Sequential chemical extractions of the mineral-associated soil organic matter: An integrated approach for the fractionation of organo-mineral complexes

Abstract

Long-term stabilisation of soil organic matter (SOM) largely depends on its interaction with the active mineral components of soils. SOM may become associated with the mineral active surfaces through a wide variety of linkages, with different strength. Thus, fractionation procedures capable of assessing the strength through which mineral-associated SOM is stabilised can be very useful. This paper presents a soil organo-mineral fractionation method (henceforth, SOF) that essentially resumes the work of classical pedologists, who aimed to quantify the different modes through which organic compounds are bound to the mineral matrix using sequential extractions with chemical reagents (0.1 M sodium tetraborate, 0.1 M sodium pyrophosphate, 0.1 M sodium hydroxide, 0.1 M sodium hydroxide after sodium dithionite pretreatment, and 0.1 M sodium hydroxide after hydrofluoric acid pretreatment). We added a previous extraction with 0.5 M potassium sulfate to remove soluble organic compounds, and a weak acid attack with 0.33 M sulfuric acid to destroy possible SOM-occluding carbonate films, which are often assumed to contribute to SOM stability in calcareous soils. The proposed sequence is applied only to the organo-mineral complexes (<20 μm), after the removal of the particulate organic matter (POM) by ultrasonic dispersion and wet sieving.We tested the SOF method on four contrasting soils: two Haplic Calcisols (under crop and forest) and two Humic Cambisols (under forest and pasture), with organic C (OC) contents ranging from 1.8 to 3.4% and pH from 3.9 to 8.0. Our results showed that the mineral-associated SOM represents the largest SOM fraction (67–72% of the total organic C content), and that a substantial part of it is weakly associated with the mineral matrix, as it can be extracted by sodium tetraborate or sodium pyrophosphate. While the sodium tetraborate extract was the main fraction in acid soils, the sodium pyrophosphate extract was the main fraction in calcareous soils, thus highlighting the role of Ca in SOM stability. In contrast, our results suggest a small role of carbonate precipitation in the stabilisation of SOM < 20 μm. The sodium hydroxide extractions after both the sodium dithionite and HF treatments released little SOM in the studied soils, but the remaining (insoluble) residue accounted for 15–30% of total OC, and deserves further study. The SOF method can be a valuable tool for splitting mineral-associated SOM into different fractions regarding their proneness for extraction, and its thoroughness may prove most useful for comparative studies about SOM stabilisation.