Prof. Yakov Kuzyakov1, Dr Ezekiel Bore2, Jun.-Prof. Michaela Dippold2
1Goettingen, Goettingen, Germany, 2University of Goettingen, Goettingen, Germany
Microbial transformation of organic substances is a key process of carbon (C) stabilization and soil organic matter (SOM) formation. Two general pathways are possible: i) convergence: means that C from initial organic compounds (e.g. in litter or rhizodeposition) are completely mixed and it is not possible to trace back their origin; or ii) divergence: means that the C fate by SOM formation completely depends on the initial organic compounds. We proved two opposite hypotheses that convergence and divergence of the fate of organic substances and of C atoms depend on microbial recycling and decomposition at two levels: 1) intermolecular: high recycling intensity leads to convergence of the C fate and is mainly important for the difference between the organic compounds, and 2) incorporation of C from various molecule positions into microbial metabolic cycles define the C fate at intramolecular level. We tested these hypotheses based on own and literature data to the fate of polymeric substances: sugars, proteins, lipids and lignin, as well as by C atoms from various positions of pentoses and hexoses by position specific 13C and 14C labeling.
The fate and functions organic compounds depends mainly on microbial recycling: C of the intensively recycled sugars and proteins, key components of microbial biomass, remains relatively long in soil, much longer than non-recycled clearly plant-specific compounds like lignin monomers. This is explained by two steps decomposition-stabilization mode of recycled compounds in contrast to one step decomposition mode of non-recycled substances. So, the convergence of the C fate is common for substances with fast microbial recycling that contrasts to divergence of slowly decomposed compounds.
For the intramolecular differences, we traced the fate of position-specific and uniformly 13C labeled glucose and ribose under field conditions for 800 days. Both sugars were simultaneously metabolized via glycolysis and pentose phosphate pathway. The similarity between position-specific 13C recovery in microbial biomass and soil reflected high contribution of microbial necromass to SOM. Despite the mean residence time (MRT) of glucose C-6 and ribose C-5 in soil were longer than of the other C positions, the MRT of uniformly labeled 13C of ribose in the soil was 3 times longer than that of glucose. Consequently, ribose and glucose were incorporated into different cellular components, defining their long-term fate in soil. The convergence of glucose C positions in soil and microbial biomass revealed that recycling dominated glucose transformation. In contrast, divergence of ribose C positions in soil revealed that intact ribose-derived cell components are reused or preserved in SOM.
Thus, convergence versus divergence distinguished the two key fates explaining the long persistence of C at inter- and intra-molecular levels: microbial recycling leads to convergence, whereas slow decomposition and preservation define the divergence of C pathways in soil.
Biography: To be confirmed.