Amino acid distribution in a Loess-Palaeosol sequence near Luochuan, Loess Plateau, China

Abstract

The distribution and abundance of potentially mobile amino acids in loess and palaeosols from the Luochuan section have been determined. Amino acids in quantities significantly above laboratory background were detected in all samples, ranging in age from Recent to upper Pliocene. The highest concentration was measured in the Recent soil forming the top 1–2 m, below which yields fell away sharply declining by 98% at the bottom of the 130 m section. Average yields were smaller than those obtained earlier using a different method of sample preparation. Differences in abundance between neutral, acidic, and basic amino acids may be explained in terms of charge characteristics, molecular weight, and mode of interaction with mineral constituents. Amino acids, as such or complexed with humus, appear to be largely associated with CaCO 3 in the loess, and with clay in the paleosols. More of these compounds could thus be released from the loess than the palaeosols. More of the amino acids was probably microbial in origin, and ultimately derived from plant residues accumulating during the formation and development of individual palaeosols under a savannah-open woodland vegetation. The bulk of amino acids in loess would have come from the overlying palaeosols by water leaching and downward migration of clay and humus particles. Potentially mobile amino acids are more abundant in the loess layers than in the palaeosol horizons, and hence can be used as one of a range of criteria for recognizing the occurrence of ancient soil development.