Figure 23: 13C enrichment in amino sugars during incubation
B: black soil R: Red soil
MurN: muramic acid; GluN: glucosamine; GalN: galactosamine
As incubation progressed, a new equilibrium was approached, including the succession between bacteria and fungi, the growth and mortality of organisms, the synthesis of new compounds and the decomposition of organic residues. Even in the case of a continuous supply of labile substrate, a significant substrate saturation point was observed during microbial assimilation of extraneous substrates. The 13C enrichment of each amino sugar in soil with low organic matter (e.g., red soil) was significantly higher than that in soil with high organic matter content (e.g., black soil), indicating that starvation not only stimulates the continual microbial assimilation of available carbon, but also significantly promotes the transformation and renewal of soil organic components.
The adaptive succession and strategies of soil microorganisms
In order to elucidate the kinetics of residue-derived amino sugar formation, we set up a microcosm experiment with agricultural topsoil under two distinct tillage management regimes (conventional tillage, CT; no-till, NT) and uniformly 13C-labeled wheat residues from different plant parts (grain, leaf and root) over a 21-day incubation period. LC-IRMS techniques were used to measure the isotopic composition of individual amino sugars.
After the addition of 13C-labelled plant materials, the muramic acid incorporated a low proportion of exogenous carbon. The residue-derived glucosamine formation could be simulated with a first-order kinetic model. These results suggested that soil fungi outperformed bacteria in efficiently incorporating the organic substrates (Figure 24). Furthermore, the abundance of 13C-glucosamine was ranked in the order of grain > leaf > root, which agreed with the order of residue quality. Such effects on residue quality suggested that it was the availability of carbon that determined the fungal assimilation capacity of the above added substrates. Compared to soils with high organic matter (e.g. no-till, NT), fungi displayed a greater capability of utilizing available high labile C than soils with low organic matter content. In contrast, in the leaf and root treatments, the abundance of 13C-glucosamine was lower in CT than in NT. Thus, less readily available substrates proved to be unfavorable to fungal development, due most likely to the adaptive strategies of energy capture and metabolism. This study was the first to utilize a first-order kinetic model to describe the formation processes of residue-derived amino sugars, in the processes providing microbial succession footprints as affected by both substrate quality and tillage histories.
Figure 24: Residue-derived amino sugar concentrations as a function of time
£：Grain ：Leaf s：Root