Abstract
Mineral-associated organic carbon (MAOC) constitutes a major fraction of global soil carbon and is assumed less sensitive to climate than particulate organic carbon (POC) due to protection by minerals. Despite its importance for long-term carbon storage, the response of MAOC to changing climates in drylands, which cover more than 40% of the global land area, remains unexplored. Here we assess topsoil organic carbon fractions across global drylands using a standardized field survey in 326 plots from 25 countries and 6 continents. We find that soil biogeochemistry explained the majority of variation in both MAOC and POC. Both carbon fractions decreased with increases in mean annual temperature and reductions in precipitation, with MAOC responding similarly to POC. Therefore, our results suggest that ongoing climate warming and aridification may result in unforeseen carbon losses across global drylands, and that the protective role of minerals may not dampen these effects.
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Data availability
The data associated with this study are publicly available via figshare (https://doi.org/10.6084/m9.figshare.24678891) (ref. 68).
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Acknowledgements
This research was funded by the European Research Council (ERC Grant agreement 647038, BIODESERT), the Spanish Ministry of Science and Innovation (PID2020-116578RB-I00) and Generalitat Valenciana (CIDEGENT/2018/041), with additional support by the University of Alicante (UADIF22-74 and VIGROB22-350). F.T.M. acknowledges support from the King Abdullah University of Science and Technology (KAUST) and the KAUST Climate and Livability Initiative. D.J.E. is supported by the Hermon Slade Foundation. H.S. is supported by a María Zambrano fellowship funded by the Ministry of Universities and European Union-Next Generation plan. L.W. acknowledges support from the US National Science Foundation (EAR 1554894). B.B. and S.S. were supported by the Taylor Family–Asia Foundation Endowed Chair in Ecology and Conservation Biology. M.B. acknowledges support from a Ramón y Cajal grant from the Spanish Ministry of Science (RYC2021-031797-I). A.L. and L.K. acknowledge support from the German Research Foundation, DFG (grant CRC TRR228) and German Federal Government for Science and Education, BMBF (grants 01LL1802C and 01LC1821A). L.K. acknowledges travel funds from the Hans Merensky Foundation. A.N. and C. Branquinho acknowledge support from FCT—Fundação para a Ciência e a Tecnologia (CEECIND/02453/2018/CP1534/CT0001, PTDC/ASP-SIL/7743/2020, UIDB/00329/2020), from AdaptForGrazing project (PRR-C05-i03-I-000035) and from LTsER Montado platform (LTER_EU_PT_001). S.C.R. was supported by NASA (NNH22OB92A) and is grateful to E. Geiger, A. Howell, R. Reibold, N. Melone and M. Starbuck for field support. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government. We thank the landowners for granting access to the sites and many people and their institutions for supporting our fieldwork activities: L. Eloff, J. J. Jordaan, E. Mudongo, V. Mokoka, B. Mokhou, T. Maphanga, D. Thompson (SAEON), A. S. K. Frank, R. Matjea, F. Hoffmann, C. Goebel, the University of Limpopo, South African Environmental Observation Network (SAEON), the South African Military and the Scientific Services Kruger National Park.
