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**Research article**
09 May 2018

**Research article** | 09 May 2018

Quantifying persistent organic carbon in temperate soils

^{1}Université Grenoble Alpes, Irstea, UR LESSEM, 38402 St-Martin-d'Hères, France^{2}Laboratoire de Géologie de l'ENS, PSL Research University, CNRS UMR 8538, 75005, Paris, France^{3}Sorbonne Université-UPMC-Univ Paris 06, Institut des Sciences de la Terre de Paris, 75005 Paris, France^{4}AgroParisTech – INRA, UMR 1402 ECOSYS, 78850 Thiverval-Grignon, France^{5}Department of Ecology, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden^{6}Department of Sustainable Agriculture Sciences, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK^{7}Earth and Environmental Science, University of Pennsylvania, Philadelphia, USA

^{1}Université Grenoble Alpes, Irstea, UR LESSEM, 38402 St-Martin-d'Hères, France^{2}Laboratoire de Géologie de l'ENS, PSL Research University, CNRS UMR 8538, 75005, Paris, France^{3}Sorbonne Université-UPMC-Univ Paris 06, Institut des Sciences de la Terre de Paris, 75005 Paris, France^{4}AgroParisTech – INRA, UMR 1402 ECOSYS, 78850 Thiverval-Grignon, France^{5}Department of Ecology, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden^{6}Department of Sustainable Agriculture Sciences, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK^{7}Earth and Environmental Science, University of Pennsylvania, Philadelphia, USA

Abstract

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Changes in global soil carbon stocks have considerable potential to influence
the course of future climate change. However, a portion of soil organic
carbon (SOC) has a very long residence time (> 100 years) and may not
contribute significantly to terrestrial greenhouse gas emissions during the
next century. The size of this persistent SOC reservoir is presumed to be
large. Consequently, it is a key parameter required for the initialization of
SOC dynamics in ecosystem and Earth system models, but there is considerable
uncertainty in the methods used to quantify it. Thermal analysis methods
provide cost-effective information on SOC thermal stability that has been
shown to be qualitatively related to SOC biogeochemical stability. The
objective of this work was to build the first quantitative model of the size
of the centennially persistent SOC pool based on thermal analysis. We used a
unique set of 118 archived soil samples from four agronomic experiments in
northwestern Europe with long-term bare fallow and non-bare fallow treatments
(e.g., manure amendment, cropland and grassland) as a sample set for which
estimating the size of the centennially persistent SOC pool is relatively
straightforward. At each experimental site, we estimated the average
concentration of centennially persistent SOC and its uncertainty by applying
a Bayesian curve-fitting method to the observed declining SOC concentration
over the duration of the long-term bare fallow treatment. Overall, the
estimated concentrations of centennially persistent SOC ranged from 5 to
11 g C kg^{−1} of soil (lowest and highest boundaries of four 95 %
confidence intervals). Then, by dividing the site-specific concentrations of
persistent SOC by the total SOC concentration, we could estimate the
proportion of centennially persistent SOC in the 118 archived soil samples
and the associated uncertainty. The proportion of centennially persistent SOC
ranged from 0.14 (standard deviation of 0.01) to 1 (standard deviation of
0.15). Samples were subjected to thermal analysis by Rock-Eval 6 that
generated a series of 30 parameters reflecting their SOC thermal stability
and bulk chemistry. We trained a nonparametric machine-learning algorithm
(random forests multivariate regression model) to predict the proportion of
centennially persistent SOC in new soils using Rock-Eval 6 thermal parameters
as predictors. We evaluated the model predictive performance with two
different strategies. We first used a calibration set (*n* = 88) and a
validation set (*n* = 30) with soils from all sites. Second, to test the
sensitivity of the model to pedoclimate, we built a calibration set with soil
samples from three out of the four sites (*n* = 84). The multivariate
regression model accurately predicted the proportion of centennially
persistent SOC in the validation set composed of soils from all sites
(*R*^{2} = 0.92, RMSEP = 0.07, *n* = 30). The uncertainty of the
model predictions was quantified by a Monte Carlo approach that produced
conservative 95 % prediction intervals across the validation set. The
predictive performance of the model decreased when predicting the proportion
of centennially persistent SOC in soils from one fully independent site with
a different pedoclimate, yet the mean error of prediction only slightly
increased (*R*^{2} = 0.53, RMSEP = 0.10, *n* = 34). This model
based on Rock-Eval 6 thermal analysis can thus be used to predict the
proportion of centennially persistent SOC with known uncertainty in new soil
samples from different pedoclimates, at least for sites that have similar
Rock-Eval 6 thermal characteristics to those included in the calibration set.
Our study reinforces the evidence that there is a link between the thermal
and biogeochemical stability of soil organic matter and demonstrates that
Rock-Eval 6 thermal analysis can be used to quantify the size of the
centennially persistent organic carbon pool in temperate soils.

How to cite

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How to cite.

Cécillon, L., Baudin, F., Chenu, C., Houot, S., Jolivet, R., Kätterer, T., Lutfalla, S., Macdonald, A., van Oort, F., Plante, A. F., Savignac, F., Soucémarianadin, L. N., and Barré, P.: A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils, Biogeosciences, 15, 2835-2849, https://doi.org/10.5194/bg-15-2835-2018, 2018.

1 Introduction

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Soils exert a key regulation on atmospheric greenhouse gas concentrations
on a decadal timescale through the net carbon source and sink status of their
organic carbon reservoir (Amundson, 2001; Eglin et al., 2010). However, a
portion of the soil organic carbon (SOC) reservoir may not contribute
significantly to the net exchange of CO_{2} and CH_{4} between
atmosphere and land during the next century because its residence time
exceeds 100 years and its rate of carbon input is low (Trumbore, 1997; He et
al., 2016). The size of the centennially persistent SOC pool is presumed to
be large (i.e., between one- and two-thirds of total SOC) and dependent on
geochemical parameters such as soil texture and mineralogy (Buyanovsky and
Wagner, 1998a; Trumbore, 2009; Mills et al., 2014; Mathieu et al., 2015).
However, the amount of centennially persistent organic carbon in soils is
highly uncertain as it cannot be estimated accurately by current analytical
methods (Post and Kwon, 2000; von Lützow et al., 2007; Bruun et al.,
2008). Physicochemical procedures attempting to isolate SOC with a high
residence time from bulk SOC have proven unsatisfactory because of
indications that such fractions are a mixture of ancient and recent SOC (von
Lützow et al., 2007; Trumbore, 2009; Lutfalla et al., 2014). Even the
well-established radiocarbon (^{14}C) analytical technique cannot
precisely determine the size of the centennially persistent SOC pool
(Schrumpf and Kaiser, 2015; Menichetti et al., 2016). The importance of
better information on the size of the centennially persistent SOC pool has
been emphasized recently (Soil Carbon Initiative, 2011; Bispo
et al., 2017; Bailey et al., 2018; Harden et al., 2018), stressing the need
for operational and standardized metrics or proxies to accurately quantify
SOC persistent at the centennial timescale. The general lack of information
on the size and turnover rate of measurable SOC pools hampers the
initialization of SOC pools in dynamic models, questioning their predictions
of the evolution of the global SOC reservoir (Falloon and Smith, 2000; Luo et
al., 2014; Feng et al., 2016; He et al., 2016; Sanderman et al., 2016). Luo
et al. (2016) and He et al. (2016) recently claimed that optimizing
parameter estimations with global datasets on SOC pools and fluxes was the
highest priority to reduce biases among Earth system models.

In the past decade, the thermal stability of organic carbon has been proposed as
a good surrogate for its biogeochemical stability in litter and soils (e.g.,
Rovira et al., 2008; Plante et al., 2009; Gregorich et al., 2015). Several
studies using thermal analysis techniques, such as thermogravimetry and
differential scanning calorimetry with ramped combustion, have shown that the
fast-cycling SOC pool determined as the amount CO_{2} respired in
laboratory incubation experiments was thermally labile (Plante et al., 2011;
Leifeld and von Lützow, 2014; Campo and Merino, 2016). Recently, studies
using thermal analysis under an oxidative or inert (pyrolysis) reaction
atmosphere coupled with evolved gas analysis have shown a high and positive
correlation between the thermally stable SOC and persistent SOC determined
using ^{14}C measurements (Plante et al., 2013) and between thermally
stable SOC and mineral-associated SOC isolated by a classical SOC physical
fractionation scheme (Saenger et al., 2015). Using long-term bare fallow
(LTBF) soils kept free of vegetation for several decades (i.e., with
negligible carbon inputs), Barré et al. (2016) recently showed that
persistent SOC was low in energy and thermally stable. While there appear to
be strong qualitative links between the thermal and biogeochemical stability of
SOC, there is to date no established quantitative link between the size of
the persistent SOC pool and SOC thermal characteristics.

The objective of this work was to design a reliable, routine method based on a thermal analysis technique (Rock-Eval 6; RE6) to quantify centennially persistent SOC in a range of temperate soil types. First, we compiled a set of reference soil samples from four long-term agronomic experiments in northwestern Europe with long-term bare fallow treatments. The SOC concentration of LTBF treatments can be used to estimate the size of the persistent SOC pool of a particular site, as proposed by Rühlmann (1999) and Barré et al. (2010). Here, we refined estimates of the persistent SOC concentration previously published by Barré et al. (2010) for the four sites used in this study. We then used these values to estimate the proportion of centennially persistent SOC in 118 archived soil samples (time series) from LTBF and non-LTBF treatments of these four sites. The last step consisted of analyzing these reference samples using RE6 thermal analysis and building a multivariate regression model to relate RE6 information on SOC thermal stability and bulk chemistry to the estimated proportion of centennially persistent SOC. In this work, we aimed to deliver a model based on thermal analysis with reliable prediction intervals around the predicted values of the size of the centennially persistent SOC pool. We thus focused on the uncertainty in the estimated proportion of centennially persistent SOC and its propagation in the multivariate regression model.

2 Materials and methods

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The reference soil sample set was built using samples from four long-term
agronomic experimental sites in northwestern Europe (Versailles and Grignon in France,
Rothamsted in the United Kingdom and Ultuna in Sweden; Supplement,
Table S1). Each of the four sites includes an LTBF
treatment, with bare fallow duration ranging from 48 years at Grignon to
79 years at Versailles. For all experimental sites, we also included non-LTBF
treatments that have increased or maintained their total SOC concentrations
over time or sustained smaller losses than the LTBF treatment. The selected
non-LTBF treatments included manure amendments (Versailles), straw or
composted straw amendments (Grignon), continuous grassland (Rothamsted) and
continuous cropland (Ultuna). Soil samples from each site and treatment have
been regularly collected and archived since the initiation of the
experiments. A total of 118 topsoil samples (0–20 to 0–25 cm of depth;
Table S1) were selected from the archives of LTBF and non-LTBF treatments to
build the reference sample set. Samples were selected from two or three field
replicate plots with a decadal frequency from the initiation of the
experiments up to 2007 (Grignon), 2008 (Versailles, Rothamsted) or 2009
(Ultuna) to obtain a sample set with the widest possible range of proportions
of centennially persistent SOC. The non-LTBF treatments and multiple sites
also added to the diversity of land-use, climate and parent material. For
each sample, total SOC concentration was measured by dry combustion with an
elemental analyzer (SOC_{EA}, g C kg^{−1} of soil) after
the removal of soil carbonates
when necessary according to NF ISO 10694 (1995).

Based on the decline in total SOC concentration over the duration of the LTBF treatment, Barré et al. (2010) estimated the concentration of centennially persistent SOC at each site using a Bayesian curve-fitting method applied to each LTBF field replicate plot. Here, we refined those site-specific estimates by (i) applying a similar Bayesian curve-fitting method to combined SOC concentration data from all LTBF field replicate plots of each site (four field replicate plots for Ultuna and Rothamsted, six field replicate plots for Versailles and Grignon) and (ii) using new SOC concentration data up to 2014 for Rothamsted and 2015 for Ultuna, increasing their LTBF duration to 55 years for Rothamsted and 59 years for Ultuna.

For each site, we assumed that the temporal evolution of LTBF SOC
concentration, *γ*(*t*), followed an exponential decay function:

$$\begin{array}{}\text{(1)}& \mathit{\gamma}\left(t\right)=a{e}^{-bt}+c,\end{array}$$

where *γ*(*t*) (unit = g C kg^{−1} of soil) is the LTBF SOC
concentration at time *t*, *t* (unit = year) is the time under bare
fallow, and *a*, *b* and *c* are fitting parameters. Parameter *a*
(unit = g C kg^{−1} of soil) corresponds to the amplitude of the decay
and *b* (unit = yr^{−1}) is the characteristic decay rate. The
parameter *c* (unit = g C kg^{−1} of soil) represents a theoretically
inert portion of SOC. We considered this parameter as a site-specific metric
of the centennially persistent SOC concentration. In our view, the
centennially persistent SOC pool is not biogeochemically inert; it has a mean
age and a mean residence time that are both assumed to be high (e.g.,
centuries) though not precisely defined in this study. As a result, its
decline is minimal at the timescale of this study and we thus considered the
centennially persistent SOC concentration at each experimental site to be
constant. We used a Bayesian inference method to compute site-specific
estimates of centennially persistent SOC concentration and associated
uncertainties (detailed in the Sect. 2.3.1).

