MOLECULAR AND MICROSCOPIC INSIGHTS INTO THE RHIZOSPHERIC PROCESSES
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Rhizosphere, the zone immediately adjacent to plant roots, represents a hotspot of carbon exudation, turnover, respiration, transformation and accumulation. The importance of roots for the formation of soil organic matter is critical, as the plant-specific substrates such as root exudates and plan litter serve as a carbon source for rhizospheric soil microorganisms, which subsequently metabolize and decompose the rhizodeposits, and transform them into more stabilized carbon forms. The objective of this research was to gain insight into the role soil microorganisms play in carbon stabilization, and to develop a molecular understanding of newly formed soil organic matter macromolecules produced by root exudates and microbes in the rhizosphere of red pine seedlings, in a controlled laboratory conditions experiment in the designed mineral matrix. We anticipated that a) microbes will contribute to the new soil organic matter pool by stabilizing root exudates and producing complex refractory carbon compounds, b) microbial residues will leave a stable signature of persistent organic matter on mineral surfaces, and c) soil organic matter composition will differ between rhizosphere soil and bulk soil. To achieve these objectives, we implemented a collective approach of imaging and analytical methods for characterizing the nature and extent of soil organic material. We categorized the organic carbon compounds based on their composition, biochemical classes, and their lability and persistence. In the subsequent experiment, we investigated these processes in the ponderosa pine forest rhizosphere field experiment to explore critical linkages among these mechanisms of mineral aggregate formation, microbial mineral weathering, and soil organic matter stabilization. We placed in-growth mesh bags containing biotite in the ponderosa pine rhizosphere and after nine months analyzed their contents by the same suite of methods as in the previous laboratory experiment. Additionally, we used high-resolution electron microscopy to examine the nature of microbially-induced soil carbon processes. Bacterial and fungal microbiomes were also analyzed, highlighting the bacterial and fungal influence on mineral weathering processes. The results suggest that the mineral aggregation and weathering were driven by the newly formed organic matter produced by microbial activity, and we present a visual conception of the complexity of these processes.