Chemical, Spectroscopic, and Biological Characteristics of the Organic Matter in Particle Size Fractions Separated from an Aquoll

Abstract

AbstractSamples from the surface horizon of a cultivated Aquoll were ultrasonically dispersed and separated into particle size fractions. Humic acids (HAs) were extracted from the whole soil and each particle size fraction. Residual humic acids (RHAs) were obtained from the residue of each HA extraction by treating with HCl‐HF and reextracting. Chemical analyses showed that the fine silt fraction had the highest C and N contents. Although δ15N values and proportions of mineralizable N increased with decreasing particle size, C/N ratios decreased. Mineralizable N appeared to be related to both the total N content and the characteristics of the minerals with which the organic matter was associated. Nitrogen in sand and silt fractions was more resistant to mineralization than in clay fractions. Extractions with 0.5 M NaOH removed more of the C and N from the fine‐clay fraction than from the whole soil or any of the other fractions. Carbon‐13 nuclear magnetic resonance (NMR) spectra showed that HAs extracted from clays were more aliphatic than HAs extracted from larger particle sizes. The major aliphatic components in the clay size fractions appeared to be alkanoic and hydroxyalkanoic acids. Compared with HAs, RHAs, were more homogeneous, contained fewer proteinaceous materials and carbohydrates, were more aromatic, richer in phenolic OH and CO2H groups, had higher molecular weights, and contained more free radicals.