THE ROLE OF MICROBIAL METABOLISM IN THE FORMATION OF MINERAL-ASSOCIATED ORGANIC MATTER IN SOIL

183 pagesThe majority of soil carbon (C) is associated with clay-size organo-mineral particles and is often observed to have longer turnover times than bulk soil C. There is mounting evidence that mineral-associated organic matter (MAOM) is of microbial origin, elevating the importance of metabolic transformations of soil C inputs during this stabilization process. My doctoral research employed a model systems approach to test linkages between substrate identity and microbial uptake preference, carbon use efficiency (CUE), and subsequent transformation of C into MAOM. In Chapter 1, I collected soils form Arnot Forest near Ithaca, NY and used exometabolomics to characterize an extracted model soil solution and culturing methods to create a microbial isolate library of fungi and bacteria (n = 20) using this solution. I then used time-resolved metabolic footprinting to phenotype the growth of a novel bacterium, P. solitsugae, in this soil extract, capturing the concentration changes of over 150 low molecular weight compounds, their temporal patterns, and determining the CUE of growth. For Chapter 2, I grew three isolates ranging in growth rate in a defined media modeled after soil extract, tracked the temporal uptake of 34 compounds and observed clustered, co-utilization of C substrates. No clear relationships between growth rate and CUE or substrate energy content and substrate use efficiency were observed, indicating a limited role of substrate energy content as a predictor of metabolic use. In Chapter 3, I conducted a batch sorption study to assess the impact of microbial and substrate source on the affinity of necromass C and N using labeled microbial necromass from Chapter 2 and goethite as a model mineral phase. I coupled this with in situ infrared spectroscopy to probe the mechanisms and kinetics of MAOM formation and destabilization. Broad differences were observed between fungal and bacterial necromass composition and resulting goethite MAOM, with bacterial MAOM containing more stable, phosphate-related groups indicative of nucleic acid contributions. Microbial source and substrate source both impacted the mineral surface affinity of necromass C and N, highlighting the importance of metabolism in driving the formation of MAOM