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:

Professor Yakov Kuzyakov worked in various universities worldwide: Germany, USA, UK, Russia and China. His expertise covers soil ecology, biogeochemistry, rhizosphere processes, C and N cycling, soil degradation, land use and agroecology. Yakov led research projects in many countries such as Germany, China, Korea, Tanzania, Indonesia, Russia and Chile.

Together with his group, Yakov focuses on soil biogeochemistry with long-term and deep expertise for processes of C and N cycles, priming effects and nutrient mobilization. His special interests are in soil – plant – microbial interactions in hotspots – locations with very high activity and abundance of microorganisms like rhizosphere, detritusphere, biopores. Yakov groundbreaking works are in the fields of rhizodeposition, low molecular weight organic substances in soil, priming effects, visualization of enzyme activities, localization of root exudation, pedogenic carbonates, biochar stability, competition of roots and microorganisms for nutrients, microbial metabolism in soil, effects of plant photosynthesis on CO₂ efflux from soil.

To understand the process mechanisms, Yakov group use a broad range of stable (¹³C, ¹⁵N) and radioactive (¹⁴C, ³³P, ¹³⁷Cs) isotopes applications including pioneering position specific labeling and combine these approaches with biomarkers as well as microbial activity parameters.

Yakov is an active member of several editorial boards, for top-ranked journals including Soil Biology and Biochemistry, Land Degradation and Development, Biogeosciences, European Journal of Soil Biology, Rhizosphere, Scientific Reports.