Changes in clay-associated organic matter quality in a C depletion sequence as measured by differential thermal analyses

• Published in: Geoderma 2005-12, Vol.129 (3), p.186-199
• Main Authors: Plante, A.F., Pernes, M., Chenu, C.
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Black carbon; Clay-size fraction; Cropping systems. Cultivation. Soil tillage; Differential scanning calorimetry; Earth sciences; Earth, ocean, space; Ecology, environment; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Hydrogen peroxide; Land use; Life Sciences; Soil organic matter; Soil tillage; Soils; Surficial geology; Thermogravimetry; Tillage. Tending. Growth control
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2004.12.043

Land use changes result in significant decreases in soil organic matter stocks due to enhanced mineralization attributed to increased tillage, and due to decreased organic matter inputs. The current paradigm of soil organic matter dynamics suggests that decreasing organic matter stocks are also associated with shifts in organic matter quality to more resistant fractions as the more labile pools are decomposed. The objectives of the current study were to characterize changes in clay-associated organic matter quality in a soil C depletion sequence in terms of thermal properties, and to thermally characterize peroxide-resistant organic matter. Clay-sized fractions were isolated from a sequence of soils ranging from native forest to long-term bare fallow, and analyzed using thermogravimetry (TG) and differential scanning calorimetry (DSC) before and after hydrogen peroxide treatment. Differential scanning calorimetry traces had exothermic maxima near 285 and 333 °C, consistent with analyses of chemically extracted organic matter and humic substances reported in the literature. Peak fitting analyses showed that these maxima consisted of several hidden peaks, but their interpretation is problematic. The qualitative shifts in the thermal properties of clay-associated organic matter with differing land use observed in the DSC traces were quantified using cultivation and fallow treatment to forest ratios. Thermogravimetric mass loss ratios were greater in the thermally labile (180–310 °C) exothermic region than in the more thermally resistant (310–450 °C) exothermic region. Similarly, ratios of peak heights and areas for the fitted DSC peaks were higher for the 324 °C peak, compared to peaks at 257 and 284 °C. The higher ratios indicate that the more thermally resistant organic matter has been retained and the more thermally labile organic matter is lost. The observed shift in the distribution of organic matter from thermally labile to thermally resistant fractions from forest to long-term bare fallow clay samples is consistent with the current paradigm of decomposition consisting of a shift to more biologically resistant fractions with increasing time under cultivation and decreasing organic matter inputs. These results suggest that the thermal properties of clay-associated organic matter are related to their biological decomposability. Hydrogen peroxide treatment of clay samples removed approximately 87% of the initial organic C in all samples, and peroxide-resis