Global patterns of radiocarbon depletion in subsoil linked to rock-derived organic carbon
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Keywords: radiocarbon, soil organic carbon, rock-derived organic carbon
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Abstract
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 Figure 1 Rock organic carbon imprints on organic carbon concentration (OC) and radiocarbon activity (Δ14C). (a) Samples from Taiwanese soils, Andes and Himalayan rivers, and black shale weathering profiles where OCpetro inputs are known, with linear fits through data shown. (b) Binary mixing model (lines) between OCbio (Δ14C = +200 ‰) and OCpetro (varying from 1 % to 0.1 % of total OC, Δ14C = −1000 ‰). The grey shaded area is where >50 % of total OC is OCpetro. (c) Samples where OCpetro is absent: soil from Hawaiian Pololu lava flow and Icelandic river samples draining basalt. |  Figure 2 (a) The distribution of radiocarbon measurements for the SED and IG profiles, with corresponding box plots depicting the median and quartiles of the distributions. (b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020) dataset for the SED and IG profiles. (c) The ISRaD soil horizons from SED soils plotted in 1/[OC] and radiocarbon (Δ14C). (d) The ISRaD soil horizons from IG soils plotted in 1/[OC] and radiocarbon activity (Δ14C). The grey shaded region defines a zone where mixing of OCpetro can produce compositions which are difficult to obtain via OC decomposition alone (Fig. 1b). (e) Locations of samples where direct measurement of OCpetro inputs have been identified (diamond, triangle), and soil profiles from the ISRaD database on igneous (IG, circles) and sedimentary rocks (SED, squares). |  Figure 3 (a) The maximum fraction of [OCpetro] is calculated via a binary mixing model between OCbio and OCpetro in the globally distributed soil profiles (ISRaD database). (b) A histogram of the maximum fraction of OCpetro values for each soil sample. The mean fpetro is 0.38 ± 0.01, the median value is 0.36. (c) A histogram of calculated maximum OCpetro (wt. %) in each SED soil horizon. The average is 0.85 ± 0.1 wt. % OC and the median is 0.19 % OCpetro in SED soils globally. |
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Introduction
Sedimentary rocks cover 64 % of Earth’s surface (Hartmann and Moosdorf, 2012
Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370
). Within the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon (14C) “dead” rock organic carbon (OCpetro) (Copard et al., 2007Copard, Y., Amiotte-Suchet, P., Di-Giovanni, C. (2007) Storage and release of fossil organic carbon related to weathering of sedimentary rocks. Earth and Planetary Science Letters 258, 345–357. https://doi.org/10.1016/j.epsl.2007.03.048
), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al., 2017Jackson, R.B., Lajtha, K., Crow, S.E., Hugelius, G., Kramer, M.G., Piñeiro, G. (2017) The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls. Annual Review of Ecology, Evolution, and Systematics 48, 419–445. https://doi.org/10.1146/annurev-ecolsys-112414-054234
). When soil forms on sedimentary rocks, OCpetro is a “bottom up” input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al., 2018Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
; Kalks et al., 2021Kalks, F., Noren, G., Mueller, C.W., Helfrich, M., Rethemeyer, J., Don, A. (2021) Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon. Soil 7, 347–362. https://doi.org/10.5194/soil-7-347-2021
). While OCpetro could be widespread in soils developed on sedimentary rocks, observations come from only a handful of weathering profiles, including OCpetro-rich rocks, such as black shales which are not globally representative (Petsch et al., 2001Petsch, S.T., Eglinton, T.I., Edwards, K.J. (2001) 14C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131. https://doi.org/10.1126/science.1058332
). In addition, estimates of global OCpetro surface exposure are based on upscaling from geological maps (Copard et al., 2007Copard, Y., Amiotte-Suchet, P., Di-Giovanni, C. (2007) Storage and release of fossil organic carbon related to weathering of sedimentary rocks. Earth and Planetary Science Letters 258, 345–357. https://doi.org/10.1016/j.epsl.2007.03.048
), but this input has not been assessed in soil carbon studies. While there is widespread evidence for OCpetro in sediments from rivers around the world (e.g., Galy et al., 2015Galy, V., Peucker-Ehrenbrink, B., Eglinton, T. (2015) Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207. https://doi.org/10.1038/nature14400
; Clarke et al., 2017Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615
), we lack constraint on the input of OCpetro to soils.Input of OCpetro to soil is important for two reasons. First, the exposure of rocks to weathering can drive OCpetro oxidation, releasing CO2 and consuming O2, with global emissions of CO2 from OCpetro oxidation similar to those from volcanism (Petsch, 2014
Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5
; Hilton and West, 2020Hilton, R.G., West, A.J. (2020) Mountains, erosion and the carbon cycle. Nature Reviews Earth & Environment 1, 284–299. https://doi.org/10.1038/s43017-020-0058-6
