Mineralogical and substrate controls on microbial-derived soil carbon accumulation and stability
Despite increasing recognition of microbial contributions to soil organic matter (SOM), there is little empirical evidence that directly demonstrates microbial inputs as a significant proportion of total SOM. Moreover, we have yet to understand the ecological context and processes that regulate the amount of microbial-derived SOM. Specifically, the upstream C source utilized by microbes for biomass sysnthesis and the mineral matrix in which microbes interact with may impact microbial biomass C allocation, or even select for communities representing inherently different physiologies, with downstream consequences on SOM abundance.
To demonstrate SOM development directly from microbial residues and to further examine the relationship between C resources, the mineral matrix, and microbial C allocation on microbial-derived SOM, a 15-mo incubation experiment was set up using constructed, initially C- and microbial-free soils. Soil microcosms were constructed with either kaolinite or montmorillonite clays followed with a natural soil microbial inoculum. For both soil mineral treatments, weekly additions of glucose, cellobiose, or syringol were carried out. Over the course of the incubation, microbial growth efficeincy (MGE), activity, biomass (MBC), community structure and SOM accumulation rates were monitored. Pyrolysis-gas chromatography/mass spectrometry was used to track the accumulation of microbial SOM and stability was measured biologically using 13C isotopes.
We show that newly developed SOM in soil microcosms orginates from microbial residues but the accumulation of these residues is determined by both substrate and mineraolgy. After one year of incubation, the constructed soils accumulated 8-13 mg C g soil-. The chemistry of this C shows a near complete utilization of the substrate and the emergence of novel compounds, largely of lipid and proteinaceous origin, indicating substrate replacement by microbial residues. Mineralogy had significant effects on microbial C utilization and community structure, where kaolinite soils exhibited greater total C, MBC, MGE, and fatty acids but lower respiration and a greater abundances of fungi and filamentous bacteria relative to montmorrilonite soils. We also saw an effect of substrate quality where syringol-treated soils supported communities with higher MGE and fungal relative abundances compared to glucose and cellobiose treated soils and also accumulated the most C. These data suggest an influence of both mineralogy and susbtrate availablity on microbial metabolism and utlimately microbially–derived SOM accumulation rates. From this work, we can develop a better understanding of the ecological context in which SOM is formed and how altering microbial community metabolism and resource inputs affects the development of stable SOM.