Revised black carbon assessment using benzene polycarboxylic acids
Introduction
Black carbon (BC), a product of incomplete combustion of biomass and fossil fuel, is found nearly everywhere due to widespread aeolian transport (Goldberg, 1985, Stoffyn-Egli et al., 1997). The BC particles comprise a wide continuum of combustion residues ranging from char and charcoal to condensates, such as soot (Jones and Chaloner, 1991, Goldberg, 1985). BC is relatively inert. Because of this, Kuhlbusch (1998), for instance, suggested that it may be a potential sink for atmospheric CO2. If so, it could well contribute to the Earth’s radiative heat balance (Crutzen and Andreae, 1990). A number of authors have developed an interest in the presumed impact of BC on the global carbon cycle and have begun intensified research into the fate of charred organic material in various environmental compartments: the atmosphere (Chang et al., 1982, Charlson and Ogren, 1982, Andreae et al., 1994, Bird, 1997), (marine) sediments (Herring, 1985, Novakov et al., 1997, Masiello and Druffel, 1998, Middelburg et al., 1999) and soils (Skjemstad et al., 1996, Schmidt et al., 1999, Schmidt and Noack, 2000, Schmidt et al., 2002).
Despite growing interest in BC, both a generally accepted definition and standard analytical procedures are lacking (Schmidt and Noack, 2000, Schmidt et al., 2001). As pointed out by Schmidt et al. (2001), comparative results from six methods revealed a non-systematic variation in soil BC content by factors of 14–571. Thermal and UV-oxidation procedures have been criticized as overestimating BC content of soils and sediments (Simpson and Hatcher, 2004). To accurately quantify BC in the environment there is, therefore, a need to improve current methods. Glaser et al. (1998) proposed the use of benzene polycarboxylic acids (BPCAs) as specific markers for BC (Fig. 1). Their method involves pre-extraction with hot 32% HCl, oxidation of condensed aromatics to BPCAs using hot HNO3, purification and silylation of the products prior to quantification using GC with FID. Some drawbacks in the method became obvious, however, when it was applied to a large number of samples. Unacceptable analytical errors arose whenever samples with a wide range of weight and containing small amounts of BC were analyzed (Brodowski et al., unpublished data). Moreover, the test was unable to reveal whether the BPCAs came from artifactual formation or additional natural sources. For instance, treating organic matter or pinoresinol, a constituent of lignin (Freudenberg et al., 1965), with strong acid resulted in a formation of BPCAs (Brunow, 1965, Rudakov et al., 1986). Strong acid treatment of bacterial biomass or of a mixture of monosaccharides and amino acids produced melanoidin-like artefacts (Allard et al., 1997). Upon permanganate oxidation at least some melanoidins yield BPCAs (Ishiwatari et al., 1986). This finding led us to suspect that artefact formation in the pre-treatment step of the method might cause further methodological errors.
Apart from artificial formation, BPCAs could, in principle, also be derived from melanoidins, the BPCA yields from which are unknown. We agree with those who say that the Maillard reaction (non-enzymatic browning) does not appear to contribute any significant amount of SOM formation (Hatcher, 2003) or degradation of algae (Zang et al., 2001). We therefore disagree with those who contend that the process is a possible humification source for soils and marine environments (Nissenbaum and Kaplan, 1972, Stevenson, 1982, Ertel and Hedges, 1983, Rubinsztain et al., 1984, Poirier et al., 2000). The reason for this is that the process involves a number of reactions, some of which take place at temperatures substantially warmer and pH substantially more alkaline than those commonly found in soil. While high temperatures are known to form heterocyclic N and polyaromatic C from soil organic residues (Almendros et al., 2003), the only time that high temperatures occur naturally in soil is during a wildfire or controlled burning. Maillard reactions are not sustained at any substantial scale when fires are absent. We therefore suggest that any melanoidin-derived aromatic structure formed in soils should be assigned as BC. Reports of BPCA formation from some of the melanoidins may warrant attention.
Other natural products in soils, upon oxidation after treatment with strong acids, can produce BPCAs. One of these is aspergillin, a black amorphous pigment from Aspergillus niger (Lund et al., 1953) that is supposed to contribute to the aromaticity of soil humic substances (Kononova and Aleksandrova, 1959). At the latter time, aspergillin had not been analyzed using the method of Glaser et al. (1998).
The main aims of the present study were (1) to eliminate the analytical problem in samples with low BC contents when analyzed using the method of Glaser et al. (1998); (2) to examine whether the HCl treatment in this method artificially forms BPCAs and, thus overestimates BC contents of soils and if so, to establish an alternative pre-treatment; (3) to determine the extent to which synthetic melanoidins and naturally occurring aspergillin yield BPCAs using this method.
Section snippets
Samples
We obtained BC-free materials from above-ground biomass (stems and leaves) of seven plants (Astragalus sp., Cymbaria dahurica L., Leperiza sp., Populus pseudosimonii var. patula T.Y. Sun, Secale cereale L., Setaria viridis (L.) P. Beauv., Zea mays L.); and from soil-inhabiting microorganisms such as the fungi Aspergillus niger DSM B202, Penicillium citrinum DSM 62830 and the bacteria Pseudomonas putida DSM 291T and Agrobacterium tumefaciens ATCC 11095.
Melanoidins were prepared by condensation
Linearity
Previous experiments using different sample amounts for analysis according to the method of Glaser et al. (1998) revealed high coefficients of variation (⩾50% of the mean). In these earlier experiments, however, a one-point calibration was used. Derivatizing standard mixtures of different concentrations revealed an apparent linearity of detection (r2 = 0.9964 and higher). The coefficient of regression was only slightly improved when using a second-order regression curve. However, as previously
Conclusions
The original method of Glaser et al. (1998), which uses BPCAs as markers for the determination of BC in complex matrices like soil, contains an intrinsic error and requires revision. The major constraint in the original method is method-induced BPCA formation from the use of hot 32% HCl for the removal of interfering cations. Minor constraints are unreliable quantification of low BPCA yields with one-point calibrations, non-adjusted HNO3 aliquots for further sample processing (it results in
Acknowledgments
The authors acknowledge the helpful support of Prof. Dr. Wolfgang Zech. We also thank Elisabeth Keese for providing the microbial biomass and Tanja Gonter and Katja Poxleitner for their help in the laboratory. We gratefully acknowledge the constructive advice of Dr. José A. González, an anonymous reviewer and Dr. Geoff Abbott. This work was funded by the Deutsche Forschungsgemeinschaft (DFG Ze 154/48-1 and 48-2) and the Hanns-Seidel-Stiftung e.V.
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