Can biochar and hydrochar stability be assessed with chemical methods? Christophe Naisse a , Marie Alexis a , Alain Plante b , Katja Wiedner c , Bruno Glaser c , Alessandro Pozzi d , Christopher Carcaillet e , Irene Criscuoli f , Cornélia Rumpel a,⇑ a Bioemco, CNRS, UPMC (UMR 7618 CNRS-UPMC-ENS-UPEC-IRD-AgroParisTech), Thiverval-Grignon, France b Department of Earth & Environmental Science, University of Pennsylvania, Philadelphia, PA, USA c Soil Biogeochemistry, Martin-Luther-University Halle-Wittenberg, Halle, Germany d Advanced Gasification Technology, Cremona, Italy e Paleoenvironments and Chronoecology (PALECO), Ecole Pratique des Hautes Etudes and Centre for Bio-Archaeology and Ecology (UMR5059 CNRS), Université Montpellier 2, Montpellier, France f Foxlab – Fondazione Edmund Mach, S. Michele all’Adige, Trento, Italy article info Article history: Received 23 November 2012 Received in revised form 23 April 2013 Accepted 26 April 2013 Available online 7 May 2013 abstract Field application of biochar is intended to increase soil carbon (C) storage. The assessment of C storage potential of biochars lacks methods and standard materials. The reactivity of biochars and hydrochars may be one possible means of evaluating their environmental stability. The aim of this study was to eval- uate the reactivity of biochar produced by gasification (GS) and hydrochar produced by hydrothermal carbonisation (HTC). The approach included analysis of the two different char types produced from the same three feedstocks. Moreover, we analysed the reactivity of Holocene charcoal (150 and 2000 yr old) to evaluate whether or not their use as standard materials to represent stable biochar is meaningful. We assessed carbon loss following oxidation with acid dichromate as well as hydrolysis with HCl. Our results showed that chemical reactivity is not a straightforward approach for characterising the stability of biochar and hydrochar. Acid hydrolysis showed little difference between HTCs and GSs, despite the contrasting elemental composition. Using acid dichromate oxidation, we determined that GSs contained ca. 70% of oxidation resistant C while the proportion for HTCs was < 10%. The different feedstocks had a slight, but significant, influence on the reactivity of GSs and HTCs. The content of oxidation resistant C decreased in the order 100 yr old charcoal = GSs > 2000 yr old charcoal > HTCs > feedstock and was related to elemental composition. This shows that acid dichromate oxidation may allow differentiation of the reactivity of modern biochars but that there is not necessarily a relationship between reactivity and age of Holocene charcoals. As the chemical reactivity of biochars may change with exposure time in soil, it is poorly suited for assessing their environmental residence time. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Biochars are solids obtained from the carbonisation of biomass (http://www.biochar-international.org/biochar), used for ameliora- tion of agricultural soil and carbon sequestration (Lehmann, 2007; Wright and Brown, 2007). They are characterised by high aromatic- ity, forming a continuum from charred biomass to soot and graphite (Keiluweit et al., 2010). The proportion of aromatic C generally in- creases with pyrolysis temperature (Pastorova et al., 1994; Ascough et al., 2008) and time (Brewer et al., 2009). The most frequently used biochar production process is slow pyrolysis at a temperature between 180 and 450 °C. Gasification is carried out at higher temperature (1000 °C) for a shorter time (seconds). An alternative process for converting biomass to a new product is hydrothermal carbonisation (Amonette and Joseph, 2009). This process affords hydrochar produced at low temperature (between 200 and 300 °C; Wiedner et al., 2013) under pressure in the aqueous phase for several hours. Hydrothermal carbonisation and gasification are different processes, giving products with contrasting morphology and structure (Fuertes et al., 2010), which may influence their reactivity, and likely their susceptibility to microbial degradation, in soil. However, few studies have been carried out to assess their reactivity towards chemical treatment. We hypothesised that gasification chars (GSs) and hydrothermal carbonisation products (hydrochars, HTCs) could be characterised by way of contrasting reactivity: low reactivity for GSs and high reactivity for HTCs. Reactivity of biochar produced by pyrolysis may be assessed by its resistance to acid dichromate (Rumpel et al., 2007; Ascough et al., 2008; Knicker, 2010; Calvelo Pereira et al., 2011), as well as acid hydrolysis (Rumpel et al., 2007; Nocentini et al., 2010). Oxidation with acid dichromate (0.1 M K 2 Cr 2 O 7 in 2 M H 2 SO 4 ) as a powerful oxidant is widely used for determination of black carbon (BC) in soil (Bird and Gröcke, 1997; Certini et al., 2011). It 0146-6380/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.orggeochem.2013.04.011 ⇑ Corresponding author. Tel.: +33 130815479. E-mail address: cornelia.rumpel@grignon.inra.fr (C. Rumpel). Organic Geochemistry 60 (2013) 40–44 Contents lists available at SciVerse ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem