Formation of soil organic matter via biochemical and physical pathways of litter mass loss

Abstract

Soil organic matter is the largest terrestrial carbon pool1. The pool size depends on the balance between formation of soil organic matter from decomposition of plant litter and its mineralization to inorganic carbon. Knowledge of soil organic matter formation remains limited2 and current C numerical models assume that stable soil organic matter is formed primarily from recalcitrant plant litter3. However, labile components of plant litter could also form mineral-stabilized soil organic matter4. Here we followed the decomposition of isotopically labelled above-ground litter and its incorporation into soil organic matter over three years in a grassland in Kansas, USA, and used laboratory incubations to determine the decay rates and pool structure of litter-derived organic matter. Early in decomposition, soil organic matter formed when non-structural compounds were lost from litter. Soil organic matter also formed at the end of decomposition, when both non-structural and structural compounds were lost at similar rates. We conclude that two pathways yield soil organic matter efficiently. A dissolved organic matter–microbial path occurs early in decomposition when litter loses mostly non-structural compounds, which are incorporated into microbial biomass at high rates, resulting in efficient soil organic matter formation. An equally efficient physical-transfer path occurs when litter fragments move into soil.

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Figure 1: Dynamics of A. gerardii litter C loss from the soil surface and recovery of litter-derived C in the mineral soil.
Figure 2: Efficiency of SOC formation from litter C loss, and relative microbial incorporation of litter-derived C during A. gerardii litter decomposition.
Figure 3: Schematic representation of biochemical and physical pathways leading to SOM formation from progressive loss of litter chemical components.

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Acknowledgements

This work was supported by the National Science Foundation-Division of Environmental Biology grant no. 0918482 and the National Science Foundation Graduate Research Fellowship Program. Additional support during data analyses and writing came from the Cotrufo–Hoppess fund for soil ecological research and the OECD’s Cooperative Research Programme.

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M.F.C. developed the concepts, designed and led the research, contributed to data analyses and wrote the paper. J.L.S. and A.J.H. conducted all the research work and analyses. E.E.C. and M.L.H. contributed to data and statistical analyses. D.H.W. and W.J.P. were co-PI on the project and contributed to the development of research concepts. All authors contributed to writing of the paper.

Corresponding author

Correspondence to M. Francesca Cotrufo.

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The authors declare no competing financial interests.

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Cotrufo, M., Soong, J., Horton, A. et al. Formation of soil organic matter via biochemical and physical pathways of litter mass loss. Nature Geosci 8, 776–779 (2015). https://doi.org/10.1038/ngeo2520

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