A Simple Approach to Isolate Slow and Fast Cycling Organic Carbon Fractions in Central European Soils—Importance of Dispersion Method

Abstract

Numerous approaches have been developed to isolate fast and slow cycling soil organic carbon (SOC) pools using physical and chemical fractionation. Most of these methods are complex, expensive, and time consuming and unsuited for high-throughput application, such as for regional scale assessments. For simpler and faster fractionation via particle size the key issue is the dispersion of soil. It is unclear how the initial dispersion of soil affects the turnover rates of isolated fractions. We investigated five commonly used dispersion methods using different intensities: shaking in water, shaking in water with glass beads, ultrasonication at 100 and 450 J ml−1 and sodium hexametaphosphate (Na-HMP). We used soils from long-term field experiments that included a change from C3 to C4 vegetation and adjacent control sites using δ13C isotope ratio mass spectrometry. We evaluated the degree of C3/C4 moieties of the fractions, mass and carbon recovery and reproducibility as well as the time expenditures of the dispersions, sieving and drying techniques to develop an efficient and cheap fractionation method. Our results indicate that ultrasonication as well as H2O treatment with and without glass beads resulted in fractions with different turnover. Moreover, isolation performances depended on soil texture. While the isolation of the fractions using water with and without glass beads was equivalent to ultrasonication in soils with low clay contents, these methods had limited potential for soils with high clay contents. Furthermore, treatment with water alone had less reproducible results than other tested methods. The SOC recovery was comparable and satisfactory amongst non-chemical dispersion methods and reached over 95% for each of these methods. The use of Na-HMP was unsuccessful due to high time expenditures and strong SOC leaching. We propose particle size fractionation combined with ultrasonic dispersion as a fast and highly reliable method to quantify slow and fast cycling SOC pools for a wide range of soil types and textures from agricultural sites in central Europe.

Organic Carbon Storage and Dynamics as Affected by the Adoption of Irrigation in a Cultivated Calcareous Mediterranean Soil

“…This fraction was chosen with the aim of keeping the soil organic matter model simple (two pools), considering it as functionally representing the fast cycling/labile organic C pool (53)(54)(55). Although mechanical dispersion methods such as shaking with glass beads have been reported to be more adequate for the isolation of this fraction, they seem not to be adequate in clay and carbonate-rich soils (56). The use of a (NaPO 3 ) 6 solution of pH 6.13 was therefore preferred, and the risk of organic C loss from this fraction by dissolution was minimized by reducing rinsing as much as possible.…”

“…The fact that the parallel model (Figure 7) was a closer approximation to actual SOC dynamics in the studied soil than the sequential model option (Figure 6) suggested that, in the soil and cropping conditions of this experiment, the protection of organic C from crop residues can be direct, once they are incorporated into the soil. This can however also been partially explained if a small fraction of the labile fraction POC 50−2,000 was transferred to the non-labile fraction in the fractionation process, as suggested by one recent re-assessment of different methodologies to isolate slow and fast cycling SOC fractions (56) We reduced rinsing, and used a non-alkaline solution, to minimize this aspect, but it is still possible that part of the most labile fraction was indeed accounted for in the non-labile fraction in our case.…”

“…In fact, the organic C in the size fraction 50-2,000 µm has been often considered as functionally representing the fast cycling /labile organic C pool (53)(54)(55). The isolation of such fraction can however be elusive, and its effectiveness depends upon soil characteristics (56).…”

Mechanisms of soil organic carbon sequestration and implications for management

“…and tested for routine analysis of large sample sets to obtain indicators for soil management and OC input strategies (Just et al, 2021;.…”

Assessing the impact of interfering organic matter on soil metaproteomic workflow

“…Different extractants have been found to systematically bias soil protein extraction (Bastida et al, 2018;Greenfield et al, 2018;Masciandaro et al, 2008). Particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions have been related to turnover times of conceptual soil organic C pools (Just et al, 2021;Peralta et al, 2022;Zimmermann et al, 2007). To date, knowledge gaps exist with regard to the relative importance of protein functions and taxonomy pertaining to the nature of extracted SOM fractions.…”