Probing carbon flux patterns through soil microbial metabolic networks using parallel position-specific tracer labeling

In order to study controls on metabolic processes in soils, we determined the dynamics of (CO2)-C-13 production from two position-specific C-13-labeled pyruvate isotopologues in the presence and absence of glucose, succinate, pine, and legume leaf litter, and under anaerobic conditions. We also compared (CO2)-C-13 production in soils along a semiarid substrate age gradient in Arizona. We observed that the C from the carboxyl group (C-1) of pyruvate was lost as CO2 much faster than its other C atoms (C-2,C-3). Addition of glucose, pine and legume leaf litter reduced the ratio between (CO2)-C-13 production from 1-C-13 pyruvate and 2,3-C-13 pyruvate (C-1/C-2,C-3 ratio), whereas anaerobic conditions increased this ratio. Young volcanic soils exhibited a lower C-1/C-2,C-3 ratio than older volcanic soils. We interpret a low C-1/C-2,C-3 ratio as an indication of increased Krebs cycle activity in response to carbon inputs, while the higher ratio implies a reduced Krebs cycle activity in response to anaerobic conditions. Succinate, a gluconeogenic substrate, reduced (CO2)-C-13 production from pyruvate to near zero, likely reflecting increased carbohydrate biosynthesis from Krebs cycle intermediates. The difference in (CO2)-C-13 production rate from pyruvate isotopologues disappeared 4-5 days after pyruvate addition, indicating that C positions were scrambled by ongoing soil microbial transformations. This work demonstrates that metabolic tracers such as pyruvate can be used to determine qualitative aspects of C flux patterns through metabolic pathways of soil microbial communities. Understanding the controls over metabolic processes in soil may improve our understanding of soil C cycling processes. (C) 2010 Elsevier Ltd. All rights reserved.