REGULAR ARTICLE Biogeochemical transformations of amino acids in soil assessed by position-specific labelling

Background and aims Amino acid turnover in soil is an important element of terrestrial carbon and nitrogen cycles. This study accounts for their driver- the micro-bial metabolism- by tracing them via the unique isoto-pic approach of position-specific labeling. Methods Three 14C isotopomers of alanine at five concentration levels combined with selective steriliza-tion were used to distinguish sorption mechanisms, exoenzymatic and microbial utilization of amino acids in soil. Results Sorption and microbial uptake occurred imme-diately. Unspecific microbial uptake followed a linear kinetic, whereas energy-dependent uptake followed Michaelis-Menten. Less than 6 % of the initially added alanine was sorbed to soil, but after microbial transfor-mation products were bound to the soil matrix at higher proportions (5–25 %). The carboxyl group (C-1) was rapidly oxidized by microorganisms, whereas C-2 and C-3 positions were preferentially incorporated into mi-crobial biomass. Dependency of C metabolization on amino acid concentration reflected individual alanine transformation pathways for starvation, maintenance and growth conditions. Conclusions This study demonstrates that position-specific labeling determines the mechanisms and rates of C cycling from individual functional groups. This approach reflected underlying metabolic pathways and revealed the formation of new organic matter. We therefore conclude that position-specific labeling is a unique tool for detailed insights into submolecular transformation pathways and their regulation factors