The proportion of centennially persistent SOC (CP_{SOC}) in each
soil sample was then calculated as the ratio of the site-specific
CP_{SOC} concentration to the total SOC concentration of the sample:

$$\begin{array}{ll}{\displaystyle}& {\displaystyle}{\mathrm{CP}}_{\mathrm{SOC}}\phantom{\rule{0.25em}{0ex}}\mathrm{proportion}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{sample}\right]=\\ \text{(2)}& {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}{\displaystyle \frac{{\mathrm{CP}}_{\mathrm{SOC}}\phantom{\rule{0.25em}{0ex}}\mathrm{concentration}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{site}\right]}{\mathrm{SOC}\phantom{\rule{0.25em}{0ex}}\mathrm{concentration}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{sample}\right]}},\end{array}$$

where the unit of CP_{SOC} concentration [site] and SOC
concentration [sample] is g C kg^{−1} of soil. The CP_{SOC}
proportions of five samples that were slightly above 1 were set to 1. In
these calculations, we assumed that at each site, the concentration of
CP_{SOC} was the same in the LTBF and non-LTBF treatments and was
constant with time. The details related to the estimation of the uncertainty
in the CP_{SOC} proportion of each sample are reported in
Sect. 2.3.2.

The 118 soil samples from the reference set were analyzed with an RE6 Turbo
device (Vinci Technologies) using the basic setup for the analysis of soil
organic matter (Behar et al., 2001; Disnar et al., 2003). The RE6 technique
provided measurements from the sequential pyrolysis and oxidation of ca.
40 mg of finely ground (< 250 µm) soil per sample (Fig. 1).
Volatile hydrocarbon effluents from pyrolysis were detected and quantified
with flame ionization detection (FID), while the evolution of CO and
CO_{2} gases was quantified by infrared detection during both the pyrolysis
and oxidation stages. Pyrolysis was carried out from 200 to 650 ^{∘}C
in an N_{2} atmosphere with a heating rate of 30 ^{∘}C min^{−1},
while the oxidation was carried out from 300 to 850 ^{∘}C in a
laboratory air atmosphere (with O_{2}) with a heating rate of
20 ^{∘}C min^{−1}. The RE6 technique generated five thermograms per
sample (Fig. 1, i.e., volatile hydrocarbon (HC) effluents during pyrolysis,
CO_{2} during pyrolysis, CO_{2} during oxidation, CO during
pyrolysis and CO during oxidation). On average, the organic carbon yield of
the RE6 analysis was greater than 96.5 % of SOC_{EA} for the
soils of the reference sample set
(SOC_{RE6} = 0.966 × SOC_{EA} + 0.403,
*R*^{2} = 0.97, *n* = 118).

For each RE6 thermogram, we determined the temperatures corresponding to each
incremental proportion of the amount of gases evolved during the pyrolysis
and oxidation stages. Upper temperatures of 850 ^{∘}C (CO oxidation
thermogram), 650 ^{∘}C (HC pyrolysis thermogram), 611 ^{∘}C
(CO_{2} oxidation thermogram) and 560 ^{∘}C (CO and CO_{2}
pyrolysis thermograms) were chosen for signal integration (Fig. 1), thereby
excluding any interference of soil carbonates (Behar et al., 2001). The thermal
decomposition of carbonates was indeed observed beyond 560 ^{∘}C (CO
and CO_{2} pyrolysis thermograms) and 611 ^{∘}C (CO_{2}
oxidation thermogram) for the site of Grignon (data not shown). For each RE6
thermogram, signal integration was performed on the offset-corrected
thermogram using sample-specific offset values estimated by the RE6 Turbo
device. For the three pyrolysis thermograms, signal integration started after
an isotherm step of 200 s at 200 ^{∘}C. Finally, we retained five of
these temperature parameters per thermogram: *T*_{10}, *T*_{30}, *T*_{50},
*T*_{70} and *T*_{90}, which respectively represent the temperatures
corresponding to the evolution of 10, 30, 50, 70 and 90 % of the amount
of evolved gases for each sample and for each of the five different
thermograms (HC, CO_{2} pyrolysis, CO_{2} oxidation, CO pyrolysis,
CO oxidation).

For the HC pyrolysis thermogram we also determined three parameters
reflecting a proportion of thermally resistant or labile hydrocarbons: a
parameter representing the proportion of hydrocarbons evolved between 200 and
450 ^{∘}C (thermo-labile hydrocarbons, TLHC index; modified from
Saenger et al., 2015), with the *I* index representing the preservation of thermally
labile immature hydrocarbons (after Sebag et al., 2016) and the *R* index
representing the proportion of hydrocarbons thermally stable at
400 ^{∘}C (after Sebag et al., 2016). Those three RE6 parameters were
calculated as follows:

$$\begin{array}{ll}{\displaystyle}& {\displaystyle}\text{TLHC index}=\\ \text{(3)}& {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}{\displaystyle \frac{\mathrm{Area}\phantom{\rule{0.25em}{0ex}}\mathrm{of}\phantom{\rule{0.25em}{0ex}}\mathrm{HC}\phantom{\rule{0.25em}{0ex}}\mathrm{pyrolysis}\phantom{\rule{0.25em}{0ex}}\mathrm{thermogram}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{200}\text{\u2013}\mathrm{450}{}^{\circ}\text{C}\right]}{\mathrm{Total}\phantom{\rule{0.25em}{0ex}}\mathrm{area}\phantom{\rule{0.25em}{0ex}}\mathrm{of}\phantom{\rule{0.25em}{0ex}}\mathrm{HC}\phantom{\rule{0.25em}{0ex}}\mathrm{pyrolysis}\phantom{\rule{0.25em}{0ex}}\mathrm{thermogram}}},{\displaystyle}& {\displaystyle}I\phantom{\rule{0.25em}{0ex}}\text{index}=\\ \text{(4)}& {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}{\mathrm{log}}_{\mathrm{10}}\left({\displaystyle \frac{{\displaystyle \begin{array}{c}\mathrm{proportion}\phantom{\rule{0.25em}{0ex}}\mathrm{of}\phantom{\rule{0.25em}{0ex}}\mathrm{HC}\phantom{\rule{0.25em}{0ex}}\mathrm{pyrolysis}\\ \mathrm{thermogram}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{200}\text{\u2013}\mathrm{400}{}^{\circ}\text{C}\right]\end{array}}}{{\displaystyle \begin{array}{c}\mathrm{proportion}\phantom{\rule{0.25em}{0ex}}\mathrm{of}\phantom{\rule{0.25em}{0ex}}\mathrm{HC}\phantom{\rule{0.25em}{0ex}}\mathrm{pyrolysis}\\ \mathrm{thermogram}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{400}\text{\u2013}\mathrm{460}{}^{\circ}\text{C}\right]\end{array}}}}\right),{\displaystyle}& {\displaystyle}R\phantom{\rule{0.25em}{0ex}}\text{index}=\\ \text{(5)}& {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}{\displaystyle \frac{\mathrm{Area}\phantom{\rule{0.25em}{0ex}}\mathrm{of}\phantom{\rule{0.25em}{0ex}}\mathrm{HC}\phantom{\rule{0.25em}{0ex}}\mathrm{pyrolysis}\phantom{\rule{0.25em}{0ex}}\mathrm{thermogram}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{400}\text{\u2013}\mathrm{650}{}^{\circ}\text{C}\right]}{\mathrm{Total}\phantom{\rule{0.25em}{0ex}}\mathrm{area}\phantom{\rule{0.25em}{0ex}}\mathrm{of}\phantom{\rule{0.25em}{0ex}}\mathrm{HC}\phantom{\rule{0.25em}{0ex}}\mathrm{pyrolysis}\phantom{\rule{0.25em}{0ex}}\mathrm{thermogram}}}.\end{array}$$

