Characteristics of amino acids in soil humic substances

Humic substances are vital environmental materials. However, their size, shape, molecular weight, and structure are still unclear. In order to better understand the factors regulating the dynamics of soil organic matter, the qualitative and quantitative distribution of amino acids in humic substances was determined after 6 M HCl hydrolysis. Amino acids were determined using an amino-acid analyzer coupled with chromatographic techniques. Humic Acids (HA) and Fulvic Acids (FA) contained the similar forms of nitrogen after acid hydrolysis, but in different proportions. Amino acid distribution was different between HA and FA. Compared with FA, HA contained relatively higher amounts of basic amino acids, e.g., arginine, histidine, and lysine, but relatively lower amounts of acidic amino acids, e.g., aspartic and glutamic acids. Our data suggest that the percentage of basic amino acid in HA may increase with an increasing degree of humification. Changes in amino acid composition may also occur when soils are subject to long-term cultivation.