Black Carbon Increases Cation Exchange Capacity in Soils

Black Carbon (BC) may significantly affect nutrient retention and play a key role in a wide range of biogeochemical processes in soils, especially for nutrient cycling. Anthrosols from the Brazilian Amazon (ages between 600 and 8700 yr BP) with high contents of biomassderived BC had greater potential cation exchange capacity (CECmeasured at pH 7) per unit organic C than adjacent soils with low BC contents. Synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy coupled with scanning transmission X-ray microscopy (STXM) techniques explained the source of the higher surface charge of BC compared with non-BC by mapping crosssectional areas of BC particles with diameters of 10 to 50 mm for C forms. The largest cross-sectional areas consisted of highly aromatic or only slightly oxidized organic C most likely originating from the BC itself with a characteristic peak at 286.1 eV, which could not be found in humic substance extracts, bacteria or fungi. Oxidation significantly increased from the core of BC particles to their surfaces as shown by the ratio of carboxyl-C/aromatic-C. Spotted and non-continuous distribution patterns of highly oxidized C functional groups with distinctly different chemical signatures on BC particle surfaces (peak shift at 286.1 eV to a higher energy of 286.7 eV) indicated that non-BCmay be adsorbed on the surfaces of BC particles creating highly oxidized surface. As a consequence of both oxidation of the BC particles themselves and adsorption of organic matter to BC surfaces, the charge density (potential CEC per unit surface area) was greater in BC-rich Anthrosols than adjacent soils. Additionally, a high specific surface area was attributable to the presence of BC, which may contribute to the high CEC found in soils that are rich in BC. B ubiquitously in soils to varyingextents as a result of deliberate vegetationburning, wild fires or energy production (Schmidt and Noack, 2000). Black C is also a product of fossil fuel burning and can occur in geological deposits, but these two forms of BC are not considered here. Worldwide BC formation from biomass burning was estimated to be 50 to 270 Tg yrwithmore than90%of theBC remaining in terrestrial ecosystems (Kuhlbusch et al., 1996). The majority of BC is believed to be stored in soils (Masiello, 2004), and can constitute a significant fraction of C buried in soils (Skjemstad et al., 1996; Schmidt et al., 1999; Schmidt and Noack, 2000). Black C may play an important role in a wide range of biogeochemical processes, such as adsorption reactions (Schmidt andNoack, 2000). The importance of adsorption of polycyclic aromatic hydrocarbons (PAH) and other organic pollutants to both biomassand fossil fuel-derived BC in soils and sediments has been established over the past years (Ghosh et al., 2000;Accardi-Dey and Gschwend, 2002; Braida et al., 2003). Limited information is available, however, on the effects of biomassderived BC on adsorption of base cations in soil. The chemical structure of BC is highly aromatic (Schmidt and Noack, 2000), yet the possibility of abiotic and microbial oxidation, and the formation of functional groups with net negative charge on BC particle surfaces cannot be ruled out (Schmidt et al., 2002). For example, charred plants showed large amounts of extractable humic and fulvic acids with high concentrations of carboxylic groups after oxidative degradation with dilute HNO3 (Trompowsky et al., 2005). Strong evidence suggests that nutrient dynamics in soil can significantly be influenced by BC (Glaser et al., 2001; Lehmann et al., 2003b). Anthrosols rich in BC were found to maintain high cation availability (Lima et al., 2002) compared with adjacent forest soils with similar mineralogy despite high leaching conditions in humid tropical Amazonia. Due to the prevalence of highly weathered clay minerals such as kaolinite in these soils, their ability to retain cations depends entirely on soil organic matter (SOM) contents (Sombroek, 1966). In addition to greater potential CEC associated with greater SOM contents, also trends of significantly higher CEC per unit soil organic C were observed in these Anthrosols compared with adjacent forest soils (Sombroek et al., 1993). Such greater CEC could be created by either of two mechanisms: (i) by a higher charge density per unit surface area which means a higher degree of oxidation of SOM; or (ii) by a higher surface area for cation adsorption sites, or a combined effect of both. Black C enrichment was believed to be the main contributor to the higher CEC in Anthrosols but the mechanism remained unclear (Glaser et al., 2001). Glaser et al. (2003) suggested the oxidation of the aromatic C and formation of carboxyl groups to be the main reason for B. Liang, J. Lehmann, D. Solomon, J. Kinyangi, J. Grossman, B. O’Neill, and J. Thies, Dep. of Crop and Soil Sciences, Cornell Univ., Ithaca, NY 14853, USA; J.O. Skjemstad, CSIRO Land and Water, Glen Osmond SA 5064, Australia; F.J. Luizão, Instituto Nacional de Pesquisa da Amazônia (INPA), 69011-970 Manaus, Brazil; J. Petersen (deceased), Dep. of Anthropology, Univ. of Vermont, Burlington, VT, 05405 USA; E.G. Neves, Museu de Arqueologia e Etnologia, Universidade de São Paulo, Sao Paulo, SP, 05508-900, Brazil. Received 28 Nov. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1719–1730 (2006).