). Second, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002Agnelli, A., Trumbore, S.E., Corti, G., Ugolini, F.C. (2002) The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C. European Journal of Soil Science 53, 147–159. https://doi.org/10.1046/j.1365-2389.2002.00432.x
), used to quantify atmospheric CO2 exchange by (Trumbore, 2000Trumbore, S. (2000) Age of Soil Organic Matter and Soil Respiration: Radiocarbon Constraints on Belowground C Dynamics. Ecological Applications 10, 399–411. https://doi.org/10.1890/1051-0761(2000)010[0399:AOSOMA]2.0.CO;2
; Shi et al., 2020Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
). Soils slow the rapid degradation of “modern” plant-derived vegetation (OCbio) through mineral and physical interactions, helping store OCbio in soils over decades to millennia (Shi et al., 2020Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
). In the topsoil, the OC pool is dominated by plant and microbial organic matter and is generally 14C-enriched (Shi et al., 2020Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
). In deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015Mathieu, J.A., Hatté, C., Balesdent, J., Parent, É. (2015) Deep soil carbon dynamics are driven more by soil type than by climate: a worldwide meta-analysis of radiocarbon profiles. Global Change Biology 21, 4278–4292. https://doi.org/10.1111/gcb.13012
; He et al., 2016He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273
; Shi et al., 2020Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
) show that 14C-depleted (“old”) OC is common around the world. These studies attribute “old” 14C ages to an increase in OC-mineral interactions, as the clay content and mineral to organic ratio increase with depth and mineral surfaces can reduce the bioavailability and thus increase the persistence of OCbio (Schmidt et al., 2011Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, D.P., Weiner, S., Trumbore, S.E. (2011) Persistence of soil organic matter as an ecosystem property. Nature 478, 49–56. https://doi.org/10.1038/nature10386
). Soil C age distributions can have very long tails, with a small amount of old or 14C-dead C skewing the mean age (Sierra et al., 2018Sierra, C.A., Hoyt, A.M., He, Y., Trumbore, S.E. (2018) Soil Organic Matter Persistence as a Stochastic Process: Age and Transit Time Distributions of Carbon in Soils. Global Biogeochemical Cycles 32, 1574–1588. https://doi.org/10.1029/2018GB005950
). However, inputs of 14C-dead OCpetro are not considered in He et al. (2016)He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273
and Shi et al. (2020)Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
, despite the recognition that OCpetro inputs result in an “apparent” ageing of soil OCbio.Here, we quantify OCpetro in globally distributed soil profiles using the International Soil Radiocarbon Database (ISRaD database, see Supplementary Information) (Lawrence et al., 2020
Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020
). We separate soil profiles developed on igneous rocks (IG: negligible OCpetro) versus sedimentary rocks (SED: potential OCpetro inputs) using global geological maps and compare bulk 14C signatures and % OC distributions. Alongside the global data set, using a mixing model analysis, we compare measurements made on river and soil OC in settings where OCpetro inputs have been demonstrated and where OCpetro is absent. Together, we assess the contribution of OCpetro to deep soils around the world and the implications for soil radiocarbon inventories and the surface carbon cycle.top
Evidence of OCpetro in soils and rivers using mixing models
The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019
Hemingway, J.D., Rothman, D.H., Grant, K.E., Rosengard, S.Z., Eglinton, T.I., Derry, L.A., Galy, V.V. (2019) Mineral protection regulates long-term global preservation of natural organic carbon. Nature 570, 228–231. https://doi.org/10.1038/s41586-019-1280-6
); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001Petsch, S.T., Eglinton, T.I., Edwards, K.J. (2001) 14C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131. https://doi.org/10.1126/science.1058332
; Hemingway et al., 2018Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
). To explore these scenarios, we examine cases where OCpetro contributes to bulk OC in river sediment loads and soils, with OCpetro inputs identified from geochemical proxies other than bulk 14C (e.g., C/N, δ13C, biomarkers, Raman spectroscopy, Ramped Pyrolysis Oxidation-14C). These can be compared to examples where OCpetro inputs are absent and OCbio decay/ageing acts alone from river sediments and soils on igneous bedrock (Fig. 1a, c). We find a notable contrast in the relationship between 1/[OC] and Δ14C in sedimentary and igneous bedrock settings. Patterns expected for mixing between OCpetro and OCbio appear to dominate in sedimentary settings.Figure 1 Rock organic carbon imprints on organic carbon concentration (OC) and radiocarbon activity (Δ14C). (a) Samples from Taiwanese soils, Andes and Himalayan rivers, and black shale weathering profiles where OCpetro inputs are known, with linear fits through data shown. (b) Binary mixing model (lines) between OCbio (Δ14C = +200 ‰) and OCpetro (varying from 1 % to 0.1 % of total OC, Δ14C = −1000 ‰). The grey shaded area is where >50 % of total OC is OCpetro. (c) Samples where OCpetro is absent: soil from Hawaiian Pololu lava flow and Icelandic river samples draining basalt.
