Molecular and microscopic insights into the formation of soil organic matter in a red pine rhizosphere

Authors: DOHNALKOVA, ALICE - Pacific Northwest National Laboratory; TFAILY, MALAKM - Pacific Northwest National Laboratory; SMITH, A. - Pacific Northwest National Laboratory; CHU, ROSALIE - Pacific Northwest National Laboratory; CRUMP, ALEX - Pacific Northwest National Laboratory; BRISLAWN, COLIN - Pacific Northwest National Laboratory; VARGA, TAMAS - Pacific Northwest National Laboratory; SHI, ZHENQING - South China University Of Technology; Thomashow, Linda; HARSH, JAMES - Washington State University; KELLER, C. - Washington State University

Submitted to: Soil Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2017
Publication Date: 8/26/2017
Citation: Dohnalkova, A.C., Tfaily, M.M., Smith, A.P., Chu, R.K., Crump, A.R., Brislawn, C.J., Varga, T., Shi, Z., Thomashow, L.S., Harsh, J.B., Keller, C.K. 2017. Molecular and microscopic insights into the formation of soil organic matter in a red pine rhizosphere. Soil Processes. 1(1):4. https://doi.org/10.3390/soils1010004.
DOI: https://doi.org/10.3390/soils1010004

Interpretive Summary: Soils play a critical role in global carbon cycling, maintaining a dynamic reservoir in the form of soil organic matter that is estimated to be four times larger than the amount of carbon stored in the atmosphere. However, knowledge is lacking about how organic carbon compounds generated by plants and introduced into the soil by roots are stabilized in soil. In this study we investigated the stability of carbon components produced on and around the roots of pine seedlings grown in a sand-mineral mixture. We hypothesized that the nutrient-limited seedlings would cause the formation of persistent carbon by root-associated microbes, and that the resulting carbon constituents would contribute to the persistence of organic matter in the soil. We used molecular and imaging techniques to identify newly formed organic compounds in this system and showed that microbes associated with the pine roots produced complex organic substances that associated with minerals in the sand, contributing to mineral breakdown. The results of this study provide new insight into how organic material of microbial origin persists in soil ecosystems and evidence that this material associated with soil minerals may be important in global carbon cycling models.

Technical Abstract: Microbially-derived carbon inputs to soils play an important role in stabilization of 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 lacking. 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 persistent C, and the new constituents would contribute to SOM formation and persistence. We utilized a suite of advanced imaging and molecular techniques to obtain a molecular-level identification of newly-formed SOM compounds, and considered implications regarding their degree of long-term persistence. 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 persistence of microbial SOM products in ecosystems, and evidence that the residual biogenic material associated with mineral matrices may be important components in current large scale carbon cycle models.