Thermal oxidation does not fractionate soil organic carbon with differing biological stabilities

Abstract

Thermal analysis techniques have been used to differentiate soil organic carbon (SOC) pools with differing thermal stability. A correlation between thermal and biological stability has been indicated in some studies, while others reported inconsistent relationships. Despite these controversial findings and no standardized method, several recently published studies used thermal analysis techniques to determine the biological stability and quality of SOC in mineral soils. This study examined whether thermal oxidation at temperature levels between 200°C and 400°C, combined with evolving gas analysis and isotope ratio mass spectrometry, is capable of identifying SOC pools with differing biological stability in mineral soils. Soil samples from three sites being under Miscanthus (C4‐plant) cultivation for more than 17 years following former agricultural cropland (only C3‐plant) cultivation were used. Due to natural shifts in 13C content, young and labile Miscanthus‐derived SOC could be distinguished from stable and old C3‐plant‐derived SOC. The proportion of Miscanthus‐derived SOC increased significantly with increasing temperatures up to 350°C in bulk soil samples, indicating increasing oxidation of labile and young SOC with increasing temperatures. Use of density fractions to validate the thermally oxidized SOC from bulk soil samples revealed that the thermal oxidation patterns did not reflect the biological stability of SOC. The suggested biologically labile particulate organic carbon (light fraction from density fractionation) was clearly enriched in Miscanthus‐derived young SOC. The thermal oxidation patterns, however, revealed preferential oxidation of these biologically labile fractions not at low temperatures, but rather at higher temperatures. The reverse was found for the biologically stable mineral‐associated density fraction (heavy fraction). Based on different soil types, it was concluded that the thermal stability of SOC between 200°C and 400°C is not a suitable indicator of the biological stability of SOC and, thus, thermal oxidation is not capable of fractionating SOC pools with differing biological stability.

Detectability of degradable organic matter in agricultural soils by thermogravimetry

“…In contrast to conclusions of Schiedung et al. () about biological stability is not reflected in thermo‐oxidative fractions, this study hints on a detectability of microbial degradation processes via recording of thermal mass losses in two independent experiments. However, these conclusions are based on methods for the assessment of thermal decay dynamics that go beyond the application of traditional peak analysis.…”

“…However, overlapping TML's caused by organic amendments ( e.g ., fresh residues or fertilizers) and SOM (clay‐dependent accumulated carbon) requires adopted evaluation approaches. A possible approach could be the use of thermal mass losses in larger predefined temperature ranges ( Kučerík et al., ) which can combine traditional fractionation of SOM with modern approaches such as density fractionation and mass spectroscopy for SOM quality determination ( Wiesmeier et al., ; Schiedung et al., ).…”

“…Thermal analysis was used to investigate the biological stability of SOM, but provided contradictory results in the assessment of SOC transformation processes (Otero et al, 2002;Peltre et al, 2013;Kuč erík and Siewert, 2014;Barros et al, 2016;Schiedung et al, 2017). Nevertheless, the consideration of natural soils not influenced by human activities and gentle soil sample preparation with standardized water content opened new opportunities, e.g., for soil property prediction (organic carbon, nitrogen, and clay content) and for monitoring microbial degradation of SOM (Siewert, 2001;Siewert, 2004;Siewert et al, 2012;Kuč erík et al, 2016).…”

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

“…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).…”

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

“…This was beyond the scope of this work, yet it remains an exciting field of research that should be addressed in the future, as highlighted by the unexpected observations discussed in Section 4.2 and by other recent works that found no relationships between the thermal oxidation of SOC between 200 °C and 400 °C and the size of SOC pools with shorter residence times in soils (below or above ca. 18 years, 5 Schiedung et al, 2017).…”