4–192 mg OC for the leaves of C. annuum L. (Fig. 1). For the C. annuum L. leaves, BPCA detection started at sample weight exceeding 5 mg OC. For the stems of Z. mays L., however, we detected B3CA and B4CA even at sample weight < 5 mg OC. We therefore concluded that, at excess OC amount in the reaction tubes, B3CA and B4CA may be artificially formed, i.e. they should not be considered in BC evaluation. We did not detected B5CA and B6CA at a sample weight < 5 mg OC. As long as the use of BPCA analysis as a proxy for combustion temperature relies solely on these BPCAs (ratio B5CA to B6CA; Wolf et al., 2013), the reconstruction of burning conditions from the BPCA pattern should not be affected. The total yield of BPCAs relative to OC increased with sample weight and hence with OC weight (R2 0.81, p < 0.001 for Z. mays L. and R2 0.91, p < 0.001 for C. annuum L., respectively). The amount of BPCAs increased systematically when the concentration of OC exceeded 10 mg OC in 2 ml HNO3. The maximum amount detected was ca. 8.7 mg BPCAs/g OC (Fig. 1). Using a conversion factor of 2.27, this accounted for up to 19.7 mg BC/g OC, i.e. a BC contribution of 2% to OC. Soil usually contains 10–30% aromatic carbon in the SOM (Kögel-Knabner and Amelung, 2014), i.e. in most cases artificial BPCA formation would remain insignificant even if the recommended threshold value for BC oxidation of 5 mg OC (Brodowski et al., 2005) were exceeded. Yet, in some fire-protected
49
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51 Table 1 BPCA production from chlorophyllin, ellagic acid and b-carotene (BDL, below detection limit).a
a b c d e
Sample (weight)
B3CAb (mg/g OC)
B4CAc (mg/g OC)
B5CAd (mg/g OC)
B6CAe (mg/g OC)
Sum (mg/g OC)
Chlorophyllin (2.95 mg OC) Chlorophyllin (6.35 mg OC) Ellagic acid (3.01 mg OC) Ellagic acid (6.41 mg OC) b-Carotene (4.25 mg OC) b-Carotene (9.37 mg OC)
BDL 1.05 ± 0.17 BDL BDL BDL 0.35 ± 0.04
BDL 11.15 ± 1.87 BDL BDL BDL 8.89 ± 0.69
BDL 7.40 ± 0.6 BDL BDL BDL 3.69 ± 0.09
BDL BDL BDL BDL BDL BDL
BDL 19.6 ± 2.61 BDL BDL BDL 12.92 ± 0.75
Substances were injected for GC with a split ratio of 1:10 and 1:50 (1:10 < 5 mg C, 1:50 > 5 mg; n, 4; BDL, below detection limit). B3CA, R hemimellitic, trimellitic, trimesic acids. B4CA, R pyromellitic, mellophanic, prehnitic acids. B5CA, benzene pentacarboxylic acid. B6CA, mellitic acid.
Fig. 1. Amounts of BPCAs from (a) Zea mays L. and (b) Capsicum annuum L. detected using GC-FID with a split ratio of 1:10 and 1:50 (B3CA, R hemimellitic, trimellitic, trimesic acids; B4CA, R pyromellitic, mellophanic, prehnitic acids; B5CA, benzene pentacarboxylic acid; B6CA, mellitic acid); n.d., not detected; BQL, below quantifiable limit (for B5CA, 0.5 g C/kg C for B6CA 0.17 g C/kg C; split 1:50).
