Changes in the molecular composition of organic matter leached from an agricultural topsoil following addition of biomass-derived black carbon (biochar) Thomas Riedel a,⇑ , Sascha Iden a , Jennifer Geilich a , Katja Wiedner b , Wolfgang Durner a , Harald Biester a a Institut für Geoökologie, Technische Universität Braunschweig, Langer Kamp 19c, 38106 Braunschweig, Germany b Bodenbiogeochemie, Martin-Luther-University Halle-Wittenberg, von-Seckendorff-Platz 3, 06120 Halle, Germany article info Article history: Received 7 August 2013 Received in revised form 13 December 2013 Accepted 1 February 2014 Available online 8 February 2014 Keywords: Soil organic carbon Biochar Ultra-high resolution mass spectrometry abstract The addition of biomass-derived black carbon or biochar to agricultural soils is attracting attention as a means for sequestering carbon and as a potentially valuable method for improving soil fertility. Recent research has shown, however, that biochar is not completely unreactive in soil as it potentially enhances microbial activity, adsorption of organic contaminants or release of polycyclic aromatic compounds, which may ultimately affect the composition of dissolved organic matter (DOM) leaching from a soil. We have tested this hypothesis using batch and soil column experiments. Addition of fresh biochar (1 wt.%) to a Gleyic Fluvisol resulted in a 22% reduction in OM mobilization from the soil. Ultrahigh resolution mass spectrometry revealed a marked change in the composition of the OM mobilized. The most saturated and reduced compounds were removed in the solutions leached from the biochar amended soil. On the other hand, while dissolved black carbon-type compounds were already leaching from the un-amended soil, their abundance increased after biochar amendment. We also detected newly-appeared ‘‘lignin-type’’ compounds with relatively high O/C ratio of ca. 0.65, indicating that non-black carbon was also leached from the amended soil in the form of highly oxygenated DOM. During a flow interruption of 1 week the soil columns turned anoxic. The DOM mobilized after the flow interruption was mostly identical to that mobilized under oxic conditions, with the exception of aromatic and polycyclic aromatic compounds being more abundant. These were probably associated with Fe and Mn oxides and released during the reductive dissolution of the oxides. In summary, the biochar amendment changed the molecular composition of soil-derived DOM. The net effect was (i) a reduction in total OC mobilization and (ii) a shift towards more oxidized and therefore less bioavailable OM that was leached to the subsoil. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Black carbon (BC), the product of incomplete combustion of bio- mass is a relatively stable form of carbon that is ubiquitously found in soils and sediments (Czimczik and Masiello, 2007). Research has focused on its stability in soils (Liang et al., 2006; Kuzyakov et al., 2009; Dittmar et al., 2012; Jaffé et al., 2013) to determine the long term fate of this relatively recalcitrant pool of soil organic matter (OM). Because the addition of biomass-derived black carbon (termed biochar) to agricultural soils has been proposed for sequestering carbon and enhancing soil fertility (Glaser et al., 2001; Lehmann et al., 2006), the impact on soil organic carbon (OC) cycling needs to be studied. Biochar may affect the composition of OM leached from amended soils in several ways. Kasozi et al. (2010) reported sorp- tion of Suwanee River fulvic acid (SRFA) and a phenolic acid (cate- chol) to pure biochar. A net removal of SRFA and catechol was observed. Whether or not SRFA was fractionated during sorption was not reported. This may be, however, of interest, because SRFA is a complex mixture of thousands of individual compounds (Sten- son et al., 2003) and it is unlikely that all of them would show sim- ilar sorption behavior. Knowledge of the compounds removed by biochar would help understand the underlying sorption mecha- nism. Only a limited number of studies exist which have investi- gated the behavior of individual compounds in the presence of biochar and most deal with organic contaminants. For example, sorption of organic contaminants to carbonaceous geosorbents http://dx.doi.org/10.1016/j.orggeochem.2014.02.003 0146-6380/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Present address: Institut für Chemie und Biologie des Meeres, Carl-von-Ossietzky Universität, 26111 Oldenburg, Germany. Tel.: +49 441 798 3768. E-mail address: thomas.riedel@uni-oldenburg.de (T. Riedel). Organic Geochemistry 69 (2014) 52–60 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem