Figure 22: The fragmentation pattern of L-alanine derivative in isotope-incubated soil samples.
(a) original sample; (b) incubated with 15N-labeled substrate; and (c) incubated with 13C-labeled substrate
Two types of techniques ¾ chromatography/mass spectrometry and chromatography/isotope ratio mass spectrometry ¾ are currently available for compound-specific isotope enrichment determination. According to the isotope quantification principle, the average isotope ratio in a molecule can be obtained for IRMS determination due to the combustion process. However, the isotope differentiation accomplished in our GC/MS techniques is based on ionization, and thus can be used for tracing the distribution of carbon and nitrogen in the molecular structure, which is especially powerful in investigating substrate metabolic pathways and the microbial transformation of soil carbon and nitrogen.
Amino sugars are one of the important microbial residue biomarkers, which are associated with the cycling of soil organic matter. However, little is known about their transformation kinetics in response to substrate availability, since living biomass only contributes a negligible amount to the total mass of amino sugars. By using the isotope tracing techniques, the newly-synthesized (labeled) amino sugars can be differentiated from the native portions in the soil matrix, making it possible to quantitatively evaluate the transformation pattern of amino sugars as well as to interpret the past and ongoing changes of microbial communities during the assimilation of extraneous 15N. Our study significantly improved the understanding of the turnover pattern of soil amino sugars and their roles in the microbial processes of soil C and N cycling. The determination of microbial biomarkers in combination with stable isotope tracer techniques can incorporate time effects within the investigation; thus the change from the instantaneous effect of the microbial biomarker to the soil organic matter dynamics can be achieved. This improvement was essential to assess the impact of microbial processes on carbon and nitrogen transformations and enhance our understanding of the role of microbial residues as biomarkers. Equally as important, the temporal pattern of the transformation and renewal of microbial-derived stable components can be inferred from the accumulated immobilization of extraneous carbon and nitrogen manipulated by microbial biomass utilization and necromass stabilization, the latter of which were recognized as “memory effect” in microbial processes. Thus stable microbial biomarkers have unique advantages in investigating microbial-driven carbon and nitrogen cycling.