environments, the specific contribution of BC to the OM may be as low as 4% (Rodionov et al., 2010; Eckmeier et al., 2013; KögelKnabner and Amelung, 2014). In this case the risk would be significant by way of artificially produced BPCAs being falsely attributed to the presence of BC. In organic-rich material the proportion of BC in OC may be even smaller, e.g. ranging around 2% in varved lake sediments (Lehndorff et al., 2015), and likely decreasing further
for BC deposited on vegetation surfaces. The exclusion of B3CA and B4CA from BC content estimation may be used to avoid over estimation (Lehndorff et al., 2015); however, the smaller the ratio of BC to OC in organic-rich environmental samples, the greater the risk of artificial BC production. Notably, the magnitude of potential artificial BPCA formation is even greater than the amount of BC that has been potentially
50
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51
ascribed to biological sources (Glaser and Knorr, 2008). Indeed Glaser and Knorr (2008) had to use a sample weight of > 5 mg OC in order to obtain a reliable quantification of BPCAs. The authors used 750 mg soil with an OC content of 12.95–47.6 g/kg, corresponding to 9.7–35.7 mg OC during HNO3 digestion. They detected a BC content of 0.25–0.64 g/kg soil (samples with low amounts of OC also contained low amounts of BC, R2 0.85), corresponding to 12.6–28 mg BC/g OC. Of this BC, 25% contained the isotope signal, i.e. 3.2–7 mg BC/OC were not attributed to a pyrogenic origin. At comparable sample weights, though with totally different samples (pure plant material, no soil present), we found that artificial BPCA production may reach 3.2–19.7 mg BC/g OC (Fig. 1). This extent of potential artificial BPCA formation in plant litter could easily add up to the portion of BC that has been potentially ascribed to biological sources in mineral soil by Glaser and Knorr (2008). At least part of the BC formerly assigned to biological sources may thus be due to carbonisation reactions. Therefore, it is of the utmost importance not to exceed 5 mg OC of sample weight in future BC studies, or at least to test the role of soil minerals on the additional BPCA formation. The results give support to the hypothesis that there are specific precursors or precursor degradation products favouring BPCA formation under the given experimental conditions. Consequently, we tested next whether artificial BPCA production needed biological precursors that were similar to BC or not. We used cyclic carbon forms, ellagic acid, the unsaturated long chain hydrocarbon, bcarotene, and a derivative of chlorophyll (chlorophyllin) for testing BPCA production. OC content was 572 g/kg for ellagic acid and 891 g/kg for b-carotene. The tests showed that BPCAs were not produced in detectable amount from chlorophyllin, ellagic acid or b-carotene, provided that OC weight during hot HNO3 treatment did not exceed 5 mg (Table 1). Our data thus agree with the findings of Ziolkowski et al. (2011), who showed that aromatic structures were fully transformed during sample treatment if they consisted of fewer than three aromatic rings. When sample weight exceeded 5 mg OC, we were able to detect BPCAs with 3, 4, 5 and 6 carboxyl groups (B3CA, B4CA, B5CA; Table 1) from chlorophyllin and b-carotene. Surprisingly, ellagic acid did not produce BPCAs. However, the structures with less similarity to BC, b-carotene and chlorophyllin, did, suggesting that these compounds were likely transformed to new precursors of BC during hot HNO3 treatment. The data in Table 1 suggest that, at given experimental conditions, the samples produced < 20 mg BPCAs/g OC, corresponding to a contribution of < 2% BPCAs (4.5% BC) relative to OC treated. There is certainly no soil with OM consisting solely of b-carotene or chlorophyll-type structures. Nevertheless, since interference from other compounds not tested here appears likely, we strongly recommend maintaining this threshold of 5 mg OC sample weight (treated with 2 ml HNO3). A potential explanation for the artificial BC production during sample treatment is hydrothermal carbonisation (HTC). HTC is an exothermic process, which converts biomass at temperatures from ca. 180–250 °C and self-generated pressure to form HTC–char (duration < 12 h; Libra et al., 2011; Poerschmann et al., 2015). With the exception of the HNO3 addition, the HTC experimental conditions have some similarity with the conditions for oxidation of BC from soil, sediment and vegetation to BPCAs (2 ml 65% NHO3, 170 °C, 8 h in a high pressure digestion apparatus). Apparently, it cannot be excluded, a priori, that at least some part of OM at a sample concentration > 5 mg OC per 2 ml HNO3 could be transferred to HTC char during our sample preparation step, which would then be oxidised to BPCAs. As each soil and sediment sample also contain vegetation residues, there is a risk that the reactions described are relevant for all environmental samples, even if the specific role of soil minerals on HTC char formation has not been explicitly tested.
