Stoichiometric Effect of Labile C and N on the Transformation Dynamics of Soil Amino Acids

Abstract

How the substrate availability, especially the stoichiometry of labile C and N, influences the transformation dynamics of amino acids (AAs) is not well understood. The transformation pattern of soil AAs can only be explored by tracing the temporal change of both newly synthesized and the native portions. Therefore, laboratory incubations of soil samples were conducted by the addition of glucose and ¹⁵N-labeled inorganic N in either NH₄ ⁺ or NO₃ ⁻ form. The ¹⁵N enrichment in soil AAs was quantified by an isotope-based HPLC/MS method. The extraneous ¹⁵N was incorporated into AAs rapidly, and the newly formed portion was accumulated in soil as microbial metabolic constituents. Higher rate of ¹⁵N enrichment derived from ¹⁵NH₄ ⁺ than ¹⁵NO₃ ⁻ indicated microbial preference to the former N due to additional energy requirement in the reduction process. Both the accumulation of ¹⁵N-labeled AAs and the decomposition of the native portion were more significant in an Alfisol (with lower SOM) than a Mollisol (with higher SOM), especially in ¹⁵NO₃ ⁻ amendment, indicating that soil AAs were apt to be decomposed for compensating the intensive C demand in the energy-poor environment.