Vegetation affects pool size and composition of amino acids in Tibetan alpine meadow soils

Abstract

Soil amino acids are of great importance for nitrogen cycling and plant nutrition, especially in the nitrogen-limited ecosystems. However, the factors that influence amino acid pools in alpine meadow soils remain unclear. The aims of this study were to answer the following questions: (1) Do soil amino acid pools under different vegetation cover show appreciable differences in their concentration and composition during the growing season? (2) Is the pool size of soil amino acids in the alpine meadow smaller or larger than those in other ecosystems? Soils were sampled from three sites dominated by three typical types of plants (Kobresia graminifolia, Polygonum viviparum and Elymus nutans) in a Tibetan alpine meadow, respectively, during the growing season. Soils were extracted with water and 2M KCl solution, respectively, and amino acids were quantitatively determined by high performance liquid chromatography. The results show that the pool size of soil exchangeable amino acids extracted by 2M KCl in this alpine meadow is comparable to those in other N-limited ecosystems. The pool size of soil amino acids across the three sites displayed temporal dynamics during the growing season, with the pool size increased from May to July and decreased in August. The P. viviparum soil had the largest amino acid pool among the three sites in May, whereas the largest pool in July and August was found in the K. graminifolia soil. The composition of the soil amino acid pools across the sites changed over the season, with acidic amino acids being abundant (63.6–84.3%) in spring, and neutral amino acids dominating (53.7–66.0%) in summer. The three sites shared five dominant amino acids (lysine, glutamic acid, leucine, alanine, and serine) through June to August; however, the other dominant amino acids (glycine, valine and threonine) varied among the sites and the months. These results suggest that the concentration and composition of amino acid pools in the Tibetan alpine meadow soils could be significantly influenced by the vegetation cover. Additionally, soil sampling time and extraction methods could also exert pronounced effect on the results. The information obtained in this study may help in understanding the influence of plant-soil feedback on soil amino acid pools, and the mechanisms of organic nitrogen uptake by alpine plants.