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F.T.M. designed and coordinated the global field survey. C.P., F.T.M. and E.M.-J. conceived this study. D.J.E., H.S., N.G., Y.L.B-P., B.G., V.O., E.G., M.G.-G., E.V., S.A., M.B., J.M.-V., B.J.M., W.F., N.E., S.C., M.A., R.J.A., J.M.A., F.A., V.A., A.I.A., K.B., F.B.S., N.B., B.B., M.A.B., D.B., C. Branquinho, C. Bu., Y.C., R. Canessa, A.P.C.-M., I.C., P.C.Q., R. Chibani, A.A.C., C.M.C., A.D.-N., B.D., C.R.D., D.A.D., A.J.D., J.D., H.E., C.E., A.F., M.F., D.F., L.H.F., J.J.G., E.G.M., R.M.H.-H., A.v.H., N.H., E.H.-S., F.M.H., O.J.-M., K.G., A.J., M.J., K.F.K., L.K., J.E.K., P.C.L.R., P.L., A.L., J.L., M.A.L., G.M.-K., T.P.M., O.M.I., E.M., P.M., A.J.M., M.P.M., J.V.S.M., J.P.M., G.M., S.M.M., A.N., G.O., G.R.O., B.O., G.P., Y.P., R.E.Q., S.C.R., V.M.R., A. Rodriguez, J.C.R., O.S., A.S., J.S., M.S., S.S., I.S., C.R.A.S., A.L.T., A.D.T., H.L.T., K.T., S.T., J.V., O.V., L.v.d.B., F.V., W.W., D.W., L.W., G.M.W., L.Y., E.Z., J.M.Z., Y.Z. and X.Z. performed field research. P.D.-M., V.O., B.G., B.J.M., S.C., N.E., J.C.G.-G., C.Z., M.P., W.F., I.B.-F., A. Rey, E.M.-J. and C.P. conducted laboratory research and analysis. P.D.-M., E.G. and C.P. carried out data analysis, after discussion, suggestions and contributions from F.T.M., E.M.-J., M.D.-B., N.G., Y.L.B-P., H.S., C.Z., M.P., P.G.-P., A. Rey., M.B. and S.M.M. P.D.-M. and C.P. wrote the original paper draft, with contributions from F.T.M., E.M.-J. and M.D.-B. All authors discussed the results and contributed to editing the paper.
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Extended data
Extended Data Fig. 1 Locations of the 326 plots surveyed across global drylands.
Locations are shown as red circles on a global aridity (1 – annual precipitation/potential evapotranspiration) map for drylands (areas with aridity > 0.35), on a less arid-to-more arid color scale.
Extended Data Fig. 2 Effects of climate on particulate organic C (POC) and mineral-associated organic C (MAOC) in dryland soils with organic C contents below and above the median.
a-d, Relationships between POC and MAOC in soils with soil organic C contents below and above the median and mean annual temperature (MAT, a and b, respectively) and precipitation (MAP, c and d, respectively). Lines and shading represent linear regressions and 95% confidence intervals. e-f, Summary of linear mixed-effects models for soils with organic C contents below (e, n = 318 POC and MAOC observations) and above (f, n = 316 POC and MAOC observations) the median, controlling for biotic factors and soil biogeochemistry (see Methods). The panel shows coefficients (circles) and 95% confidence intervals (CI, bars) for main and interaction effects of C fraction type (binary variable, either POC or MAOC) and climate (MAT and MAP) on POC and MAOC contents. The variance explained (R2) by the fixed and random effects relative to the total variance was 53% and 25%, respectively (n = 318), for soils with organic C content below the median, and 62% and 13%, respectively (n = 316), for soils with high organic C content above the median. Carbon fraction contents were natural-logarithm transformed, and all the predictors were standardized.
Extended Data Fig. 3 Importance of climate, biotic factors, and soil biogeochemistry in random forest models of particulate organic carbon C (POC) and mineral-associated organic carbon C (MAOC) in global drylands.
Climate predictors included mean annual temperature and mean annual precipitation; biotic factors included net primary productivity, type of vegetation, woody cover, plant richness, grazing pressure, and herbivore richness; and soil biogeochemistry included clay and silt, pH, chemical index of alteration, exchangeable Ca, non-crystalline Al and Fe, available N and P, and microbial biomass C. Importance was quantified as the increase in mean squared error (MSE) when a predictor was permuted. The variance explained by random forest models was 71% for POC and 85% for MAOC, respectively.
Extended Data Fig. 4 Effects of soil biogeochemistry on particulate organic C (POC) and mineral-associated organic C (MAOC) contents across global dryland soils.
Coefficients (dots) and 95% confidence intervals (CI, bars) for the effects of soil biogeochemical variables in linear mixed-effects models for POC and MAOC contents. The variance explained by the fixed and random effects relative to the total variance was 69% and 20% for POC (n = 317) and 84% and 11% for MAOC (n = 317), respectively.
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Díaz-Martínez, P., Maestre, F.T., Moreno-Jiménez, E. et al. Vulnerability of mineral-associated soil organic carbon to climate across global drylands. Nat. Clim. Chang. 14, 976–982 (2024). https://doi.org/10.1038/s41558-024-02087-y
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DOI: https://doi.org/10.1038/s41558-024-02087-y
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