An Andean Mountain river draining shales (Clark et al., 2017
Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615
) has river sediment OC Δ14C values that ranged from −0.5 ‰ to −839 ‰. Clark et al. (2017)Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615
concluded that, on average, 0.37 ± 0.03 % of the suspended sediment mass was OCpetro. There is a significant linear trend between Δ14C values and 1/[OC] (p < 0.05, R2 = 0.75, Fig. 1a) that was attributed to binary mixing of OCpetro and biospheric OC supplied by erosion processes. In Himalayan rivers, the river OC also has a linear relationship between these variables (p < 0.05, R2 = 0.37), albeit with a shallower slope due to lower OCpetro contents (Galy et al., 2015Galy, V., Peucker-Ehrenbrink, B., Eglinton, T. (2015) Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207. https://doi.org/10.1038/nature14400
). In soils, OCpetro has been recognised in only a few studies (Petsch, 2014Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5
; Hemingway et al., 2018Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
; Hilton et al., 2021Hilton, R.G., Turowski, J.M., Winnick, M., Dellinger, M., Schleppi, P., Williams, K.H., Lawrence, C.R., Maher, K., West, M., Hayton, A. (2021) Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering. Water Resources Research 57, e2021WR029844. https://doi.org/10.1029/2021WR029844
; Kalks et al., 2021Kalks, F., Noren, G., Mueller, C.W., Helfrich, M., Rethemeyer, J., Don, A. (2021) Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon. Soil 7, 347–362. https://doi.org/10.5194/soil-7-347-2021
). In Taiwan, high erosion rates continuously supply fresh bedrock to the surface and keep surface soils young, and these samples have a linear relationship between Δ14C and 1/[OC]. Mixing models show that OCpetro input to sediments and soils result in a linear trend between 1/[OC] and Δ14C (Supplementary Information, Eq. S-8; Fig. 1b). These can explain the main patterns seen in the Andes and Himalayan rivers and Taiwanese soil data.In contrast to the river sediments draining sedimentary rocks, rivers from a basaltic catchment, the Efri Haukadalsá River in Iceland (Torres et al., 2020
Torres, M.A., Kemeny, P.C., Lamb, M.P., Cole, T.L., Fischer, W.W. (2020) Long-Term Storage and Age-Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland. Geochemistry, Geophysics, Geosystems 21, e2019GC008632. https://doi.org/10.1029/2019GC008632
), have 14C value ranging from −60 ‰ to −395 ‰ and display no relationship between 1/[OC] and Δ14C (Fig. 1c). Torres et al. (2020)Torres, M.A., Kemeny, P.C., Lamb, M.P., Cole, T.L., Fischer, W.W. (2020) Long-Term Storage and Age-Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland. Geochemistry, Geophysics, Geosystems 21, e2019GC008632. https://doi.org/10.1029/2019GC008632
attributed the 14C-depletion to erosion of millennial-aged OC from soils. In tropical soils, formed along a climate gradient underlain by a 450 ka lava flow from Hawaii, [OC] remains high while OC ages due to Fe-oxide mineral-carbon stabilisation, and these materials have an almost vertical array in 1/[OC] and Δ14C (Fig. 1c; Grant et al., 2022Grant, K.E., Galy, V.V., Haghipour, N., Eglinton, T.I., Derry, L.A. (2022) Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions. Chemical Geology 587, 120629. https://doi.org/10.1016/j.chemgeo.2021.120629
). These patterns in the data are more consistent with the paired evolution of 1/[OC] and Δ14C expected for OC loss and ageing of OC, explored here using a simple organic matter degradation model (Supplementary Information sections 4.1 and 4.2).Overall, the river and soil data here show how 1/[OC] and Δ14C can be used to examine the role of OCpetro input. A domain of OC and Δ14C values can most easily be reached by OCpetro addition to the organic matter pool (Fig. 1b), as seen in the grey “mixing zone” of sedimentary soils. In the igneous soils, this mixing domain is harder to populate. We note the importance of OCpetro is likely to be most relevant in soils with low OCbio inputs or relatively old 14C mean ages, which could be particularly important in deep soils (Rumpel and Kögel-Knabner, 2011
Rumpel, C., Kögel-Knabner, I. (2011) Deep soil organic matter—a key but poorly understood component of terrestrial C cycle. Plant and Soil 338, 143–158. https://doi.org/10.1007/s11104-010-0391-5
; Shi et al., 2020Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
).top
Widespread OCpetro input to global soils
To assess a potential, wider input of OCpetro into soils, we use the ISRaD database (Lawrence et al., 2020
Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020
) (see Supplementary Information). All database samples used have reported Δ14C values and measured (not extrapolated or modelled) [OC] (Fig. 2a). We characterise the parent material for each geo-located profile from the global lithological map database (GLiM) (Hartmann and Moosdorf, 2012Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370