4. Conclusions We tested the performance of plant material and defined structures (ellagic acid, b-carotene and the synthetic chlorophyll derivative chlorophyllin) on potential BPCA formation at elevated sample concentration. We provide strong evidence that BPCAs – especially B3CA and B4CA – may not originate only from BC, but may be produced from OM during sample treatment and measurement when sample weight exceeds 5 mg OC. The use of BPCAs as markers for BC quality and as a proxy for combustion temperature is thus only reliable when OC weight during treatment is maintained at < 5 mg. Our data imply that care has to be taken when applying the BPCA method to organic rich matrices in the environment or to vegetation surfaces (e.g. for biomonitoring purposes), especially when a low amount of BC is assumed. Acknowledgments The authors acknowledge the Deutsche Forschungsgemeinschaft, Germany (German Research Council; DFG), Collaborative Research Centre 806 ‘‘Our Way to Europe: Culture-Environment Interaction and Human Mobility in the Late Quaternary”, and the Deutsche Bundesstiftung Umwelt, Germany (German Federal Environmental Foundation; DBU) for financial support. We also thank two anonymous reviewers for constructive comments. Associate Editor—I. Kögel-Knabner References Brodowski, S., Rodionov, A., Haumaier, L., Glaser, B., Amelung, W., 2005. Revised black carbon assessment using benzene polycarboxylic acids. Organic Geochemistry 36, 1299–1310. Brodowski, S., Amelung, W., Haumaier, L., Zech, W., 2007. Black carbon contribution to stable humus in German arable soils. Geoderma 139, 220–228. Cornelissen, G., Gustafsson, O., Bucheli, T.D., Jonker, M.T.O., Koelmans, A.A., Van Noort, P.C.M., 2005. Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation. Environmental Science and Technology 39, 6881–6895. Crutzen, P.J., Andreae, M.O., 1990. Biomass burning in the tropics – impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669–1678. Czimczik, C.I., Masiello, C.A., 2007. Controls on black carbon storage in soils. Global Biogeochemistry Cycles 21. Eckmeier, E., Mavris, C., Krebs, R., Pichler, B., Egli, M., 2013. Black carbon contributes to organic matter in young soils in the Morteratsch proglacial area (Switzerland). Biogeosciences 10, 1265–1274. Glaser, B., Amelung, W., 2003. Pyrogenic carbon in native grassland soils along a climosequence in North America. Global Biogeochemistry Cycles 17. Glaser, B., Knorr, K.-H., 2008. Isotopic evidence for condensed aromatics from nonpyrogenic sources in soils – implications for current methods for quantifying soil black carbon. Rapid Communications in Mass Spectrometry 22, 935–942. Glaser, B., Haumaier, L., Guggenberger, G., Zech, W., 1998. Black carbon in soils: the use of benzene carboxylic acids as specific markers. Organic Geochemistry 29, 811–819. Glaser, B., Balashov, E., Haumaier, L., Guggenberger, G., Zech, W., 2000. Black carbon in density fractions of anthropogenic soils of the Brazilian amazon region. Organic Geochemistry 31, 669–678. Goldberg, E.D., 1985. Black Carbon in the Environment: Properties and Distribution. Wiley, New York. Gustafsson, Ö., Gschwend, P.M., 1998. The flux of black carbon to surface sediments on the New England continental shelf. Geochimica et Cosmochimica Acta 62, 465–472. Hammes, K., Schmidt, M.W.I., Smernik, R.J., Currie, L.A., Ball, W.P., Nguyen, T.H., Louchouarn, P., Houel, S., Gustafsson, Ö., Elmquist, M., Cornelissen, G., Skjemstad, J.O., Masiello, C.A., Song, J., Peng, P., Mitra, S., Dunn, J.C., Hatcher, P.G., Hockaday, W.C., Smith, D.M., Hartkopf- Fröder, C., Böhmer, A., Lüer, B., Huebert, B.J., Amelung, W., Brodowski, S., Huang, L., Zhang, W., Gschwend, P.M., Flores-Cervantes, D.X., Largeau, C., Rouzaud, J.-N., Rumpel, C., Guggenberger, G., Kaiser, K., Rodionov, A., Gonzalez-Vila, F.J., Gonzalez-Perez, J.A., de la Rosa, J.M., Manning, D.A.C., López-Capél, E., Ding, L., 2007. Comparison of black carbon quantification methods using reference materials from soil, water, sediment and the atmosphere, and implications for the global carbon cycle. Global Biogeochemical Cycles 21, 2–18. Haumaier, L., 2010. Benzene polycarboxylic acids – a ubiquitous class of compounds in soils. Journal of Plant Nutrition and Soil Science 173, 727–736.