Using the HC pyrolysis thermogram, we determined a parameter reflecting SOC
bulk chemistry, the hydrogen index (HI, mg HC g^{−1}C) that corresponds to
the quantity of pyrolyzed hydrocarbons relative to SOC_{RE6}. Using
the CO and CO_{2} pyrolysis thermograms, we determined another parameter
reflecting SOC bulk chemistry, the oxygen index (OI_{RE6},
mgO_{2} g^{−1}C) corresponding to the oxygen yield as CO and CO_{2}
during the thermal pyrolysis of soil organic matter divided by the total SOC
(SOC_{RE6}) of the sample. The HI correlates with the elemental
H : C atomic ratio of SOC and the OI_{RE6} correlates with the
elemental O : C atomic ratio of SOC (Espitalié et al., 1977).

Overall, we thus calculated for each soil sample a series of 30 RE6 parameters reflecting SOC thermal stability and bulk chemistry to be used in subsequent statistical and modeling analyses.

The signal integration of the RE6 thermograms and the calculation of the RE6 temperature parameters were performed with R v.3.4.3 (R Core Team, 2017) and the hyperSpec (Beleites and Sergo, 2014), pracma (Borchers, 2015) and stringr (Wickham, 2015) packages.

At each site, the CP_{SOC} concentration was estimated as the model
parameter *c* of the exponential decay function described in Eq. (1). To
estimate the value of this parameter and assess its uncertainty, we sampled
the posterior probability density function (PDF) of the model parameters in
Eq. (1), which is given by Bayes' theorem as a function of the prior PDF
(i.e., what we know before collecting data) and the likelihood (i.e., how
likely is it to predict the data given a set of parameters). The posterior
PDF is the combination of our prior knowledge and the information carried
by the data, including measurement uncertainties. For a model vector
** m** (containing the parameters

We chose uniform PDFs for the model parameters *a*, *b* and *c* to be as
uninformative as possible. We use the Gaussian form of the likelihood
function, such as $P\left(\mathit{m}\right|\mathit{d})\propto {e}^{-\frac{\mathrm{1}}{\mathrm{2}}{\left(\mathit{d}-\mathit{\gamma}\left(\mathit{t}\right)\right)}^{\mathrm{T}}{\mathbf{C}}_{\mathrm{d}}^{-\mathrm{1}}\left(\mathit{d}-\mathit{\gamma}\left(\mathit{t}\right)\right)}$, where ** t** is
the vector of all observation times and

Based on our assessment of the uncertainties in SOC concentration data and
site-specific CP_{SOC} concentrations (see above), we propagated
these errors to estimate the uncertainty in the CP_{SOC} proportion
in each soil sample. This was estimated by calculating the standard deviation
(SD) of the CP_{SOC} proportion for each sample as
follows:

$$\begin{array}{ll}{\displaystyle}& {\displaystyle}\mathrm{SD}\phantom{\rule{0.25em}{0ex}}\left({\mathrm{CP}}_{\mathrm{SOC}}\phantom{\rule{0.25em}{0ex}}\mathrm{proportion}\phantom{\rule{0.25em}{0ex}}\right[\mathrm{sample}\left]\right)=\\ {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}{\mathrm{CP}}_{\mathrm{SOC}}\phantom{\rule{0.25em}{0ex}}\mathrm{proportion}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{sample}\right]\\ {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}\times \sqrt{{\left({\displaystyle \frac{\mathrm{SD}\left({\mathrm{CP}}_{\mathrm{SOC}}\phantom{\rule{0.25em}{0ex}}\mathrm{concentration}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{site}\right]\right)}{{\mathrm{CP}}_{\mathrm{SOC}}\phantom{\rule{0.25em}{0ex}}\mathrm{concentration}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{site}\right]}}\right)}^{\mathrm{2}}}\\ \text{(6)}& {\displaystyle}& {\displaystyle}\phantom{\rule{1em}{0ex}}\stackrel{\mathrm{\u203e}}{+{\left({\displaystyle \frac{\mathrm{SD}\left(\mathrm{SOC}\phantom{\rule{0.25em}{0ex}}\mathrm{concentration}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{sample}\right]\right)}{\mathrm{SOC}\phantom{\rule{0.25em}{0ex}}\mathrm{concentration}\phantom{\rule{0.25em}{0ex}}\left[\mathrm{sample}\right]}}\right)}^{\mathrm{2}}}.\end{array}$$

The reference sample set was randomly split into a calibration set
(*n* = 88 samples) and a validation set (*n* = 30 samples), each one
containing soils from all sites. The correlations between the 30 RE6
parameters and the CP_{SOC} proportion were assessed with a
nonparametric Spearman's rank correlation test on the calibration set
(*n* = 88). A principal component analysis (PCA) of the 30 centered and
scaled RE6 parameters was performed for the calibration set to (i) summarize
the variance of SOC thermal stability and bulk chemistry on a single
factorial map and (ii) illustrate the correlations among RE6 parameters.
Correlations between the CP_{SOC} proportion in calibration soils
and their principal component scores were determined using Spearman's rank
correlation tests, and its relationships with the 30 RE6 parameters were
further illustrated by projecting the CP_{SOC} proportion variable
in the PCA correlation plot. The RE6 data of the soils from the validation
set were projected on the same PCA factorial map to check that the validation
set was representative of the calibration set.

A multivariate regression model was built to relate the CP_{SOC}
proportion in the reference samples from the calibration set with soils from
all sites (response vector or dependent variable ** y**,

The performance of the random forests regression model for predicting
CP_{SOC} proportion was assessed by statistics comparing the
RF-predicted vs. reference (estimated) values of the sample set. The
performance statistics were calculated on (i) the “out-of-bag” soil
samples of the calibration set and (ii) the soil samples of the validation
set. Out-of-bag samples are observations from the calibration set not
included in the learning sample set for a specific tree that can be used as a
“built-in” test set for calculating its prediction accuracy (Strobl et al.,
2009). The performance statistics included the coefficient of determination
(pseudo-*R*^{2} for the calibration set or *R*^{2} for the validation set) and
the root mean square error of calibration or prediction (RMSEC for the
calibration set or RMSEP for the validation set). The ratio of performance to
deviation (RPD) and the bias of the random forests regression model were
additionally calculated for the validation set. The relative importance
(i.e.,
ranking) of each of the 30 RE6 parameters for the prediction of the
proportion of CP_{SOC} in the RF regression model was computed as
the unscaled permutation accuracy (Strobl et al., 2009).

