Molecular and Imaging Insights into the Formation of Soil Organic Matter in a Red Pine Rhizosphere

Microbially-derived carbon inputs to soils play an important role in forming soil organic matter (SOM), but detailed knowledge of basic mechanisms of carbon (C) cycling, such as stabilization of organic C compounds originating from rhizodeposition, is limited. The objective of this study aimed to investigate the stability of rhizosphere-produced carbon components in a model laboratory mesocosm of Pinus resinosa grown in a designed mineral soil mix. We hypothesized that nutrient limitation would cause formation of microbially-produced C constituents that would contribute to SOM stabilization. We focused on the processes of rhizodeposition in the rhizosphere, and we utilized a suite of advanced imaging and molecular techniques to obtain a molecular-level identification of the microbial community and the newly-formed SOM compounds in the rhizosphere and the bulk soil. We considered implications regarding their degree of long-term stability. The microbes in this controlled, nutrient-limited system, without pre-existing organic matter, produced extracellular polymeric substances that formed associations with nutrient-bearing minerals and contributed to the microbial mineral weathering process. Electron microscopy revealed unique ultrastructural residual signatures of biogenic C compounds, and the increased presence of an amorphous organic phase associated with the mineral phase was evidenced by X-ray diffraction. These findings provide insight into the various degrees of stability of microbial SOM products in ecosystems and evidence that the residual biogenic material associated with mineral matrices may be important components in current carbon cycle models.