) if no parent material is specified in the database (79.4 % of entries). The soil radiocarbon profiles are split into sedimentary (SED) (80.9 %) and igneous (IG) (19.4 %) parent material, with 557 soil profiles and 2978 radiocarbon measurements (Supplementary Information) (Fig. 2e). There are more SED profiles (397 profiles, 2260 measurements) than IG profiles (160 profiles, 718 measurements), broadly mirroring the fact that ∼60 % of Earth’s terrestrial surface is composed of sedimentary rock (Hartmann and Moosdorf, 2012Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370
). An important caveat is that we do not capture all Quaternary deposits such as loess or ashfall, which are important parent materials for soil (Baisden and Parfitt, 2007Baisden, W.T., Parfitt, R.L. (2007) Bomb 14C enrichment indicates decadal C pool in deep soil? Biogeochemistry 85, 59–68. https://doi.org/10.1007/s10533-007-9101-7
). Rather than filter the database further, we note some of the SED samples on loess or ashfall may have very low OCpetro inputs.Figure 2 (a) The distribution of radiocarbon measurements for the SED and IG profiles, with corresponding box plots depicting the median and quartiles of the distributions. (b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020)
Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
dataset for the SED and IG profiles. (c) The ISRaD soil horizons from SED soils plotted in 1/[OC] and radiocarbon (Δ14C). (d) The ISRaD soil horizons from IG soils plotted in 1/[OC] and radiocarbon activity (Δ14C). The grey shaded region defines a zone where mixing of OCpetro can produce compositions which are difficult to obtain via OC decomposition alone (Fig. 1b). (e) Locations of samples where direct measurement of OCpetro inputs have been identified (diamond, triangle), and soil profiles from the ISRaD database on igneous (IG, circles) and sedimentary rocks (SED, squares).Despite the complexities inherent in any global soil assessment (Lawrence et al., 2020
Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020
), characterising samples from ISRaD as from either SED or IG sites reveals patterns which can be accounted for by OCpetro inputs. First, the median and lower quartile Δ14C values of SED samples are lower than IG sites (Fig. 2c, Table S-2). Second, the distribution of SED and IG soil Δ14C values are significantly different at the 0.05 level (Kolomogorov-Smirnov Test). Third, the calculated profile-averaged Δ14C values and modelled mean soil age are significantly different between SED and IG profiles (p > 0.05, Supplementary Information section 1.3). Finally, the SED samples that reach lower Δ14C values (Fig. 2c) define the grey zone suggested by a binary mixing model (Fig. 1b). IG profiles do not commonly reach this range of compositions, with the 14C depleted samples either having retained higher % OC, or remain fairly 14C-enriched (Δ14C > −200 ‰) as OC is lost, i.e. have a shallower trajectory between 1/[OC] and Δ14C (Fig. 2d).Having established potential OCpetro inputs at SED sites, the radiocarbon data can be used to quantify a maximum permissible OCpetro input. To do so, we assume all 14C-depletion is driven by OCpetro addition (i.e. no OCbio ageing) (Supplementary Information section 5.1). We calculate an average maximum proportion of OCpetro = 0.38 or 38 % (Fig. 3b) for all SED soils. This corresponds to an average 0.85 ± 0.1 % OCpetro in soils (Fig. 3c). This maximum value is reasonable given known OC contents of sedimentary rocks (Graz et al., 2011
Graz, Y., Di-Giovanni, C., Copard, Y., Elie, M., Faure, P., Laggoun Defarge, F., Lévèque, J., Michels, R., Olivier, J.E. (2011) Occurrence of fossil organic matter in modern environments: Optical, geochemical and isotopic evidence. Applied Geochemistry 26, 1302–1314. https://doi.org/10.1016/j.apgeochem.2011.05.004
; Partin et al., 2013Partin, C.A., Bekker, A., Planavsky, N.J., Scott, C.T., Gill, B.C., Li, C., Podkovyrov, V., Maslov, A., Konhauser, K.O., Lalonde, S.V., Love, G.D., Poulton, S.W., Lyons, T.W. (2013) Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales. Earth and Planetary Science Letters 369–370, 284–293. https://doi.org/10.1016/j.epsl.2013.03.031
).Figure 3 (a) The maximum fraction of [OCpetro] is calculated via a binary mixing model between OCbio and OCpetro in the globally distributed soil profiles (ISRaD database). (b) A histogram of the maximum fraction of OCpetro values for each soil sample. The mean fpetro is 0.38 ± 0.01, the median value is 0.36. (c) A histogram of calculated maximum OCpetro (wt. %) in each SED soil horizon. The average is 0.85 ± 0.1 wt. % OC and the median is 0.19 % OCpetro in SED soils globally.