A. Kappenberg et al. / Organic Geochemistry 94 (2016) 47–51 Jaffé, R., Ding, Y., Niggemann, J., Vähätalo, A.V., Stubbins, A., Spencer, R.G.M., Campbell, J., Dittmar, T., 2013. Global charcoal mobilization from soils via dissolution and riverine transport to the oceans. Science 340, 345–347. Kögel-Knabner, I., Amelung, W., 2014. Dynamics, chemistry, and preservation of organic matter in soils. In: Turekian, H.D.H.K. (Ed.), Treatise on Geochemistry, second ed. Elsevier, Oxford, pp. 157–215. Lehndorff, E., Wolf, M., Litt, T., Brauer, A., Amelung, W., 2015. 15,000 years of black carbon deposition – a post-glacial fire record from maar lake sediments (Germany). Quaternary Science Reviews 110, 15–22. Libra, J.A., Ro, K.S., Kammann, C., Funke, A., Berge, N.C., Neubauer, Y., Titirici, M.-M., Fühner, C., Bens, O., Kern, J., Emmerich, K.H., 2011. Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels 1, 89–124. Möller, W.A.A., Scharf, B.W., 1986. The content of chlorophyll in the sediment of the volcanic maar lakes in the Eifel region (Germany) as an indicator for eutrophication. Hydrobiologia 143, 327–329. Möller, A., Kaiser, K., Amelung, W., Niamskul, C., Udomsri, S., Puthawong, M., Haumaier, L., Zech, W., 2000. Forms of organic C and P extracted from tropical soils as assessed by liquid-state 13C- and 31P-NMR spectroscopy. Australian Journal of Soil Research 38, 1017–1035. Poerschmann, J., Weiner, B., Wedwitschka, H., Zehnsdorf, A., Koehler, R., Kopinke, F.D., 2015. Characterization of biochars and dissolved organic matter phases obtained upon hydrothermal carbonization of Elodea nuttallii. Bioresource Technology 189, 145–153.
51
Rodionov, A., Amelung, W., Peinemann, N., Haumaier, L., Zhang, X., Kleber, M., Glaser, B., Urusevskaya, I., Zech, W., 2010. Black carbon in grassland ecosystems of the world. Global Biogeochemical Cycles 24. Roth, P.J., Lehndorff, E., Brodowski, S., Bornemann, L., Sanchez-Garcia, L., Gustafsson, Ö., Amelung, W., 2012. Differentiation of black carbon pools in soils: method comparisons and perspectives. Organic Geochemistry 46, 66–75. Santín, C., Doerr, S.H., Kane, E.S., Masiello, C.A., Ohlson, M., de la Rosa, J.M., Preston, C.M., Dittmar, T., 2015. Towards a global assessment of pyrogenic carbon from vegetation fires. Global Change Biology. Schneider, M.P., Hilf, M., Vogt, U.F., Schmidt, M.W.I., 2010. The benzene polycarboxylic acid (BPCA) pattern of wood pyrolyzed between 200 °C and 1000 °C. Organic Geochemistry 41, 1082–1088. Schneider, M.P.W., Pyle, L.A., Clark, K.L., Hockaday, W.C., Masiello, C.A., Schmidt, M. W.I., 2013. Toward a ‘‘molecular thermometer” to estimate the charring temperature of wildland charcoals derived from different biomass sources. Environmental Science and Technology 47, 11490–11495. Smith, M.W., Baran, P.S., 2015. As simple as [2 + 2]. Science 349, 925–926. Wolf, M., Lehndorff, E., Wiesenberg, G.L.B., Stockhausen, M., Schwark, L., Amelung, W., 2013. Towards reconstruction of past fire regimes from geochemical analysis of charcoal. Organic Geochemistry 55, 11–21. Ziolkowski, L.A., Chamberlin, A.R., Greaves, J., Druffel, E.R.M., 2011. Quantification of black carbon in marine systems using the benzene polycarboxylic acid method: a mechanistic and yield study. Limnology Oceanography: Methods 9, 140.