13C isotopic fractionation during biodegradation of agricultural wastes

Abstract

Significant differences in δ13C signatures occur within and between plant tissues and their constituent biochemical entities, and also within and between heterotrophic bacteria and fungi and their metabolic products. Furthermore, 13C isotopic fractionation occurs during the biodegradation of organic molecules as seen in the substrate, respired CO2 and the microbial biomass, which could be related to substrate composition and/or microbial metabolism. The 13C isotopic fractionation observed during the decomposition of a single defined C substrate appears to be due to the intra-molecular heterogeneity in 13C in the substrate and to 13C isotopic fractionation during microbial metabolism. Very limited data suggest that the latter may be quantitatively more important than the former. Studies with defined fungi in culture media have highlighted the complexities associated with the interpretation of the observed patterns of 13C isotopic fractionation when a single defined C source is added to the culture medium which itself contains one or more C sources. Techniques involving 13C enrichment or paired treatments involving an equivalent C3- and C4-derived substrate have been devised to overcome the problem of background C in the culture medium and 13C isotopic fractionation during metabolism. Studies with complex substrates have shown an initial 13C depletion phase in respired CO2 followed by a 13C enrichment phase which may or may not be followed by another 13C depletion phase. Basic studies involving an integrated approach are required to gain a new insight into 13C isotopic fractionation during organic residue decomposition, by simultaneous measurements of δ13C in all C moieties. New analytical tools to measure real-time changes in δ13CO2 and the intra-molecular δ13C distribution within plant biochemical entities offer new opportunities for unravelling the complex interactions between substrate and microbial metabolism with respect to 13C isotopic fractionation during biodegradation.