Glucose and ribose stabilization in soil: Convergence and divergence of carbon pathways assessed by position-specific labeling

Transformation of sugars by microorganisms is a key process influencing carbon (C) stabilization in soil. To reveal the mechanisms responsible for the persistence of labile C in soil, the fates of position-specific and uniformly 13C labeled glucose and ribose were studied under field conditions for 800 days. We hypothesized a convergence of the fate of individual C positions and substances because of the long-term C recycling by microorganisms. Position-specific data revealed that both sugars were simultaneously metabolized via glycolysis and the pentose phosphate pathway (PPP). The position-specific 13C recovery pattern in soil and in microbial biomass was similar. This similarity demonstrated significant contribution of microbial products and necromass to soil organic matter (SOM) formation. Based on a biexponential model, the mean residence times (MRTs) of glucose C-6 and ribose C-5 in the soil were longer than the other C positions. However, the MRT of uniformly labeled 13C from ribose in the soil was 3 times longer than that from 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 and modification of recycled components 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 of individual C positions distinguished the two key stabilization mechanisms explaining the long persistence of C from easily available sources in the soil: sustained microbial recycling (convergence) versus preservation (divergence) in long-term stabilized compound classes.