Additionally, the sensitivity of the RF regression model to pedoclimate was
assessed by examining its predictive performance for a calibration set based
on soils from three sites (Versailles, Grignon, Rothamsted, *n* = 84) and
a fully independent validation set based on soils from a different
pedoclimate (Ultuna, *n* = 34) but with similar RE6 thermal
characteristics to those of the calibration set (see Sect. 3.2). Soils from
Ultuna indeed have a higher clay content (from 11 to 20 % more clays)
and experience a lower mean annual temperature (from 4 to 5.2 ^{∘}C
lower temperature) and a lower mean annual precipitation (from 116 to
179 mm lower precipitation) than the soils of the three other sites
(Table S1).

Since our objective was to deliver a model based on thermal analysis with
reliable prediction intervals around the predicted values of the
CP_{SOC} proportion, we estimated the prediction uncertainty of the
random forests model for new soil samples. We used a methodology recently
published by Coulston et al. (2016) to approximate prediction uncertainty for
random forests regression models and adapted it to explicitly take into
account the uncertainty in the reference values of the CP_{SOC} proportion
(Eq. 6) that were used to build the model (Supplement,
Fig. S1).

Briefly, we sampled with replacement (i.e., bootstrapped) the calibration set
(** y**,

$$\begin{array}{}\text{(7)}& \mathit{\tau}=\sqrt{{\displaystyle \frac{(y-\overline{\widehat{y}}{)}^{\mathrm{2}}}{\mathrm{var}\left(\widehat{y}\right)}}}.\end{array}$$

A Monte Carlo approach was used to estimate $\widehat{\mathit{\tau}}$, the 95th
percentile of all calculated *τ* values for all out-of-bag observations
of the 2000 bootstraps (Fig. S1). This $\widehat{\mathit{\tau}}$ value was such that
95 % of the predictions of the CP_{SOC} proportion lie within
$\widehat{\mathit{\tau}}$ × $\mathrm{SD}\left(\widehat{y}\right)$ of the true value of the
CP_{SOC} proportion (i.e., 95 % prediction intervals). As
$\mathrm{SD}\left(\widehat{y}\right)$, the standard deviation of the predictions of the
CP_{SOC} proportion across 1000 regression trees can be calculated
by the random forests regression model for any soil sample; this approach
allows for the calculation of 95 % prediction intervals on new soil
samples for which only **X** data (30 RE6 parameters) are available. We
calculated the 95 % prediction intervals
($\overline{\widehat{\mathit{y}}}$ ± $\widehat{\mathit{\tau}}$ × $\mathbf{SD}\left(\widehat{\mathit{y}}\right)$) for the
validation set (*n* = 30) to examine whether those intervals included the
true (estimated) values of the CP_{SOC} proportion. More details on the
procedure to approximate prediction uncertainty for random forests regression
models are provided in Coulston et al. (2016). We finally checked how the
error in the CP_{SOC} proportion propagated into the random forests
regression model by (i) comparing the value of $\widehat{\mathit{\tau}}$ with or without
incorporating the uncertainty in the reference values of the CP_{SOC}
proportion in the algorithm and (ii) by comparing the sizes of the 95 %
prediction intervals calculated for the validation soil samples with their
respective 95 % confidence intervals (determined by multiplying their
standard deviation calculated in Eq. 6 by 1.96).

The Bayesian inference method was performed with Python 2.7 and the PyMC library (Patil et al., 2010). All other statistical analyses were performed with R v.3.4.3 (R Core Team, 2017) and the factoextra package for running PCA (Kassambara, 2015), the randomForest package for running the random forests regression models (Liaw and Wiener, 2002) and the boot package for bootstrapping (Davison and Hinkley, 1997; Canty and Ripley, 2015).

3 Results

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The Bayesian inference of the parameter *c* of the exponential decay function
(Eq. 1) yielded site-specific estimates of the CP_{SOC}
concentration with 95 % confidence intervals (Eq. 1, Table 1, Fig. 2).
Estimated CP_{SOC} concentrations ranged from
6.22 g C kg^{−1} of soil at Versailles to 10.46 g C kg^{−1} of soil at
Rothamsted. The uncertainty in CP_{SOC} concentration was lower at
Rothamsted (standard deviation of 0.27 g C kg^{−1} of soil) and Versailles
(standard deviation of 0.31 g C kg^{−1} of soil) than at Ultuna (standard
deviation of 0.88 g C kg^{−1} of soil) and Grignon (standard deviation of
1.00 g C kg^{−1} of soil).

Overall, the wide range in total SOC concentrations within and across sites
(from 5 to 46 g C kg^{−1} of soil; Table 1), combined with an assumed
constant CP_{SOC} concentration within each site, resulted in a
reference sample set with a wide spectrum of CP_{SOC} proportions
ranging from 0.14 to 1 (Eq. 2, Table 1). The uncertainty (standard deviation)
in the values of CP_{SOC} proportion ranged from 0.01 to 0.15 for
the reference sample set (Eq. 6, Fig. S2). High
uncertainties were found for high values of CP_{SOC} proportion
(i.e., samples with longer time periods under bare fallow treatment), with a
modulation by the site-specific CP_{SOC} concentration uncertainty
(Grignon > Ultuna > Versailles > Rothamsted; Table 1), as
expected from Eq. (6) (Fig. S2).

The random splitting of the reference sample set generated calibration and
validation sample sets with similar mean values, range of values and standard
deviations for both total SOC concentration and CP_{SOC} proportion
(Table 1).

The 30 RE6 parameters showed very different and even contrasting correlations
with the CP_{SOC} proportion in the calibration set with soils from
all sites (Table 2). Most RE6 temperature parameters showed positive
correlations with the CP_{SOC} proportion, with Spearman's *ρ*
above 0.8 for four of them (the RE6 temperature parameter corresponding to
50 % of CO_{2} gas evolution during the pyrolysis stage,
${T}_{\mathrm{50}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{PYR}}$, and the RE6 temperature parameters
corresponding to 30, 50 and 70 % of CO_{2} gas evolution during the
oxidation stages ${T}_{\mathrm{30}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{OX}}$,
${T}_{\mathrm{50}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{OX}}$,
${T}_{\mathrm{70}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{OX}}$;
Table 2).