Taken together, our analysis places a maximum bound on the OCpetro content to deep and 14C depleted soils forming on sedimentary rocks. While this estimation is an upper bound because OCbio ageing will occur in deep soils (Grant et al., 2022
Grant, K.E., Galy, V.V., Haghipour, N., Eglinton, T.I., Derry, L.A. (2022) Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions. Chemical Geology 587, 120629. https://doi.org/10.1016/j.chemgeo.2021.120629
), and does not consider loss of OCpetro during weathering, it is a reasonable starting point to understand how incorporation of OCpetro can influence mean soil age.top
OCpetro in soils leads to mean age overestimation
Input of OCpetro in soils formed on sedimentary rocks is consistent with patterns in the dataset (Fig. 2c, d). If OCpetro contributes to deep soils, how much could it alter carbon residence times based on 14C measurements? To provide a first constraint, we reconsider the Shi et al. (2020)
Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
global weighted Δ14C accounting for a fraction of OCpetro. The mean age in subsoils (30−100 cm depth) was Δ14C = −391 ± 56 ‰ (∼3970 14C years). Accounting for our calculated maximum fraction of 0.38 OCpetro could mean the OCbio age calculated in the Shi et al. (2020)Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
dataset is equivalent to −17.7 ‰, or 150 14C years, a difference of ∼3800 14C years. Using a more modest mass fraction of 0.10 for OCpetro would shift the biospheric radiocarbon age younger by 800 14C years (from 3900 to 3100 14C yr). While a 38 % OCpetro contribution is likely an overestimation, the analysis shows that even modest OCpetro inputs could shift assessments of OCbio residence time in soils, yet OCpetro inputs are not considered in global assessments (He et al., 2016He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273
; Shi et al., 2020Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
). An “apparent” increase in OCbio age by failing to account for OCpetro addition would imply soil carbon exchange fluxes with the atmosphere are underestimated. Accounting for the fate of OCpetro in soils is important for reducing uncertainty on the present and future role of soils in the anthropogenic carbon budget.The input of OCpetro to soils is central to the long-term carbon cycle. It is important to assess the OCpetro input to soils, its oxidation during soil formation that can release CO2, and the potential influence of OCpetro on soil OC stabilisation mechanisms. We can compare our maximum OCpetro contents (Fig. 3) from the SED soil profiles from ISRaD to global sedimentary rock OC values (Fig. S-3). These sedimentary rock samples have higher OC contents than sedimentary soils, suggesting the widespread loss of OCpetro in global soils (Partin et al., 2013
Partin, C.A., Bekker, A., Planavsky, N.J., Scott, C.T., Gill, B.C., Li, C., Podkovyrov, V., Maslov, A., Konhauser, K.O., Lalonde, S.V., Love, G.D., Poulton, S.W., Lyons, T.W. (2013) Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales. Earth and Planetary Science Letters 369–370, 284–293. https://doi.org/10.1016/j.epsl.2013.03.031
). Traditionally, OCpetro is considered inert or unreactive on anthropogenic timescales. However, in Taiwanese soils, Hemingway et al. (2018)Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
found that ∼67 % of OCpetro was lost during soil formation, even during rapid erosion which limits time for soil weathering. This extensive OCpetro oxidation was attributed to microbial assimilation of OCpetro (Hemingway et al., 2018Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
). To move forward, we must recognise that, like OCbio in soil, OCpetro is a continuum of compounds which differ in chemistry, linked to the history of past sedimentation, diagenesis, and metamorphism (Galy et al., 2008Galy, V., Beyssac, O., France-Lanord, C., Eglinton, T. (2008) Recycling of Graphite During Himalayan Erosion: A Geological Stabilization of Carbon in the Crust. Science 322, 943–945. https://doi.org/10.1126/science.1161408
; Petsch, 2014Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5
) which may have a distribution of reactivities and residence times in soils. Radiocarbon measurements suggest that OCpetro contribution to soils is a global feature and so needs to be considered to properly constrain soil carbon storage and turnover. Quantifying the fate of OCpetro in soils is then crucial to understanding the evolution of the long-term carbon and oxygen cycles.top
Acknowledgements
This work was performed under the European Research Council (ERC) Starting Grant (678779, ROC-CO2) and partly under an ERC Consolidator Grant (101002563, RIV-ESCAPE) to RGH. Partial writing of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 (LLNL-JRNL-837045).
Editor: Liane G. Benning
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References
Agnelli, A., Trumbore, S.E., Corti, G., Ugolini, F.C. (2002) The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C. European Journal of Soil Science 53, 147–159. https://doi.org/10.1046/j.1365-2389.2002.00432.x
Show in contextSecond, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002), used to quantify atmospheric CO2 exchange by (Trumbore, 2000; Shi et al., 2020).
View in article
Baisden, W.T., Parfitt, R.L. (2007) Bomb 14C enrichment indicates decadal C pool in deep soil? Biogeochemistry 85, 59–68. https://doi.org/10.1007/s10533-007-9101-7
Show in contextAn important caveat is that we do not capture all Quaternary deposits such as loess or ashfall, which are important parent materials for soil (Baisden and Parfitt, 2007).
View in article
Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615
Show in contextWhile there is widespread evidence for OCpetro in sediments from rivers around the world (e.g., Galy et al., 2015; Clarke et al., 2017), we lack constraint on the input of OCpetro to soils.
View in article
An Andean Mountain river draining shales (Clark et al., 2017) has river sediment OC Δ14C values that ranged from −0.5 ‰ to −839 ‰.
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Clark et al. (2017) concluded that, on average, 0.37 ± 0.03 % of the suspended sediment mass was OCpetro
View in article
Copard, Y., Amiotte-Suchet, P., Di-Giovanni, C. (2007) Storage and release of fossil organic carbon related to weathering of sedimentary rocks. Earth and Planetary Science Letters 258, 345–357. https://doi.org/10.1016/j.epsl.2007.03.048
Show in contextWithin the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon (14C) “dead” rock organic carbon (OCpetro) (Copard et al., 2007), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al., 2017).
