Characterization of Organic Carbon Fractions in a Southwestern Nigerian Soil: an Acid Hydrolysis Approach.

Soil organic carbon characterization using acid hydrolysis is commonly performed at high temperatures but rarely at room temperature. This study used acid hydrolysis at 95 ºC and 25 ºC to quantify recalcitrant (non-hydrolyzable carbon, NHC) and labile C fractions of three depths in a planted fallow (Mucuna jaspedea) soil. Across depths, SOC fractions were: total SOC (11.83 – 21.57 g kg-1) > NHC 25 ºC (8.43 – 16.51 g kg-1) > NHC 95ºC (5.09 – 11. 41 g kg-1) ≥ labile C (5.03 – 10.16 g kg-1). Recalcitrant C fraction isolated at room temperature (NHC 25ºC) was not significantly different (P= 0.05) from total SOC concentrations across the soil depths, and contributed to a large amount of total SOC (71 – 92%), indicating incomplete hydrolysis of the labile C fraction. Conversely, recalcitrant C fraction isolated at 95 ºC (NHC 95ºC) contributed about 43 – 66 % to the total SOC and was significantly lower (P = 0.05) than total SOC, suggesting a meaningful separation between the recalcitrant and labile C fractions. The labile C fractions accounted for 34 – 57 % of total SOC and these amounts were generally similar but not significantly different from those of the NHC 95ºC. The recalcitrant C fractions (NHC 25ºC and NHC 95ºC) were significantly correlated with total SOC (R2 = 0.88 and 0.87 respectively) and with each other (R2 = 0.97) across the soil depths, but labile C was weakly correlated (R2 = 0.38; 0.20) with total SOC and NHC 25ºC. The stratification ratio (concentration at the soil surface/concentration at a lower depth) of SOC fractions were below the threshold value of 2 and was 1.0 to 1.7 for SOC, 1.1 to 1.8 for NHC, and 0.7 to 1.7 for labile C, suggesting a disturbed or degraded condition. The information provided from this study showed that acid hydrolysis at room temperature (25ºC) cannot be used as an alternative to hydrolysis performed at 95ºC.