Although the overall contribution of decaying plants, available substrate, and microbes to the buildup of soil organic matter (SOM) is well recognized, their individual contributions are not as clearly understood. Analytical shortcomings have constrained a thorough study that can distinguish the amount of SOM attributable to plants and the amount attributable to microbes. Using pyrolysis-GC/MS, the authors investigated the chemistry of carbon and microbe-depleted soils after 18 months, after inoculation by various substrates and with two clays (montmorillonite and kaolinite).
By six months, active microbial communities were present in all inocula save one, and SOM molecular diversity increased across all model soil systems. Soils treated with either sugar or syringol (a structural component of cell walls) contained substantial concentrations of lipids and proteins after 18 months. Soil organic carbon (SOC) also increased over time. Syringol-treated montmorillonite soils accumulated the most carbon, and had lower bacterial and higher fungal abundance than sugar-treated soils. Higher fungal abundance was positively correlated with carbon use efficiency (CUE) across treatments.
The authors concluded that the microbial community may be a stronger driver of SOM development than the soil’s mineralogy. They also found that their sugar- and syringol-treated samples provided a chemical diversity that was as rich as natural soil. This article contributes to an expanding scientific knowledge base regarding soil ecosystems and the critical role of soil microorganisms vis a vis the carbon cycle.
Kallenbach, Cynthia M., Serita D. Frey, & A. Stuart Grandy, 2016, Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls, Nature Communications 7, https://www.nature.com/articles/ncomms13630.