Abstract
Abstract. Evolution of organic carbon content in soils has the potential to be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding soil carbon dynamics is a challenge due to a wide range of residence times of soil organic matter and limited constraints on the mechanisms influencing its persistence. In particular, large uncertainties exist regarding the persistence of pyrogenic organic carbon in soils. In order to characterize organic matter with varying degrees of persistence and to distinguish pyrogenic organic carbon, we combined Rock-Eval analysis, a thermo-chemical method, with the benzene polycarboxylic acid molecular marker method and Raman spectroscopy to characterize samples from long-term bare-fallow experiments, progressively depleted in the most labile organic carbon over time. Considering the heterogeneity of soil samples, size fractions have been separated to distinguish pools of organic carbon with distinct properties. We observe that organic carbon dynamics is dependent on granulometry. A pool of organic carbon with intermediate residence times, from years to a few decades, representing ca. 65 % of the bulk soil organic carbon stock, is mainly associated with fine fractions (< 20 µm). With time under bare fallow, this organic carbon is progressively transferred towards finer fractions through the breakdown of organic matter. Coarse fractions (> 20 µm) are rich in centennially persistent organic carbon, representing ca. 20 % of the initial organic carbon stock, due to the chemical recalcitrance of organic matter in these fractions, dominated by pyrogenic organic carbon. A second pool of persistent organic carbon, representing ca. 15 % of the initial organic carbon stock, is associated with the clay fraction, indicating mechanisms of protection occurring at the submicron scale (< 2 µm). This persistent organic carbon only represents 30 % of the organic carbon initially present in the clay fraction. Persistent organic carbon exhibits heterogeneous chemical signatures depending on the considered pool but a consistent thermal signature demonstrating the relationship between thermal stability and biogeochemical stability of soil organic carbon. This gives the possibility of assessing the size of the persistent organic carbon pool in soils using thermal parameters. The persistence of pyrogenic organic carbon in the clay fraction is similar to the one of total organic carbon. The different persistence of coarse and fine pyrogenic organic carbon resides in the highly condensed nature of sand-sized pyrogenic carbon, which may result from burning temperatures over 700 ∘C. Pyrogenic organic carbon is not inert but exhibits a consistent increase in the level of condensation with time in all size fractions, showing the role of chemical quality in pyrogenic organic carbon persistence. Overall, this study helps improve the separation, evaluation and characterization of carbon pools with distinct residence times in soils and gives insight into the mechanistic origin of soil organic carbon dynamics.
Highlights
Improved management and forecast of soil organic carbon (SOC) stock evolution is a crucial challenge to soil security (e.g. Lal, 2004; Koch et al, 2013) as well as to climate change models and the associated mitigation strategies (e.g. Eglin et al, 2010; Ciais et al, 2014)
The objectives and hypotheses of this work are threefold. (i) We aim to identify SOC pools with distinct dynamics as the heterogeneous properties of the studied fractions should lead to varied SOC residence times, giving the opportunity to separate SOC into distinct pools. (ii) We want to determine the thermo-chemical characteristics of each pool as their dynamics is expected to depend on their stoichiometry (e.g. H and O content), chemistry and thermal stability. (iii) We wish to deduce the mechanistic origin of their dynamics
We show that two-thirds of the SOC stock is constituted of OC with intermediate residence time, from years to decades, which is progressively transferred towards finer fractions from the breakdown of coarse organic compounds
Summary
Improved management and forecast of soil organic carbon (SOC) stock evolution is a crucial challenge to soil security (e.g. Lal, 2004; Koch et al, 2013) as well as to climate change models and the associated mitigation strategies (e.g. Eglin et al, 2010; Ciais et al, 2014). A wide range of SOC residence times are observed in soils, from days to millennia (Trumbore, 2000). The processes behind this remain elusive due to the varied chemical nature of organic compounds and complex interactions with the mineral matrix and microbial decomposers (ii) The interaction between SOC and mineral particles determines an energy barrier inducing the physico-chemical protection of organic matter (OM) against decomposition (Mikutta and Kaiser, 2011). Even if chemical recalcitrance is regarded as a secondary parameter to explain bulk SOC persistence (Amelung et al, 2008), it could be a relevant parameter for a specific form of SOC: pyrogenic organic carbon (PyOC; Schmidt et al, 2011). The processes responsible for PyOC persistence and its contribution to the persistent SOC pool remain to be determined (Lutfalla et al, 2017)
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