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Microbial formation of stable soil carbon is more efficient from belowground than aboveground input

Nature Geoscience (2018) | Download Citation


The relative contributions of aboveground versus belowground plant carbon inputs to the stable soil organic carbon pool are the subject of much debate—with direct implications for how the carbon cycle is modelled and managed. The belowground rhizosphere pathway (that is, carbon exiting the living root) is theorized to form stable soil carbon more efficiently than the aboveground pathway. However, while several mechanisms have been invoked to explain this efficiency, few have been empirically tested or quantified. Here, we use soil microcosms with standardized carbon inputs to investigate three posited mechanisms that differentiate aboveground from belowground input pathways of dissolved organic carbon—through the microbial biomass—to the mineral-stabilized soil organic carbon pool: (1) the physical distance travelled, (2) the microbial abundance in the region in which a carbon compound enters (that is, rhizosphere versus bulk soil) and (3) the frequency and volume of carbon delivery (that is, infrequent ‘pulse’ versus frequent ‘drip’). We demonstrate that through the microbial formation pathway, belowground inputs form mineral-stabilized soil carbon more efficiently than aboveground inputs, partly due to the greater efficiency of formation by the rhizosphere microbial community relative to the bulk soil community. However, we show that because the bulk soil has greater capacity to form mineral-stabilized soil carbon due to its greater overall volume, the relative contributions of aboveground versus belowground carbon inputs depend strongly on the ratio of rhizosphere to bulk soil.

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We thank E. Iturbe for assistance with the figures, M. Keiluweit for input on microcosm design and J. Sanderman, D. Rasse and C. Chenu for discussion during the planning stages of the experiment. We also thank E. Karlsen-Ayala and C. Lombroso for assistance with laboratory work. Funding was provided to N.W.S. from the National Science and Engineering Research Council of Canada, the Yale Institute for Biospheric Studies and a Doctoral Dissertation Improvement Grant from the National Science Foundation.

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    • Noah W. Sokol

    Present address: Lawrence Livermore National Laboratory, Physical and Life Sciences Directorate, Livermore, CA, USA


  1. Yale University, School of Forestry & Environmental Studies, New Haven, CT, USA

    • Noah W. Sokol
    •  & Mark A. Bradford


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N.W.S. and M.A.B. conceived the project. N.W.S. designed and led the research and analysed the data. N.W.S. wrote the manuscript, with contributions from M.A.B.

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The authors declare no competing interests.

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Correspondence to Noah W. Sokol.

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