View in article
In addition, estimates of global OCpetro surface exposure are based on upscaling from geological maps (Copard et al., 2007), but this input has not been assessed in soil carbon studies.
View in article
Galy, V., Beyssac, O., France-Lanord, C., Eglinton, T. (2008) Recycling of Graphite During Himalayan Erosion: A Geological Stabilization of Carbon in the Crust. Science 322, 943–945. https://doi.org/10.1126/science.1161408
Show in contextTo move forward, we must recognise that, like OCbio in soil, OCpetro is a continuum of compounds which differ in chemistry, linked to the history of past sedimentation, diagenesis, and metamorphism (Galy et al., 2008; Petsch, 2014) which may have a distribution of reactivities and residence times in soils.
View in article
Galy, V., Peucker-Ehrenbrink, B., Eglinton, T. (2015) Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207. https://doi.org/10.1038/nature14400
Show in contextWhile there is widespread evidence for OCpetro in sediments from rivers around the world (e.g., Galy et al., 2015; Clarke et al., 2017), we lack constraint on the input of OCpetro to soils.
View in article
In Himalayan rivers, the river OC also has a linear relationship between these variables (p < 0.05, R2 = 0.37), albeit with a shallower slope due to lower OCpetro contents (Galy et al., 2015).
View in article
Grant, K.E., Galy, V.V., Haghipour, N., Eglinton, T.I., Derry, L.A. (2022) Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions. Chemical Geology 587, 120629. https://doi.org/10.1016/j.chemgeo.2021.120629
Show in contextIn tropical soils, formed along a climate gradient underlain by a 450 ka lava flow from Hawaii, [OC] remains high while OC ages due to Fe-oxide mineral-carbon stabilisation, and these materials have an almost vertical array in 1/[OC] and Δ14C (Fig. 1c; Grant et al., 2022).
View in article
While this estimation is an upper bound because OCbio ageing will occur in deep soils (Grant et al., 2022), and does not consider loss of OCpetro during weathering, it is a reasonable starting point to understand how incorporation of OCpetro can influence mean soil age.
View in article
Graz, Y., Di-Giovanni, C., Copard, Y., Elie, M., Faure, P., Laggoun Defarge, F., Lévèque, J., Michels, R., Olivier, J.E. (2011) Occurrence of fossil organic matter in modern environments: Optical, geochemical and isotopic evidence. Applied Geochemistry 26, 1302–1314. https://doi.org/10.1016/j.apgeochem.2011.05.004
Show in contextThis maximum value is reasonable given known OC contents of sedimentary rocks (Graz et al., 2011; Partin et al., 2013).
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Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370
Show in contextSedimentary rocks cover 64 % of Earth’s surface (Hartmann and Moosdorf, 2012).
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We characterise the parent material for each geo-located profile from the global lithological map database (GLiM) (Hartmann and Moosdorf, 2012) if no parent material is specified in the database (79.4 % of entries).
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There are more SED profiles (397 profiles, 2260 measurements) than IG profiles (160 profiles, 718 measurements), broadly mirroring the fact that ∼60 % of Earth’s terrestrial surface is composed of sedimentary rock (Hartmann and Moosdorf, 2012).
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He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273
Show in contextIn deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015; He et al., 2016; Shi et al., 2020) show that 14C-depleted (“old”) OC is common around the world.
View in article
However, inputs of 14C-dead OCpetro are not considered in He et al. (2016) and Shi et al. (2020), despite the recognition that OCpetro inputs result in an “apparent” ageing of soil OCbio
View in article
While a 38 % OCpetro contribution is likely an overestimation, the analysis shows that even modest OCpetro inputs could shift assessments of OCbio residence time in soils, yet OCpetro inputs are not considered in global assessments (He et al., 2016; Shi et al., 2020).
View in article
Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
Show in contextWhen soil forms on sedimentary rocks, OCpetro is a “bottom up” input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al., 2018; Kalks et al., 2021).
View in article
The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001; Hemingway et al., 2018).
View in article
In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article
However, in Taiwanese soils, Hemingway et al. (2018) found that ∼67 % of OCpetro was lost during soil formation, even during rapid erosion which limits time for soil weathering.
View in article
This extensive OCpetro oxidation was attributed to microbial assimilation of OCpetro (Hemingway et al., 2018).
View in article
Hemingway, J.D., Rothman, D.H., Grant, K.E., Rosengard, S.Z., Eglinton, T.I., Derry, L.A., Galy, V.V. (2019) Mineral protection regulates long-term global preservation of natural organic carbon. Nature 570, 228–231. https://doi.org/10.1038/s41586-019-1280-6
Show in contextThe 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001; Hemingway et al., 2018).
View in article
Hilton, R.G., West, A.J. (2020) Mountains, erosion and the carbon cycle. Nature Reviews Earth & Environment 1, 284–299. https://doi.org/10.1038/s43017-020-0058-6
Show in contextInput of OCpetro to soil is important for two reasons. First, the exposure of rocks to weathering can drive OCpetro oxidation, releasing CO2 and consuming O2, with global emissions of CO2 from OCpetro oxidation similar to those from volcanism (Petsch, 2014; Hilton and West, 2020).
