Lability of soil organic carbon in tropical soils with different clay minerals Thilde Bech Bruun a, * , Bo Elberling a , Bent T. Christensen b a Department of Geographyand Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark b Department of Agroecology and Environment, Faculty of Agricultural Sciences, Aarhus University, P.O. Box 50. DK-8830 Tjele, Denmark article info Article history: Received 12 July 2009 Received in revised form 23 November 2009 Accepted 18 January 2010 Available online 4 February 2010 Keywords: Basal soil respiration SOC Carbon stabilization Clay mineralogy Fe and Al (hydr-) oxides Tropical soils abstract Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 mm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithioniteecitrateebicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammoniumeoxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The central role of soil organic carbon (SOC) in determining soil fertility has been recognized for centuries (Lal, 2008). The beneficial role of SOC is especially important in tropical farming systems, where soil productivity often relies on limited external inputs thus main- tenance of the SOC pool plays a key role in sustainable management of these soils (Tiessen et al., 1994). However, most research on SOC dynamics relates to temperate conditions while the mechanisms that control SOC turnover in tropical soils are less well researched (Wang and Hsieh, 2002). The SOC pool is a function of the amount and quality of the C input and its subsequent rate of mineralization, which can be reduced by stabilization processes that protect SOC against further decomposition. Thus, changes in input as well as in degree of stabilization will influence the mineralization rate as determined e.g. by basal soil respiration (BSR). Interactions between soil minerals and SOC may lead to stabilization of SOC by entrapment in soil micro-pores (Baldock and Skjemstad, 2000) and by intermolecular interactions between SOC and the surface of clay particles and Fe and Al (hydr-) oxides (Basile-Doelsch et al., 2007; Oades et al., 1989). However, the relative importance of the SOC stabilization mecha- nisms in different soil types and the influence of clay contents and mineralogy on SOC turnover rates in tropical soils remains only partially understood (Gonzalez and Laird, 2003). Our understanding of the influence of clay on SOC stabilization arises largely from early laboratory experiments involving soils amended with pure mined clay types, while more recent studies have addressed the effects of clay content and mineralogy on SOC turnover in soils in situ e.g. by studying textural gradients within a field (Gregorich et al., 1991; Thomsen et al., 1999). Other studies have relied on samples from different locations, but these studies are often confounded by contrasting climate or differences in land use of the investigated sites (Barthés et al., 2008; Wattel-Koekkoek et al., 2003). The majority of studies on organo-mineral interactions have been conducted as laboratory experiments using ultrasonic treatment to obtain complete soil dispersion and either applying chemicals to selectively extract different SOC pools or employing oxidation agents to imitate natural decomposition processes (Bruun et al., 2010; Paul et al., 2008b). Studies relating clay mineralogy to basal soil respiration * Corresponding author. Tel.: þ45 35333412; fax: þ45 35332175. E-mail address: thbb@life.ku.dk (T.B. Bruun). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2010.01.009 Soil Biology & Biochemistry 42 (2010) 888e895