Microbially-derived soil carbon: Experimental evidence links accumulation rates with microbial response to resource quality

Background/Question/Methods

Soil microbial communities, which use plant C as a primary energy source, make significant direct contributions to soil organic matter (SOM) when the communities turn over. Still, there is an empirical gap in understanding the pathways from plant inputs to SOM that has not kept pace with recent conceptual and quantitative models that source most stable SOM to microbial biomass. For example, it remains unclear how resource quality effecting microbial biomass production and C allocation influences the accumulation of microbially-derived SOM. To better understand the links between resource inputs and microbial SOM, we used a combination of laboratory and field-level approaches.  A 15-mo incubation using artificial, initially C- and microbial-free soils was set up with different clay mineralogies and a C resource gradient (glucose, cellobiose, or syringol). Over the course of the incubation, microbial growth efficiency (MGE), activity and biomass, and SOM accumulation rates were monitored. Pyrolysis-GC/MS was used to track microbial transformation of added substrates into complex SOM and stability was measured biologically using ¹³C isotopes. A similar experimental approach was applied to conventional and organic agricultural field treatments in Michigan to determine if higher soil C concentrations observed at the organic sites can be explained by differences in microbial physiologies.

Results/Conclusions

The long-term incubation demonstrated a significant influence of both soil mineralogy and substrate quality on microbial physiology with subsequent effects on total newly formed soil C concentrations. Microbial turnover of simple compounds resulted in soil C concentrations between 0.5 and 1% of which more than half was biologically stable. Py-GC/MS results showed a transformation of simple substrates into chemically complex SOM, rich in proteins and lipids. Treatments with lower respiration (e.g. kaolinite) and greater soil C also exhibited higher MGEs but were not always linked to high resource quality. Similar results between SOM formation and microbial physiology were found between an organic and conventional agricultural field experiment. Early results show the organic agricultural system has higher microbial growth rates in combination with historically higher C accumulation rates.  Preliminary ¹³C data show that these differences in soil C also relate to treatment effects on the fate of added C into microbial biomass, lipids, and silt-clay C pools. This work not only shows the direct contributions of microbes to SOM formation and chemical complexity but also demonstrates the importance of resource quality on SOM accumulation by the degree to which resources optimize microbial biomass production.