A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils

Abstract

Abstract. 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 15 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 thermal analysis based model of the 20 size of the centennially persistent SOC pool. We used a unique set of 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 on the observed declining SOC concentration over the duration of the long-term 25 bare fallow treatment. Overall, the estimated concentrations of centennially persistent SOC ranged from 5 to 11 gC.kg-1 soil (lowest and highest boundaries of four 95% confidence intervals). Then, by dividing site-specific concentrations of persistent SOC by the total SOC concentration of 118 archived soil samples from long-term bare fallow and non-bare fallow treatments, we could estimate the proportion of centennially persistent SOC in the 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 30 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. The sample set was split into a calibration set (n = 88) and a validation set (n = 30). We trained a non-parametric machine learning algorithm (random forests multivariate regression model) that accurately samples from similar pedoclimates. Our study strengthens the evidence for 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.

Environmental factors controlling soil organic carbon stability in French forest soils

“…This modulation of the effect of depth by soil class could be linked to different types of SOM moieties developed by very different pedogenetic processes, eutric Cambisols showing a relatively more oxidized SOC than other soil classes (higher OIRE6; specifically down to 40 cm depth). In the deep layer, dystric Cambisols were characterized by high OIRE6 values, which could be linked to larger stable SOC pools in this soil class, likely associated with more oxidized SOC moieties (Cécillon et al, 2018).…”

“…Among them, Rock-Eval 6 (RE6) analysis has shown promising results in the determination of SOM biogeochemical stability (Barré et al 2016;Gregorich et al 2015;Saenger et al 2015;Sebag et al 2016). RE6-derived parameters are reliable indicators of the stable SOC pool (Barré et al 2016;Cécillon et al 2018) and can be a useful complement to the aforementioned usual indicators of the labile SOC pool (Soucémarianadin et al 2018). Specifically, one RE6-derived parameter, T50_HC_PYR, which corresponds to the temperature at which 50% of the hydrocarbons released as pyrolysis effluents during SOM pyrolysis have evolved, was linked to the highly-labile and the labile SOC pools (Gregorich et al 2015;Soucémarianadin et al, 2018).…”

“…T50_HC_PYR could thus be used as an indicator of the labile SOC pool defined above, similarly to the POM-C fraction. Another RE6-derived parameter, the temperature at which 50% of the CO2 resulting from SOM thermal oxidation has evolved (T50_CO2_OX) was positively related to an increasing proportion of persistent SOC (Barré et al 2016;Cécillon et al 2018) and to a POM-C depletion in temperate forest soils (Soucémarianadin et al 2018). T50_CO2_OX could thus be used as an indicator of the stable SOC pool with mean residence times greater than several decades (> 50-100 years; Cécillon et al 2018).…”

“…T50_HC_PYR was used as an indicator of the labile SOC pool with mean residence time generally up to 20 years (negative correlation with the labile SOC pool according toGregorich et al (2015) andSoucémarianadin et al (2018)), while T50_CO2_OX was used as an indicator of the stable SOC pool with mean residence time typically greater than 50-100 years, followingBarré et al (2016) andCécillon et al (2018). Signal processing of the RE6 thermograms, i.e., signal integration and calculation of T50_HC_PYR and T50_CO2_OX, was performed with the R environment software v.3.3 (R Core Team 2016) using the hyperSpec(Beleites and Sergo 2015) and pracma (Borchers 2015) R packages.…”

Reservation on the rationale of the DRIFTS stability index of soil organic matter (SOM) in mineral soil, and its use for partitioning the C kinetic pools of SOM dynamics models

“…Finally, as Laub and colleagues benefit from soil samples from two long-term bare fallow sites in Europe, I suggest that they compute the Spearman's Rho coefficient of their DRIFTS stability index with the proportion of centennially persistent soil organic carbon (CPsoc), that may be derived from the SOC evolution in the bare fallow plots, as shown by Cécillon et al (2018). A higher Spearman's rho coefficient of the DRIFTS stability index with CPsoc than the Spearman's rho coefficient of SOC concentration with CPsoc, would suggest an added value of the index compared to SOC concentration, in its current state.…”