Influence of straw disposal on distribution of amino acids in soil particle size fractions

Abstract

Clay (<2 μm), silt (2–20 μm) and sand (20–2000 μm) size samples (isolated by ultrasonic dispersion and gravity sedimentation) and whole soils were hydrolyzed in 6 N HC1 and concentrations of amino acids (excluding S-containing but including diaminopimelic acid) determined. Two soils from an experiment in which straw had either been burned or incorporated annually for 18 yr were used. Amino acid N made up 31–39% of the total N in whole soils, clay and silt. The sand fraction was lower in hydrolyzable amino acid N (straw burned, 21–25%; straw incorp., 26–30%). Straw incorporation increased total N contents, the hydrolyzability of sand N, and the concentrations of most amino acids. Amino acid contents in clay and silt were 5–10 and 2–5 times higher than in whole soils, respectively, while the sand fraction contained 10–20 times less amino acids. The relative distribution of individual amino acids was almost the same for clay, silt, sand and whole soil. Glycine, argin ine. aspartic acid, histidine, glutamic acid and lysine were quantitatively the most important, making up 63–69% of the total amino acid N. Method of straw disposal or differences in decomposability of organic matter in different size fractions were not reflected in the distribution of hydrolyzable amino acids. The distribution of diaminopimelic acid suggested a higher (past or present) microbial biomass in the clay size fraction than in the silt.