The increase in KOC on HF-treatment

The increase in KOC on HF-treatment varied widely, and consequently the correlation between KOC of the whole soils and their isolated organic matter (HF-treated residues) is poor ( Fig. 1). This contrasts with the findings of Ahangar et al. (2008b), who reported a much tighter range of increases in KOC on HF-treatment; this is unsurprising given that Ahangar et al. (2008b) used a much less diverse range of soils. Neither soil clay content nor soil C content appears to control the magnitude of the organic matter–mineral interactions, since the ratio of KOC before and after HF-treatment correlates poorly with each of these parameters (data not shown). Clay mineralogy is likely to be important, based on the results from humic acid studies ( Murphy et al., 1994, Celis et al., 1998, Laor et al., 1998, Feng et al., 2005, Celis et al., 2006 and Feng et al., 2006). Indeed, a recent study ( Ketrot et al., 2013) reported the strength of interactions between organic matter and clay minerals was influenced by clay mineralogy across a range of tropical soils; in this study these interactions were found to influence cation exchange capacity and electrophoretic mobility of the soil clays.