Conversely, five RE6 temperature parameters showed significant negative
correlations with the CP_{SOC} proportion
(*T*_{10_HC_PYR},
*T*_{10_CO_PYR},
*T*_{30_CO_PYR},
*T*_{50_CO_PYR},
*T*_{70_CO_PYR}; Table 2). The three RE6 parameters
reflecting a proportion of thermally resistant or labile hydrocarbons
(TLHC index, *I* index and the *R* index) showed no correlations with the
CP_{SOC} proportion (Table 2). The two RE6 parameters reflecting SOC
bulk chemistry showed highly significant correlations with the
CP_{SOC} proportion (Table 2), the HI being negatively correlated
and the OI_{RE6} being positively correlated.

The PCA of the centered and scaled RE6 parameters illustrates the
correlations among those 30 variables in the calibration set with soils from
all sites (Fig. 3). A continuum of CP_{SOC} proportion values was
observed in the reference samples along the first two principal components
(Fig. 3a), and projecting the CP_{SOC} proportion in the PCA
correlation circle further highlighted the relationships between this
variable and the 30 RE6 parameters (Fig. 3b). The CP_{SOC}
proportion variable had a strongly negative projected loading score on PC1
(Fig. 3b), as well as negative projected loadings on PC2 (Fig. 3b) and PC3
(data not shown). The scores of the calibration soils on the first three
principal components were indeed significantly and negatively correlated with
the CP_{SOC} proportion (*ρ* = −0.61, *p* value < 0.001
for PC1, *ρ* = −0.49, *p* value < 0.001 for PC2,
*ρ* = −0.25, *p* value < 0.05 for PC3) such that a large part
(82 %) of the variance in the 30 RE6 parameters was linked to the
CP_{SOC} proportion in the calibration set.

The random splitting of the reference sample set generated calibration and
validation sample sets with similar RE6 thermal characteristics as
illustrated by their similar distribution on the factorial map of the first
two principal components of the PCA (Fig. 3a). Soils from the site of Grignon
(with carbonates) showed RE6 thermal characteristics different from the other
sites (Fig. 3a). Some soils from the sites of Rothamsted and Versailles with
high CP_{SOC} proportions also showed specific RE6 thermal
signatures (Fig. 3a). Conversely, all soils from the site of Ultuna showed
similarities regarding their RE6 thermal characteristics with certain soil
samples from other sites (Versailles and Rothamsted; Fig. 3a).

The random forests regression model performed very well in predicting the
CP_{SOC} proportion in the reference sample set using the 30 RE6
parameters as predictors (Fig. 4). Both performance statistics on the
calibration set with soils from all sites (pseudo-*R*^{2} = 0.89,
RMSEC = 0.07, *n* = 88) and on the validation set with soils from all
sites (*R*^{2} = 0.92, RMSEP = 0.07, *n* = 30) demonstrated the
good predictive power of the regression model based on RE6 thermal analysis.
The predictive performance of the random forests model based on RE6 thermal
analysis (RE6-RF) was altered when soil samples from a pedoclimate different
(site of Ultuna, *n* = 34) from the calibration set (Versailles,
Rothamsted, Grignon), but with similar RE6 thermal characteristics (see
Sect. 3.2), were used for validation (Fig. 5). The coefficient of
determination of the model decreased (*R*^{2} = 0.53), yet its mean error
of prediction did not increase strongly (RMSEP = 0.10; Fig. 5).

Propagating the estimated uncertainties in the values of CP_{SOC}
proportion increased the size of the prediction intervals of the RE6-RF
regression model. Indeed, the value of $\widehat{\mathit{\tau}}$ increased from 1.83 to
2.12 when the uncertainty in CP_{SOC} proportion was integrated into
the algorithm described in Sect. 2.3.5. The horizontal and vertical error
bars in Fig. 4 illustrate the global error propagation of the
CP_{SOC} proportion estimates in the RE6-RF regression model for the
validation soil sample set. The values of the total width of the 95 %
confidence interval (reference estimations of CP_{SOC} proportion;
horizontal error bars in Fig. 4) were 0.03 (minimum total width), 0.58
(maximum total width) and 0.24 (mean total width) for the soil samples of the
validation set (*n* = 30). For the 95 % prediction intervals (RE6-RF
predictions of CP_{SOC} proportion; vertical error bars in Fig. 4),
the uncertainties increased to 0.11 (minimum total width), 0.66 (maximum
total width) and 0.37 (mean total width). The 30 total 95 % prediction
intervals for RE6-RF predictions of the CP_{SOC} proportion in the
validation set all included their respective reference estimation of the
CP_{SOC} proportion (Fig. 4).

Out of the 30 RE6 parameters tested by the random forests model as possible
predictor variables of the CP_{SOC} proportion in the calibration
set with soils from all sites, the RE6 temperature parameters corresponding
to 50 and 70 % of CO_{2} gas evolution during the pyrolysis stage
(${T}_{\mathrm{50}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{PYR}}$,
${T}_{\mathrm{70}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{PYR}}$) and to 30 % of CO_{2} gas
evolution during the oxidation stage (${T}_{\mathrm{30}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{OX}}$)
showed the highest importance scores (based on permutation accuracy
importance calculations; Table 2). The eight most important RE6 parameters
for predicting the CP_{SOC} proportion were temperature parameters
calculated on the five different RE6 thermograms (Table 2). The two RE6
parameters reflecting SOC bulk chemistry (OI_{RE6} and HI) were of
medium importance to predict the CP_{SOC} proportion, while the RE6
parameters reflecting a proportion of thermally resistant or labile
hydrocarbons (*I* index, *R* index and TLHC index) were of weak importance
(Table 2).

4 Discussion

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Adding new SOC concentration data for Rothamsted (up to 2014) and Ultuna (up
to 2015) and combining SOC concentration data from all LTBF field replicate
plots of each site decreased the uncertainty in the site-specific estimates
of the CP_{SOC} concentration (Fig. 2) compared with the previous
estimations published by Barré et al. (2010). Indeed, the total width of
the 95 % confidence interval around the estimation of the site-specific
CP_{SOC} concentration slightly decreased from 1.4 to
1.2 g C kg^{−1} of soil at Versailles and from 4.96 to
3.92 g C kg^{−1} of
soil at Grignon; it also strongly decreased from 7.24 to 3.46 g C kg^{−1} of soil
at Ultuna and from 5.98 to 1.06 g C kg^{−1} of soil at Rothamsted (Table 1,
Fig. 2; Barré et al., 2010). The mean estimated values of the
CP_{SOC} concentration were marginally changed at Versailles (6.22
vs. 6.12 g C kg^{−1} of soil in Barré et al., 2010) and Grignon (7.12 vs.
6.80 g C kg^{−1} of soil in Barré et al., 2010), but strongly modified
(increased) at Ultuna (6.95 vs. 3.90 g C kg^{−1} of soil in Barré et al.,
2010) and Rothamsted (10.46 vs. 2.72 g C kg^{−1} of soil in Barré et al.,
2010; Table 1).

