Rapid decomposition of traditionally produced biochar in an Oxisol under savannah in Northeastern Brazil Edvaldo Sagrilo a,1 , Tatiana F. Rittl b, ⁎ ,1 , Ellis Hoffland b , Bruno J.R. Alves c , Herony U. Mehl a , Thomas W. Kuyper b a Embrapa Mid-North, Av. Duque de Caxias 5650, 64006-220 Teresina, Piauí,Brazil b Department of Soil Quality, Wageningen University, PO Box 47, Wageningen 6700 AA, The Netherlands c Embrapa Agrobiology, km 47, Antiga Rodovia Rio — São Paulo, Seropédica 23890-000, Rio de Janeiro, Brazil abstract article info Article history: Received 26 March 2015 Received in revised form 12 August 2015 Accepted 13 August 2015 Available online 18 August 2015 Keywords: Pyrogenic organic matter Soil organic carbon Carbon sequestration Recalcitrance Stable isotope Oxisols Soil amendment with biochar has been claimed as an option for carbon (C) sequestration in agricultural soils. Most studies on biochar/soil organic carbon (SOC) interactions were executed under laboratory conditions. Here we tested the stability of biochar produced in a traditional kiln and its effects on the stocks of native SOC under field conditions. The biochar was characterized using pyrolysis–gas chromatography–mass spectrometry, and then added to an Oxisol under savannah climate. This soil was amended with 0, 5, 10, 20 and 40 Mg ha -1 of biochar in a randomized complete block design with four replications and cultivated with soybean over four cropping seasons (CSs; 120 days each). Soil samples from the 0–10 cm top layer were collected at the end of the first and fourth CSs and analyzed for CO 2 emissions, isotopic C abundance ( 13 C/ 12 C ratio) and enzymatic activity (fluorescein diacetate and dehydrogenase). The biochar showed a low degree of thermal modification. Its relative decomposition rate was higher (k = 0.32–1.00 year -1 ) than generally claimed (k = 0.005– 0.0005 year -1 ), and higher than the decomposition of native SOC (k = 0.22 year -1 ). Addition of biochar did not affect the stocks of native SOC. Our findings highlight the need for critically reviewing the potential of locally produced biochar to sequester C. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Biochar is the solid product of carbonization of organic materials at low oxygen concentration, intentionally produced to be applied in the soil (Lehmann and Joseph, 2009). The deliberate production and addition of biochar in soil distinguishes it from other carbonized products like charcoal and pyrogenic organic materials (PyOM). Soil amendment with biochar has been advocated as a climate-smart solution for agriculture, reducing atmospheric concentrations of carbon (C) dioxide (Woolf et al., 2010), thereby attracting the interest of the carbon market (Lehmann, 2007). When incorporated into the soil, bio- char is expected to contribute to the recalcitrant soil organic carbon (SOC) pool (Lehmann et al., 2006). The decomposition rate of biochar depends on the soil environment to which biochar is applied. Carbon- ized products decompose faster under warmer and drier conditions than under cooler and moister conditions (Glaser and Amelung, 2003; Nguyen and Lehmann, 2009). In well-aerated tropical sandy soils, charcoal can be degraded in decades (Bird et al., 1999; Zimmermann et al., 2012). These decomposition studies used charcoal produced by fire events (Bird et al., 1999; Glaser and Amelung, 2003; Nguyen et al., 2008; Zimmermann et al., 2012). It is not immediately evident that find- ings on charcoal degradation are relevant for the biochar debate, because one may assume that they are distinct types of materials. How- ever, regarding to the physico-chemical properties, these materials are essentially the same, and therefore the mechanisms that control their decomposition in the soil. Furthermore, findings of biochar produced and incubated under controlled conditions cannot be easily extrapolat- ed to field conditions. In order to apply and to scale up biochar use to mitigate climate change, biochar produced by traditional methods (e.g., in brick kilns) has to be tested under field conditions. This is espe- cially relevant in countries like Brazil, where such charring methods still predominate (Duboc et al., 2007). Currently, there is a lack of field data on decomposition rates of biochar produced in traditional kilns. Increases in CO 2 emission following biochar additions in soils or PyOM produced from wildfires may result from the decomposition of part of these materials (Cross and Sohi, 2011; Hilscher et al., 2009; Jones et al., 2011; Méndez et al., 2013; Sagrilo et al., 2014; Smith et al., 2010; Zimmerman et al., 2011). This suggests that some carbonized products (PyOM, biochar or charcoal) may be less recalcitrant than ex- pected (Knicker et al., 2013). In most studies however, it is not possible to distinguish whether the increased production of CO 2 after biochar addition is due to degradation of biochar and/or of SOC. Isotope analysis Geoderma Regional 6 (2015) 1–6 ⁎ Corresponding author. E-mail addresses: edvaldo.sagrilo@embrapa.br (E. Sagrilo), tatarittl@gmail.com (T.F. Rittl), ellis.hoffland@wur.nl (E. Hoffland), bruno.alves@embrapa.br (B.J.R. Alves), herony.mehl@embrapa.br (H.U. Mehl), thom.kuyper@wur.nl (T.W. Kuyper). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.geodrs.2015.08.006 2352-0094/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geoderma Regional journal homepage: www.elsevier.com/locate/geodrs