Rhizosphere Leaves a Chemical “Fingerprint” on Mineral-associated Carbon

The Science

The field of soil carbon dynamics has faced a perplexing question when it comes to the role of microbial and plant root interactions in soil carbon stability: Why does carbon persist in some mineral-associated soil and not in others? Understanding this is critical to mitigating carbon cycling and soil health. A multi-institutional team recently examined how biogeochemical differences in plant roots and microbes found in the rhizosphere versus those found in bulk soil might affect the early stages of carbon association with minerals. Their findings indicated that while the total amount of carbon deposited on minerals was not significantly different in the presence or absence of roots, the chemistry of root-derived carbon bore a distinct chemical fingerprint.

The Impact

The team’s findings highlight the importance of understanding plant root–microbe–mineral interactions in the rhizosphere and the mechanism of carbon transformation and persistence in soil. Specifically, the presence of roots influences the type of organic matter that binds to minerals. This research also suggests that the presence of compounds in the soil that are derived from plant roots may be short-lived because of the rapid exchanges with mineral surfaces. Also demonstrated are that carbon transformation and persistence in soil are dependent on the interactions among plant roots, microbes, and minerals.

Summary

It is important to understand the role of microbial and plant root interactions with minerals in the rhizosphere because their interactions influence the stability of carbon in soils, and that stability is important for mitigating carbon cycling and improving soil health. To investigate these interactions, a multi-institutional team of scientists investigated the interactions between plant roots and how minerals affect soil organic matter (SOM) chemistry. For microcosms that contained the annual grass plant, Avena barbata, the team collected soil in close proximity to roots, or rhizosphere soil. For microcosms that didn’t have plants, the researchers collected bulk soil. The team incubated permeable mineral bags in soil microcosms, with and without plants, inside a carbon dioxide chamber. Using nuclear magnetic resonance spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and lipidomics capabilities available from EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility located at PNNL, the scientists traced carbon deposition onto minerals over three growth stages of the grass.

Experimental research determined that the dynamic rhizosphere environment and its associated microbial community create different rates of turnover and compositional characteristics of mineral-associated carbon. Rhizosphere soils containing roots from Avena barbata were found to influence the chemistry of mineral-associated SOM, displaying a more diverse array of compounds than the minerals incubated in bulk soil even though the total amount of carbon deposited on the minerals from either soil type was not significantly different. The data also suggests that the presence of compounds in soil that are derived from plant roots may be short-lived because of the rapid exchanges with mineral surfaces. Finally, the research implies that diverse rhizosphere-derived compounds may interact rapidly with mineral SOM.

Contacts 

Rachel A. Neurath, University of California, Berkeley, rneurath@berkeley.edu  

Malak Tfaily, Environmental Molecular Sciences Laboratory and the University of Arizona, Tfaily@email.arizona.edu 

Mary K. Firestone, University of California, Berkeley, mkfstone@berkeley.edu 

Funding

Funding for this work was provided by the DOE Biological and Environmental Research program through the Genomic Sciences program and by the Lawrence Livermore National Laboratory (LLNL) soil microbiome science focus area. Additional funding was also provided by an LLNL fellowship and a National Science Foundation grant. Experiments were performed at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility. Some research was also conducted at Lawrence Berkeley National Laboratory (LBNL), and at the University of California, Berkeley.

Publication

Neurath, RA, J Pett-Ridge, I Chu-Jacoby, D Herman, T Whitman, P Nico, A Lipton, J Kyle, M Tfaily, A Thompson, and M Firestone. 2021. “Root Carbon Interaction with Soil Minerals Is Dynamic, Leaving a Legacy of Microbially Derived Residues.” Environmental Science & Technology 2021 55 (19), 13345-13355 DOI: 10.1021/acs.est.1c00300