Adding biochar to the soil caused a significant increase in the CEC
and in the amount of exchangeable cations in the amended soils,
suggesting an improvement in soil fertility and nutrient retention.
The improvement of the CEC can be attributed to the high SSA
(340 m2 g−1) of the biochar, which resulted from its porous structure
(Fig. 1a). Additionally, slow oxidation of the biochar increased the
number of carboxylic groups, which in turn increased the CEC of the
amended soil. These results agreed with those of Lehmann (2007),
who indicated that the CEC of the biochar increased with aging, primarily
because of the increased carboxylation of carbon through abiotic
oxidation (Cheng et al., 2006). Our results confirmed that biochar
can improve the exchangeable cation status of the soil, especially for
calcium, which correlated with the results of Lehmann et al. (2003),
and Chan et al. (2008), who believed that original nutrients in the
biochar itself supplied the exchangeable cations in degraded soils.