View in article
Hilton, R.G., Turowski, J.M., Winnick, M., Dellinger, M., Schleppi, P., Williams, K.H., Lawrence, C.R., Maher, K., West, M., Hayton, A. (2021) Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering. Water Resources Research 57, e2021WR029844. https://doi.org/10.1029/2021WR029844
Show in contextIn soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article
Jackson, R.B., Lajtha, K., Crow, S.E., Hugelius, G., Kramer, M.G., Piñeiro, G. (2017) The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls. Annual Review of Ecology, Evolution, and Systematics 48, 419–445. https://doi.org/10.1146/annurev-ecolsys-112414-054234
Show in contextWithin the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon (14C) “dead” rock organic carbon (OCpetro) (Copard et al., 2007), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al., 2017).
View in article
Kalks, F., Noren, G., Mueller, C.W., Helfrich, M., Rethemeyer, J., Don, A. (2021) Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon. Soil 7, 347–362. https://doi.org/10.5194/soil-7-347-2021
Show in contextWhen soil forms on sedimentary rocks, OCpetro is a “bottom up” input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al., 2018; Kalks et al., 2021).
View in article
In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
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Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020
Show in contextHere, we quantify OCpetro in globally distributed soil profiles using the International Soil Radiocarbon Database (ISRaD database, see Supplementary Information) (Lawrence et al., 2020).
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To assess a potential, wider input of OCpetro into soils, we use the ISRaD database (Lawrence et al., 2020) (see Supplementary Information).
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Despite the complexities inherent in any global soil assessment (Lawrence et al., 2020), characterising samples from ISRaD as from either SED or IG sites reveals patterns which can be accounted for by OCpetro inputs.
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Mathieu, J.A., Hatté, C., Balesdent, J., Parent, É. (2015) Deep soil carbon dynamics are driven more by soil type than by climate: a worldwide meta-analysis of radiocarbon profiles. Global Change Biology 21, 4278–4292. https://doi.org/10.1111/gcb.13012
Show in contextIn deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015; He et al., 2016; Shi et al., 2020) show that 14C-depleted (“old”) OC is common around the world.
View in article
Partin, C.A., Bekker, A., Planavsky, N.J., Scott, C.T., Gill, B.C., Li, C., Podkovyrov, V., Maslov, A., Konhauser, K.O., Lalonde, S.V., Love, G.D., Poulton, S.W., Lyons, T.W. (2013) Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales. Earth and Planetary Science Letters 369–370, 284–293. https://doi.org/10.1016/j.epsl.2013.03.031
Show in contextThis maximum value is reasonable given known OC contents of sedimentary rocks (Graz et al., 2011; Partin et al., 2013).
View in article
These sedimentary rock samples have higher OC contents than sedimentary soils, suggesting the widespread loss of OCpetro in global soils (Partin et al., 2013).
View in article
Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5
Show in contextInput of OCpetro to soil is important for two reasons. First, the exposure of rocks to weathering can drive OCpetro oxidation, releasing CO2 and consuming O2, with global emissions of CO2 from OCpetro oxidation similar to those from volcanism (Petsch, 2014; Hilton and West, 2020).
View in article
In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article
To move forward, we must recognise that, like OCbio in soil, OCpetro is a continuum of compounds which differ in chemistry, linked to the history of past sedimentation, diagenesis, and metamorphism (Galy et al., 2008; Petsch, 2014) which may have a distribution of reactivities and residence times in soils.
View in article
Petsch, S.T., Eglinton, T.I., Edwards, K.J. (2001) 14C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131. https://doi.org/10.1126/science.1058332
Show in contextWhile OCpetro could be widespread in soils developed on sedimentary rocks, observations come from only a handful of weathering profiles, including OCpetro-rich rocks, such as black shales which are not globally representative (Petsch et al., 2001).
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The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001; Hemingway et al., 2018).
View in article
Rumpel, C., Kögel-Knabner, I. (2011) Deep soil organic matter—a key but poorly understood component of terrestrial C cycle. Plant and Soil 338, 143–158. https://doi.org/10.1007/s11104-010-0391-5
Show in contextWe note the importance of OCpetro is likely to be most relevant in soils with low OCbio inputs or relatively old 14C mean ages, which could be particularly important in deep soils (Rumpel and Kögel-Knabner, 2011; Shi et al., 2020).
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Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, D.P., Weiner, S., Trumbore, S.E. (2011) Persistence of soil organic matter as an ecosystem property. Nature 478, 49–56. https://doi.org/10.1038/nature10386
Show in contextThese studies attribute “old” 14C ages to an increase in OC-mineral interactions, as the clay content and mineral to organic ratio increase with depth and mineral surfaces can reduce the bioavailability and thus increase the persistence of OCbio (Schmidt et al., 2011).
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Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
Show in context
Second, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002), used to quantify atmospheric CO2 exchange by (Trumbore, 2000; Shi et al., 2020).
View in article
Soils slow the rapid degradation of “modern” plant-derived vegetation (OCbio) through mineral and physical interactions, helping store OCbio in soils over decades to millennia (Shi et al., 2020).
