Characterization of soil organic carbon pools by acid hydrolysis

Chemically stable fractions of the soil organic carbon (SOC) contribute to the soil C sequestration and may have an important role for the global C budget. Soil organic carbon characterization was studied using acid hydrolysis methods with 1 M and 6 M HCl and hot-water extraction in two coastal plain soils collected from different landscape positions. The residues after the chemical treatments were analyzed by solid state 13C nuclear magnetic resonance (NMR) spectroscopy to investigate the changes in the structural composition. This study demonstrates the effectiveness of acid hydrolysis treatment with HCl for estimating the labile pools of SOC. Between 1 to 4% of total C was extracted by hot water in both soils compared to 18 to 32% of total C hydrolyzed by HCl. Although there were quantitative differences in the amount of C released, both soils showed similar percentages of non-hydrolyzable carbon (18–25% of total C for upland soil, and 24–32% of total C for wetland soil). The NMR spectra of the residues after acid hydrolysis indicated the removal of labile components (i.e. carbohydrates and amino acids), and the ability of 6 M HCl to characterize SOC quality appeared to be superior compared to the 1 M HCl hydrolysis and hot-water extraction. The best two-component model (R2 = 0.994) identified by stepwise regression analysis for hydrolyzable C pool included microbial C biomass and water-extractable C. Our data demonstrate that acid hydrolysis with HCl can provide meaningful estimates of C pools in sandy, low C coastal plain soils.