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Soil carbon in tropical savannas mostly derived from grasses

Nature Geoscience volume 16, pages 710–716 (2023)Cite this article

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Abstract

Tropical savannas have been increasingly targeted for carbon sequestration by afforestation, assuming large gains in soil organic carbon (SOC) with increasing tree cover. Because savanna SOC is also derived from grasses, this assumption may not reflect real changes in SOC under afforestation. However, the exact contribution of grasses to SOC and the changes in SOC with increasing tree cover remain poorly understood. Here we combine a case study from Kruger National Park, South Africa, with data synthesized from tropical savannas globally to show that grass-derived carbon constitutes more than half of total SOC to a soil depth of 1 m, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (>70% of total SOC). Across the tropics, SOC concentration was not explained by tree cover. Both SOC gain and loss were observed following increasing tree cover, and on average SOC storage within a 1-m profile only increased by 6% (s.e. = 4%, n = 44). These results underscore the substantial contribution of grasses to SOC and the considerable uncertainty in SOC responses to increasing tree cover across tropical savannas.

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Fig. 1: Geographic locations of soil samples across tropical and subtropical savannas.
Fig. 2: Contributions of grass-derived SOC to total SOC across tropical and subtropical savannas.
Fig. 3: Changes in SOC storage following woody encroachment across tropical and subtropical grassy ecosystems.

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Data availability

Data supporting the results can be found at Dryad Digital Repository (https://doi.org/10.5061/dryad.c59zw3rbg). Source data are provided with this paper.

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Acknowledgements

Y.Z. was supported by Utah State University and a G. Evelyn Hutchinson Environmental Postdoctoral Fellowship from Yale Institute for Biospheric Studies. L.C.R.S. was partially supported by the National Science Foundation (AGS #1602958), the Convergence Accelerator Program #1939511, and by CNPq (PPBio-457602/2012–0 and PELD-441244/2016–5).

Author information

Author notes

  1. Madelon F. Case

    Present address: US Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, OR, USA

Authors and Affiliations

  1. Department of Wildland Resources and the Ecology Center, Utah State University, Logan, UT, USA

    Yong Zhou

  2. Yale Institute for Biospheric Studies, Yale University, New Haven, CT, USA

    Yong Zhou & A. Carla Staver

  3. Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT, USA

    Yong Zhou & A. Carla Staver

  4. Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Barbara Bomfim

  5. Department of Biological Sciences, University of Cape Town, Rondebosch, South Africa

    William J. Bond, Edmund C. February & Emma F. Gray

  6. Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, USA

    Thomas W. Boutton

  7. Institute of Ecology and Evolution, University of Oregon, Eugene, OR, USA

    Madelon F. Case, Lucas C. R. Silva & Jamie L. Wright

  8. Scientific Services, Kruger National Park, Skukuza, South Africa

    Corli Coetsee

  9. School of Natural Resource Management, Nelson Mandela University, George, South Africa

    Corli Coetsee

  10. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA

    Andrew B. Davies

  11. Environmental Studies Program, Department of Biology, University of Oregon, Eugene, OR, USA

    Lucas C. R. Silva & Jamie L. Wright

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Contributions

Y.Z. and A.C.S. conceived this study. A.C.S. conducted field sampling in Kruger National Park, and Y.Z. performed laboratory analyses. B.B., W.J.B., T.W.B., M.F.C., C.C., A.B.D., E.C.F., E.F.G., L.C.R.S. and J.L.W. contributed to conceptualization and data curation. Y.Z. collected data from the literature, performed data analysis and drafted the paper with substantial inputs from A.C.S. All authors provided paper feedback.

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Correspondence to Yong Zhou.

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Nature Geoscience thanks Aisling Devine, William Hoffmann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Xujia Jiang, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Model predictors for soil organic carbon (SOC) concentration (g C/kg) and grass-derived SOC (%) across Kruger National Park, South Africa.

(a) The correlation coefficient matrix among predictors used to model SOC concentration and grass-derived SOC. (b) Conditional regression plots from the best fitted linear model of changes in SOC concentration in response to grass biomass (kg/ha) and soil sand content (%). (c) Conditional regression plots from the best fitted linear model of changes in grass-derived soil C in response to grass biomass (kg/ha), soil sand content (%), mean annual rainfall (mm), and elephant density (kg/ha). All plots in panel (b) and (c) represent 72 sites across Kruger National Park (that is, n = 72). Shaded areas represent 95% confidence intervals. See Extended Data Table 1 for more details on model specifications.

Extended Data Fig. 2 Comparison of field measured soil sand content with soil sand content extracted from soil Grid.

(a) A scatter plot between field measured soil sand content (%) from Kruger National Park and soil sand content (%) extracted from SoilGrid (R2 = 0.03, p = 0.08, n = 98). (b) A scatter plot between soil sand content extracted from literature across tropical/subtropical savannas and soil sand content extracted from SoilGrid (R2 = 0.09, p = 0.015, n = 72). Dashed lines in panels (a) and (b) indicate the 1: 1 ratio line. Soil organic carbon (SOC) concentration (g/kg) throughout the entire 1-m profile across tropic and subtropical savannas as a function of soil sand content (%) extracted from the related literature (c). Regression line indicates a linear fit (R2 = 0.29, p < 0.0001, n = 72). Shaded areas indicate the 95% confidence intervals of the fit.

Extended Data Fig. 3 Model predictors for soil organic carbon (SOC) concentration (g/kg) and grass-derived SOC (%) across tropical and subtropical savannas.

(a) The correlation coefficient matrix among predictors used to model SOC concentration and grass-derived SOC. (b) Conditional regression plots from the best fitted linear model of changes in grass-derived soil C in response to different continent, mean annual rainfall (mm), and tree cover (%). (c) Conditional regression plots from the best fitted linear model of changes in SOC concentration in response to different continent, mean annual rainfall, and slope (◦). All plots in panel (b) and (c) represent 148 sites across tropical and subtropical savannas (that is, n = 148). Box plots display medians (that is, 50th percentile), 25th and 75th percentiles. Shaded areas represent 95% confidence intervals. See Extended Data Table 2 for more details on model specifications.

Extended Data Fig. 4 Grass-derived SOC as a function of mean annual rainfall across tropical and subtropical savannas.

(a) Boxplot distribution of mean annual rainfall (mm) for tropical and subtropical savanna sites across different continents (n = 81 for Africa, n = 5 for Australia, n = 6 for North America, and n = 56 for South America). Box plots display medians (that is, 50th percentile), 25th and 75th percentiles, and 95% confidence intervals. (b) Grass-derived SOC (%) throughout the 1-m soil profile as a function of mean annual rainfall (n = 148). The dotted black line indicates where grass- and tree-derived C contributed half of the total SOC (that is, 50%). Dashed blue lines indicate the upper bound and lower bound of the 95% quantile regressions.

Extended Data Table 1 Model specifications and model selections for soil organic carbon (SOC) concentration (g C/kg) and grass-derived SOC (%) across Kruger National Park, South Africa
Full size table
Extended Data Table 2 Model specifications and model selections for soil organic carbon (SOC) concentration (g/kg) and grass-derived SOC (%) across tropical and subtropical savannas
Full size table

Supplementary information

Supplementary Information

Supplementary Table 1

Source data

Source Data Fig. 2

Statistical source data.

Source Data Fig. 3

Statistical source data.

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Zhou, Y., Bomfim, B., Bond, W.J. et al. Soil carbon in tropical savannas mostly derived from grasses. Nat. Geosci. 16, 710–716 (2023). https://doi.org/10.1038/s41561-023-01232-0

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