View in article
In the topsoil, the OC pool is dominated by plant and microbial organic matter and is generally 14C-enriched (Shi et al., 2020).
View in article
In deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015; He et al., 2016; Shi et al., 2020) show that 14C-depleted (“old”) OC is common around the world.
View in article
However, inputs of 14C-dead OCpetro are not considered in He et al. (2016) and Shi et al. (2020), despite the recognition that OCpetro inputs result in an “apparent” ageing of soil OCbio
View in article
We note the importance of OCpetro is likely to be most relevant in soils with low OCbio inputs or relatively old 14C mean ages, which could be particularly important in deep soils (Rumpel and Kögel-Knabner, 2011; Shi et al., 2020).
View in article
(b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020) dataset for the SED and IG profiles.
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If OCpetro contributes to deep soils, how much could it alter carbon residence times based on 14C measurements? To provide a first constraint, we reconsider the Shi et al. (2020) global weighted Δ14C accounting for a fraction of OCpetro
View in article
Accounting for our calculated maximum fraction of 0.38 OCpetro could mean the OCbio age calculated in the Shi et al. (2020) dataset is equivalent to −17.7 ‰, or 150 14C years, a difference of ∼3800 14C years.
View in article
While a 38 % OCpetro contribution is likely an overestimation, the analysis shows that even modest OCpetro inputs could shift assessments of OCbio residence time in soils, yet OCpetro inputs are not considered in global assessments (He et al., 2016; Shi et al., 2020).
View in article
Sierra, C.A., Hoyt, A.M., He, Y., Trumbore, S.E. (2018) Soil Organic Matter Persistence as a Stochastic Process: Age and Transit Time Distributions of Carbon in Soils. Global Biogeochemical Cycles 32, 1574–1588. https://doi.org/10.1029/2018GB005950
Show in contextSoil C age distributions can have very long tails, with a small amount of old or 14C-dead C skewing the mean age (Sierra et al., 2018).
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Torres, M.A., Kemeny, P.C., Lamb, M.P., Cole, T.L., Fischer, W.W. (2020) Long-Term Storage and Age-Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland. Geochemistry, Geophysics, Geosystems 21, e2019GC008632. https://doi.org/10.1029/2019GC008632
Show in contextIn contrast to the river sediments draining sedimentary rocks, rivers from a basaltic catchment, the Efri Haukadalsá River in Iceland (Torres et al., 2020), have 14C value ranging from −60 ‰ to −395 ‰ and display no relationship between 1/[OC] and Δ14C (Fig. 1c.
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Torres et al. (2020) attributed the 14C-depletion to erosion of millennial-aged OC from soils.
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Trumbore, S. (2000) Age of Soil Organic Matter and Soil Respiration: Radiocarbon Constraints on Belowground C Dynamics. Ecological Applications 10, 399–411. https://doi.org/10.1890/1051-0761(2000)010[0399:AOSOMA]2.0.CO;2
Show in contextSecond, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002), used to quantify atmospheric CO2 exchange by (Trumbore, 2000; Shi et al., 2020).
View in article
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Supplementary Information
The Supplementary Information includes:
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Figures
Figure 1 Rock organic carbon imprints on organic carbon concentration (OC) and radiocarbon activity (Δ14C). (a) Samples from Taiwanese soils, Andes and Himalayan rivers, and black shale weathering profiles where OCpetro inputs are known, with linear fits through data shown. (b) Binary mixing model (lines) between OCbio (Δ14C = +200 ‰) and OCpetro (varying from 1 % to 0.1 % of total OC, Δ14C = −1000 ‰). The grey shaded area is where >50 % of total OC is OCpetro. (c) Samples where OCpetro is absent: soil from Hawaiian Pololu lava flow and Icelandic river samples draining basalt.
Figure 2 (a) The distribution of radiocarbon measurements for the SED and IG profiles, with corresponding box plots depicting the median and quartiles of the distributions. (b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020)
Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
dataset for the SED and IG profiles. (c) The ISRaD soil horizons from SED soils plotted in 1/[OC] and radiocarbon (Δ14C). (d) The ISRaD soil horizons from IG soils plotted in 1/[OC] and radiocarbon activity (Δ14C). The grey shaded region defines a zone where mixing of OCpetro can produce compositions which are difficult to obtain via OC decomposition alone (Fig. 1b). (e) Locations of samples where direct measurement of OCpetro inputs have been identified (diamond, triangle), and soil profiles from the ISRaD database on igneous (IG, circles) and sedimentary rocks (SED, squares).
Figure 3 (a) The maximum fraction of [OCpetro] is calculated via a binary mixing model between OCbio and OCpetro in the globally distributed soil profiles (ISRaD database). (b) A histogram of the maximum fraction of OCpetro values for each soil sample. The mean fpetro is 0.38 ± 0.01, the median value is 0.36. (c) A histogram of calculated maximum OCpetro (wt. %) in each SED soil horizon. The average is 0.85 ± 0.1 wt. % OC and the median is 0.19 % OCpetro in SED soils globally.