Amino acids in grassland soils: Climatic effects on concentrations and chirality

The response of soil organic nitrogen (SON) dynamics to climate may partly be deduced from changes in the concentration and origin of the major N constituents in soil, such as amino acids. In this study, we determined the enantiomers of bound amino acids in 18 native grassland soils (0-10 cm) that were sampled along a transect from central Saskatchewan, Canada, to Southern Texas, USA. Mean annual temperature (MAT) ranged from 0.9 to 23.4 degrees C and mean annual precipitation (MAP) from 300 to 1308 MM. D-alanine and D-glutamic acid served as markers for the bacterial origin of SON. The D-content of lysine, phenylalanine, and aspartic acid indicated an ageing of the respective SON forms. Deuterium labeling was applied to account for hydrolysis-induced racemization reactions. We found that the concentration of the bacterial biomarkers was weakly but significantly parabolically related to MAT, as previously reported for microbial-derived amino sugars. The age markers D-lysine, D-phenylalanine, and D-aspartic acid comprised 2-15% of the respective L-form. The presence of these compounds demonstrated that the structures that contained these D-enantiomers had survived microbial attack, i.e., these hydrolyzable SON forms were conserved in soil despite a living environment. First estimates indicate that the mean residence time of the lysine-containing organic matter forms extend beyond a century. Within this time-scale we did not find that climate significantly affects the degree of ageing of SON constituents in the studied topsoils. (C) 2005 Elsevier B.V. All rights reserved.