Our results obtained under four contrasting pedoclimates of northwestern
Europe indicate a minimum value of 5 g C kg^{−1} of soil (the lowest boundary of
the four 95 % confidence intervals; Table 1) and a maximum value of
11 g C kg^{−1} of soil (the highest boundary of the four 95 % confidence
intervals; Table 1) for CP_{SOC} concentration in topsoils (0–20 to
0–25 cm of depth). These estimates are close, yet below the CP_{SOC}
concentration value of 12 g C kg^{−1} of soil estimated by Buyanovsky and
Wagner (1998b) for the topsoil (0–20 cm of depth) of the Sanborn long-term
(100 years) agronomic experiment (Columbia, Missouri, USA). Our estimates of
topsoil CP_{SOC} concentration are also well below the value of
16 g C kg^{−1} of soil estimated by Franko and Merbach (2017) in the topsoil
(0–30 cm of depth) of the long-term (28 years) bare fallow experiment of Bad
Lauchstädt (central Germany). The soil type in Bad Lauchstädt (Haplic
Chernozem) and its high concentration of slow-cycling black carbon (estimated
at 2.5 g C kg^{−1} of soil; Brodowski et al., 2007) may explain this
difference, as well as the relatively short time period under bare fallow
(higher uncertainty in the inferred CP_{SOC} concentration).

Among the wide range of CP_{SOC} proportions (0.14 to 1) of our
reference sample set, high values of CP_{SOC} proportions (> 0.6)
were obtained only for soils that had been under bare fallow for a long
period of time: after several years or decades with negligible C inputs and
sustained SOC decomposition (Table 1). Similarly, the low values of
CP_{SOC} proportions (< 0.25) of our reference sample set were
obtained for soils without vegetation but receiving high amounts of manure
amendments at Versailles (Table 1). It could be argued that the CP_{SOC}
proportion values obtained for bare soils with or without organic matter
amendments may not be representative of the CP_{SOC} proportions of
soils under conventional management practices. However, it is interesting to
note that the soils of the reference sample set with vegetation and
experiencing classical management practices (grassland at Rothamsted,
cropland at Ultuna) also showed a wide range of CP_{SOC}
proportions, from 0.25 to 0.56 (Table 1). Moreover, other studies have shown
the high variability of CP_{SOC} proportions in soils. For instance,
Falloon et al. (1998) listed a series of published values of
CP_{SOC} proportions ranging from 0.13 to 0.59. More recently, Mills
et al. (2014) published a large dataset of CP_{SOC} proportions in
uncultivated topsoils (ca. 15 cm of depth). They estimated CP_{SOC}
proportions using a global dataset of topsoil radiocarbon (^{14}C)
data and a steady-state SOC turnover model with a fixed mean residence time
of 1000 years for persistent SOC. Their estimates of CP_{SOC}
proportions varied greatly from 0.03 to 0.98 (mean = 0.48, standard
deviation = 0.22, *n* = 232; soils with inconsistent negative modeled
SOC pools values were removed), with significantly higher CP_{SOC}
proportions in non-forest than in forest uncultivated ecosystems (Mills et
al., 2014).

Overall, those combined results illustrate the wide range of
CP_{SOC} concentrations and proportions in topsoils that may depend
upon pedoclimate, land-use and management practices.

This work reinforces the evidence that there is a link between SOC
persistence in ecosystems and its thermal stability, providing evidence of
the first quantitative link between thermal and in situ long-term
(> 100 years) biogeochemical SOC stability (Fig. 4). The regression model
yields accurate RE6-RF predictions of CP_{SOC} proportions with
95 % prediction intervals that fully propagate the uncertainties
originating from the calibration set that was used to build the model.
Predictions on the validation set illustrate that the error propagation
scheme provides highly conservative 95 % prediction intervals of the
CP_{SOC} proportion in new samples at all intervals including their
respective reference estimate of CP_{SOC} proportion (Fig. 4).
Despite rather large prediction intervals, the RE6-RF regression model
clearly discriminates soils with small CP_{SOC} proportions from
samples with large CP_{SOC} proportions (Fig. 4). This model based
on RE6 thermal analysis can thus be used to predict the size of the
CP_{SOC} pool with known uncertainty in new soil samples from
similar pedoclimates and with thermal characteristics (i.e., RE6 predictor
variables) similar to those of the reference sample set.

Our results also highlight the sensitivity of the RE6-RF regression model to
pedoclimate. Decreased predictive performance of the model (as assessed by
the coefficient of determination) was indeed observed when predicting the
CP_{SOC} proportion in new soils with similar RE6 thermal
characteristics but from a different pedoclimate (Fig. 5). However, the mean
error of prediction of the model only slightly increased when predicting the
CP_{SOC} proportion in soils from the fully independent site of
Ultuna (Fig. 5). Overall, those results illustrate the potential of the model
based on RE6 thermal analysis to predict the proportion of CP_{SOC}
in new soil samples from different pedoclimates, at least for sites that have
similar RE6 thermal characteristics to those of the calibration set.

Our results also illustrate the complex relationships between thermal-analysis-based parameters of SOC stability and the CP_{SOC}
proportion. The hypothesis behind the use of SOC thermal stability as a proxy
for its biogeochemical stability implies positive correlations between the
size of the CP_{SOC} pool and temperature parameters derived from
thermal analysis such as the 25 RE6 temperature parameters calculated in this
study. Significant positive correlations with the CP_{SOC}
proportions were indeed found for the majority (14 out of 25) of RE6
temperature parameters, with very high and positive Spearman's *ρ* values
for some of them (Table 2). This was notably the case of the RE6 temperature
parameter corresponding to 50 % of CO_{2} gas evolution during the
oxidation stage, ${T}_{\mathrm{50}\mathit{\_}{\mathrm{CO}}_{\mathrm{2}}\mathit{\_}\mathrm{OX}}$, which had been
previously shown to systematically increase with bare fallow duration in the
same soils by Barré et al. (2016). This study extends the results of
Barré et al. (2016) towards a quantitative link between RE6 temperature
parameters and SOC persistence (direct correlations and predictions of the
size of the CP_{SOC} pool rather than time under bare fallow
treatment). It also extends those results to non-bare fallow soils: bare
soils receiving organic amendments (at Grignon and Versailles), cropland
soils (Ultuna) and grassland soils (Rothamsted). Conversely, 11 RE6
temperature parameters showed no significant correlation or significant
negative correlations with the CP_{SOC} proportion. Weak or negative
correlations occurred principally for temperature parameters calculated on
thermograms of the pyrolysis stage of the RE6 analysis: for parameters of the
HC and CO thermograms (except *T*_{90_HC_PYR}) and
the lowest temperature parameter of the CO_{2} thermogram (Table 2).
Negative correlations contradict the abovementioned hypothesis, with the
evolution of a similar proportion of the total amount of gases (HC pyrolysis
effluents or CO) occurring at lower temperatures for samples with high
CP_{SOC} proportions than for soils with low CP_{SOC}
proportions. A possible explanation for this unexpected observation could be
that the pyrolysis of SOC in samples with a high proportion of
CP_{SOC} may undergo an enhanced pyrolysis catalyst effect by soil
minerals (Auber, 2009), which are relatively more abundant in those samples
generally characterized by low total SOC concentrations.

