Particle Size Fractions and Their Use in Studies of Soil Organic Matter: I. The Nature and Distribution of Forms of Carbon, Nitrogen, and Sulfur

Abstract

Organo‐mineral complexes in various size fractions from the surface horizons of two Chernozemic soils (Typic Argiboroll and Udic Haploboroll) were separated without chemical pretreatment by ultrasonic dispersion in water, followed by sieving and centrifugation. The organic carbon (C), nitrogen (N), and sulfur (S) composition in the size fractions and the degree of polycondensation of humic materials extracted by an alkaline pyrophosphate technique were compared. Fifty‐five to 58% of the organic C was in the clay fraction, with greatest absolute amounts in the coarse clay (2–0.2 µm). Carbon/nitrogen ratios narrowed as particle size decreased. The organic matter separated from the coarse‐clay and fine‐silt fractions (5–2 µm) was dominated by conventional humic acids (HA‐A), which based on their strong adsorption at 280 nm and resistance to acid hydrolysis, were considered strongly aromatic and recalcitrant in soil. In contrast, the organic matter associated with the fine clay (<0.2 µm) was largely fulvic acids (FA‐A, FA‐B) and humic acids (HA‐B) that were less aromatic than conventional humic acids and contained considerable amounts of hydrolyzable N. The fine‐ and coarse‐clay fractions (<2 µm) contained >70% of the total soil S, >80% of the HI‐reducible S, and >64% of the carbohydrate C. In relation to C and N, S was preferentially associated with the fine‐clay fractions. The C/S and N/S ratios decreased substantially from maximum values in the fine‐silt fraction (approximately 120:1 and 10:1, respectively) to minimum values in fine clay (approximately 33:1 and 4.5:1, respectively). The distinct differences between the humus of the coarse clay‐fine silt and the fine‐clay fractions indicate that size fractionation following ultrasonic dispersion in water is a promising method of isolating stable and labile forms of soil organic matter. The data also support earlier hypotheses on the nature of soil S that were based on studies of chemical separation of organic matter from complete soils.