Bioavailable Amino Acids in Soils of North-South and West-East Transects of Continental United States.

Soluble amino acids are those dissolved in soil solutions and are bioavailable to plant and organisms. In spite of their low fraction in the soil soluble organic N (SON) pool, normally less than 5% of SON, soluble amino acids play vital role in plant nutrition and nitrogen fluxes in terrestrial ecosystems. However, there is a paucity regarding their bioavailability and variability across a variety of soil ecosystems. The primary objective of this study is to assess the bioavailable amino acids composition and concentrations in soils of different climate zones of north-south and west-east transects of continental United States. A subset (149 out of 4871 sites) of surface (0-5cm, i.e A horizon) and C horizon soil samples collected from 2007 to 2010 for the USGS Geochemical Landscapes Project were analyzed in this study. The bioavailable amino acids were extracted using 0.01M KCl. The extracted bioavailable amino acids were derivatized with the Waters AQC-Fluor reagent, followed by detection of the derivatized amino acids on a high performance liquid chromatography coupled with a fluorescence detector (HPLC/FLD). A total of 24 amino acids were analyzed for the soil samples. The result showed significant variations for the levels of total bioavailable amino acids among soils from different sites. The concentration of total bioavailable amino acids ranged from 0.7 to 251.9 mg/kg in A horizon and 0.11 to 21.2 mg/kg in C horizon. The concentration of individual and total amino acids was significant larger in A horizon than in C horizon (p<0.0001). Dominant bioavailable amino acids in most of soils were glutamic acid, glutamine, aspartic acid, leucine, alanine, threonine, glycine and valine. The percentage composition of amino acids was significant different between the two horizons for most of the individual amino acids but less different for classified groups (acid, neutral, basic, aromatic, etc). Neutral amino acids were more predominant compared to acid, basic, aromatic and other amino acids.