Isotopic and Spectroscopic Investigations into the Microbial Formation of Soil Organic Matter.

Soil carbon is a vital to soil health, food security, and climate change mitigation, but understanding the underlying mechanisms controlling the stabilization and destabilization of soil carbon are still poorly known. There has been a conceptual paradigm shift in how soil organic matter is formed that now emphasizes the importance of microbial activity to build stable (i.e. long-lived) and mineral-associated soil organic matter. In this conceptual model, the consumption of plant carbon by micoorganisms, followed by subsequent turnover of microbial bodies closely associated with mineral particles, produces a layering of amino acid and lipid residues on the surfaces of soil minerals, that remains protected from destabilization from mineral-association and aggregation processes. We tested this new model by examining how do isotopically labeled plant and microbial C differ in their fundamental stabilization and destabilization processes on different soil minerals through a soil profile. We used a combination of laboratory and field-based approached to bridge multiple spatial scales, and used soil depth as well as synthetic minerals to create gradients of soil mineralogy. We used Raman microscopy as a tool to probe organic matter association with mineral surfaces, as it allows for the simultaneous quantification and identification of living microbes, carbon, minerals, and isotopes in 2-D space through time. As expected, we found that the type of minerals present had a strong influence on the amount of C retained, but the stabilization of new C critically depends on growth, death, and turnover of microbial cells. The destabilization of microbial residue C on mineral surfaces was unaffected by flushes of dissolved labile C. We believe this new insight into microbial mechanisms of carbon stabilization in soils will eventually lead to new avenues for measuring and modeling SOM dynamics and soils and aid in the management of soil carbon to mediate global challenges.