Despite the fact that the TLHC index, the *I* index and the *R* index had
originally been proposed as useful qualitative metrics of soil carbon
dynamics, reflecting a proportion of thermally resistant or labile
hydrocarbons (Disnar et al., 2003; Sebag et al., 2006, 2016; Saenger et al.,
2013, 2015), those parameters were not correlated with the CP_{SOC}
proportion. Furthermore, they also had a weak importance in the random
forests model predictions of the CP_{SOC} proportion (Table 2). The
poor link between those three RE6 parameters and the CP_{SOC}
proportion may be explained by the high residence time of CP_{SOC}
(> 100 years). Indeed, so far those parameters have been related to the
proportion of SOC present in the particulate organic matter fraction (size
> 50 µm, density < 1–1.6), an SOC pool characterized by a
residence time in soils generally below 20 years (Saenger et al., 2015;
Soucémarianadin et al., 2018).

The two RE6 parameters reflecting SOC bulk chemistry showed highly
significant correlations with the CP_{SOC} proportion. This
confirms, and extends to vegetated soils, the observed decreasing trend for
HI and increasing trend for OI_{RE6} (except at Versailles where
soils have a high pyrogenic C content) with bare fallow duration observed by
Barré et al. (2016) in the bare fallow treatments of the same
experimental sites. Soils with high proportions of CP_{SOC} are thus
characterized by an oxidized and H-depleted organic matter.

Future developments of this work must extend the Rock-Eval 6 thermal analysis
regression model to a wider range of pedoclimates and to other biomes. As
sites with LTBF treatments are not widespread, complementing the reference
sample set may be achieved by using soils that have different soil forming
factors (e.g., climate, parent material) and (i) which are sampled from
long-term (> 50 to 100 years) experiments with contrasting SOC inputs,
enabling the estimation of their CP_{SOC} concentration (Buyanovsky
and Wagner, 1998a, b), or (ii) for which the mean SOC age is known from
radiocarbon data, enabling the estimation of the size of their persistent SOC
pool (Trumbore, 2009; Mills et al., 2014).

Another development of this work will involve elucidating the fundamental
mechanisms linking the biogeochemical stability of SOC with its thermal
stability (e.g., Leifeld and von Lützow, 2014). This was beyond the scope
of this work, yet it constitutes an exciting field of research that should be
addressed in the future, as highlighted by the unexpected observations
discussed in Sect. 4.2 and by other recent works that found no relationships
between the thermal oxidation of SOC between 200 and 400 ^{∘}C and the
size of SOC pools with shorter residence times in soils (below or above ca.
18 years; Schiedung et al., 2017).

Overall, this work demonstrates the value of Rock-Eval 6 as a routine method
for quantifying the size of the centennially persistent SOC pool with known
uncertainty in temperate soils. The relatively low cost of the Rock-Eval 6
technique and the robustness of the thermal analysis regression model make
it possible to apply it to soil monitoring networks across a continuum of
scales as a reliable proxy for SOC persistence. This may be part of the
framework proposed by O'Rourke et al. (2015) to better understand SOC
processes at the biosphere to biome scale and should be added to the soil
carbon cycling proxies recently listed by Bailey et al. (2018). Mapping
persistent SOC at large scales may allow for the identification of regional
hotspots of centennially persistent SOC that may contribute little to climate
change by 2100. It may also provide information on the sustainability of
additional SOC storage from soil carbon sequestration strategies such as
those promoted by the international 4 per 1000 initiative in agriculture and
forestry (https://www.4p1000.org/, last access: 6 May 2018; Dignac et al., 2017; Minasny et al.,
2017; Soussana et al., 2018). A global map of centennially persistent SOC
based on this empirical RE6 thermal analysis model could also be useful for
improving the parameterization of SOC dynamics in Earth system models
(Falloon and Smith, 2000; Luo et al., 2014; He et al., 2016). Indeed, this
model based on RE6 thermal analysis appears as a robust and operational
alternative to existing techniques used to initialize the size of the
CP_{SOC} pool in models of SOC dynamics (such as the methods of
Falloon et al. (1998) or Zimmermann et al. (2007) that estimate the size of
the inert SOC pool in the RothC model). The integration of large-scale
information on the size of SOC kinetic pools may provide an adequate
complement to the global datasets on SOC fluxes that are currently under
development and restructuring (Hashimoto et al., 2015; Luo et al., 2016;
Harden et al., 2018).

Data availability

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Data availability.

Data can be accessed upon request to Lauric Cécillon and Pierre Barré.

Supplement

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Supplement.

The supplement related to this article is available online at: https://doi.org/10.5194/bg-15-2835-2018-supplement.

Author contributions

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Author contributions.

LC and PB designed the study. PB and CC collected the archived soils. TK, SH, FvO and AM provided the archived soils and associated metadata. FB and FS performed the RE6 thermal analyses. RJ wrote the Python code. LC wrote the R codes and performed all statistical analysis. All authors contributed to the interpretation of the results. LC prepared the paper with contributions from all coauthors.

Competing interests

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Competing interests.

The authors declare that they have no conflict of interest.

Acknowledgements

Back to toptop
Acknowledgements.

This work was funded by ADEME and EC2CO (CARACAS project). Pierre Barré,
Lauric Cécillon, Laure Soucémarianadin and Suzanne Lutfalla thank
the Mairie de Paris (Emergences Programme) for financial support. We thank
Thomas Eglin (ADEME), Nicolas Bouton and Jean Espitalié (Vinci
Technologies), David Sebag (Normandie Univ., University of Lausanne), the
associate editor and two anonymous reviewers for their valuable suggestions
on this work. We thank INRA and AgroParisTech for access to and maintenance
of the Versailles 42 plots and the Grignon 36 plots with their long-term experiments and
sample archives. We thank Rothamsted Research and the Lawes
Agricultural Trust for access to archived samples and the BBSRC for support
under the Institute National Capabilities program grant (BBS/E/C/000J0300).
We gratefully acknowledge the Faculty of Natural Resources and Agricultural
Sciences of the Swedish University of Agricultural Sciences (SLU) for
providing funds for the maintenance of the Ultuna long-term field experiment and
for its sample archive.

Edited by: Marcel van
der Meer

Reviewed by: two anonymous referees

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