Table 1 lists the properties of the soil and the biochar. The soil was
very acidic (pH b4.0) and had low levels of total organic carbon (TOC)
(4.37%) and soil organic carbon (SOC) (b2.0%), which is typical for
soils in humid tropical regions. A low CEC might be the result of low
organic matter content and low clay activity in the soil. In addition,
heavy rainfall (N2500 mm yr−1) in the study area resulted in intensive
leaching.
Biochar made from the wood of white lead trees (L. leucocephala
(Lam.) de Wit) has an extremely high pH (N9.0) and a high liming potential
on acid soils. The biochar in this study had 78.3% TC and 0.64%
total nitrogen (TN), but relatively low levels of SOC (b2.0%) which
was determined by Walkley–Black method. The SOC estimated in
this study was considered as oxidisable carbon contents in the soil
and the biochar. This indicates the recalcitrant nature of the biochar
in the soils. The Fourier-transform infrared spectra (FTIR) of the
biochar (Fig. 1) show large proportions of OH stretching vibrations
of the H-bonded hydroxyl (O–H) group of phenol (aromatic compound),
implying that most of the C was stable in the biochar. The high specific
surface area (SSA) and porous characteristics (Fig. 1) of the biochar might be the reason for the higher CEC (22.3 cmol (+) kg−1) in the
biochar than in the study soil. Table 1 shows that the exchangeable cations
of the biochar were all higher than those of the study soils, especially
in calciumand potassium. This finding is consistent with the EDS results
of the biochar (Fig. 1).