Is thermal oxidation at different temperatures suitable to isolate soil organic carbon fractions with different turnover?

Abstract

Findings of previous studies suggest that there are relations between thermal stability of soil organic matter (SOM), organo-mineral associations, and stability of SOM against microbial decay. We aimed to test whether thermal oxidation at various temperatures (200°C, 225°C, 275°C, 300°C, 400°C, or 500°C) is capable of isolating SOM fractions with increasing stability against microbial degradation. The investigation was carried out on soils (Phaeozem and Luvisol) under different land-use regimes (field, grassland, forest). The stability of the obtained soil organic carbon (SOC) fractions was determined using the natural-13 C approach for continuously maize-cropped soils and radiocarbon dating. In the Luvisol, thermal oxidation with increasing temperatures did not yield residual SOC fractions of increasing microbial stability. Even the SOC fraction resistant to thermal oxidation at 300°C contained considerable amounts of young, maize-derived C. In the Phaeozem, the mean 14 C age increased considerably (from 3473 y BP in the mineral-associated SOC fraction to 9116 y BP in the residual SOC fraction after thermal oxidation at 300°C). An increasing proportion of fossil C (calculated based on 14 C data) in residual SOC fractions after thermal oxidation with increasing temperatures indicated that this was mainly due to the relative accumulation of thermally stable fossil C. We conclude that thermal oxidation with increasing temperature was not generally suitable to isolate mineral-associated SOC fractions of increasing microbial stability.

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

“…4). Similarly, other studies have found no clear correlation between thermal stability and SOC degradability using isotopic analysis and different thermal techniques such as the TG‐DSC method ( Kuzyakov et al, 2006; Dorodnikov et al, 2007) or other thermal oxidations methods ( Helfrich et al, 2010) at similar temperatures. Kuzyakov et al (2006) used TG‐DSC to study the thermal stability of SOC in only one soil (loamy Gleyic Cambisol) which had been cultivated with Miscanthus for 10.5 years.…”

“…Several studies demonstrated a relationship between thermal and biological stability of SOC for pyrolysis ( Leinweber and Schulten 1992; Plante et al, 2005; Gregorich et al, 2015) and thermal oxidation ( Siewert , 2001; Lopez‐Capel et al, 2005; Peltre et al, 2013). However, other studies found no consistent relationship ( Dorodnikov et al, 2007) or even no relationship ( Kuzyakov et al, 2006; Helfrich et al, 2010) using thermal oxidation. Despite these contradictory findings, thermal oxidation techniques have been used in recent studies to determine biologically labile and stable pools of SOC in mineral soils ( Gregorich et al, 2015; Katsumi et al, 2016; Ma et al, 2016; Miao et al, 2016).…”

“…The EGA was found to have great sensitivity in identifying and characterizing SOC pools within bulk soil samples ( Plante et al, 2009; Fernández et al, 2012b). The temperatures used here were between 200°C and 400°C, which are reported to be the optimal range for characterization of SOC stability ( Plante et al, 2005; Dorodnikov et al, 2007; Plante et al, 2009; Helfrich et al, 2010). Using the widely established combination of thermogravimetry (TG; sample weight is measured as a function of temperature) and differential scanning calorimetry (DSC; the energy added to a sample and a reference is measured in order to determine exothermic and endothermic mass losses), Plante et al (2009) reported SOC being sensitive to temperatures between 200°C and 350°C as thermo‐labile SOC components and SOC oxidized at a temperature of around 450°C as thermo‐stable components.…”

“…For differentiation of labile and stable thermally oxidized SOC from bulk soil samples, this study applied thermal oxidation to density fractions from soils which had undergone a shift in δ 13 C as a result of a vegetation change from C 3 ‐plant to C 4 ‐plant cultivation ( Poeplau and Don , 2014). It has been suggested that the particulate organic matter (POM) density fraction, representing a biologically labile SOC pool ( Helfrich et al, 2010; Schrumpf et al, 2013) can be used to identify similarly labile SOC of bulk soil samples oxidized at low temperatures (between 200°C and 350°C) due to its specific isotopic signature. Likewise, the stable mineral‐associated organic matter (MOM) density fraction can be expected to be correlated with higher oxidation temperatures (> 400°C) upon thermal oxidation of bulk soil SOC.…”

Methods for determination of labile soil organic matter: An overview

“…An improved characterisation -both chemical and functional -of the individual fraction is needed (Gregorich et al 2006, Broos et al 2007, Helfrich et al 2009, Prechtel et al 2009). …”