Abstract:
Purpose Humic substances, which are integral components of total organic carbon (TOC), influence soil quality. The study
aimed to investigate whether humic and non-humic fractions exhibit early, consistent, and measurable changes and affect TOC
sensitivity and storage in a tropical sandy loam soils amended with corn cob biochar.
Materials and methods There were four treatments with four replicates established in a randomized complete block design.
Composite soil samples were taken from plots without biochar (CT), from plots incorporated with 15 t biochar ha-1 (BC-15), and
30 t biochar ha-1 without or with phosphate fertilizer (BC-30 and BC-30+P). The TOC, and humin, humic acid (HA), and fulvic
acid (HA) fractions of soil organic carbon were determined for each treatment. The optical densities (400–700 nm) were
measured on the soil-free extracts by spectrophotometry; the densities measured at 465 and 665 nm were used to calculate the
E465/E665 ratios.
Results and discussion The BC-30 and BC-30+P plots recorded the highest TOC, humin, humic acid (HA), and fulvic acid (FA)
contents with respect to the lowest in the CT. The total exchangeable carbon stratification was significantly higher in all the
biochar-treated plots relative to the CT. Spectral analysis showed higher values of E465/E665 (5.02 and 5.15) in the CT and BC-15-
treated soils, respectively, compared with the BC-30 and BC-30+P-amended soils with E465/E665 ratios of 2.76 and 2.98,
respectively.
Conclusions Corn cob biochar applied to a tropical sandy loam:
• increased the concentrations of HA and FA and led to increased stratification of TOC, with a stronger effect on HA compared
with FA;
• significantly lowered E465/E665 at the high biochar application rate of 30 t ha-1, implying the dominance of high molecular
weight humic acid-like substances, and increased degree of aromaticity of the TOC
