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

Proteins or peptides are significant contributors to soil organic nitrogen pool. Their stabilization and bioavailability are considered to be critical points in terrestrial N cycle. Studies on their abundance, composition, and turnover in detail, however, are lacking especially in soils across a wide range of ecosystems. Our aims were: 1) to quantify hydrolysable amino acid contents in surface and subsurface horizons of soils from North-South and West-East Transects of Continental United States; 2) to investigate the relationship between hydrolysable amino acid abundance/composition and soil properties; and 3) to explore the impact of environmental factors such as temperature and vegetation cover on hydrolysable amino acids. Soil samples were hydrolyzed using 6 N HCl at 115℃ for 24 h. Amino acids in the hydrolysates were derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC), followed by analysis on a High Performance Liquid Chromatography (HPLC) equipped with a fluorescence detector (FLD). Seventeen amino acids were characterized in each soil sample. The concentrations of hydrolysable amino acids were significantly higher in A horizon soils than in C horizon soils.  The levels of hydrolysable amino acids were highly correlated with soil organic carbon contents. Between 58 to 88% of the total hydrolysable amino acids were Asp, Ser, Glu, Gly, Thr, Ala, Pro and Val. Neutral hydrolysable amino acids were more abundant while acidic hydrolysable amino acids were less abundant in A horizon soils than in C horizon soils. Principle component analysis (PCA) showed that hydrolysable amino acid composition was similar in soils throughout the two transects with various vegetation. However, the hydrolysable amino acid composition varied significantly between surface and subsurface soil horizons. The results suggested selective transformation and sorption of organic N in different soil depth.