Reproducibility of a soil organic carbon fractionation method to derive RothC carbon pools

Abstract

Fractionation of soil is undertaken to isolate organic carbon with distinct functional properties, such as stability and turnover times. Soil organic carbon (SOC) fractionation helps us to understand better the response of SOC to changes in land use, management or climate. However, fractionation procedures are often poorly defined and there is little information available on their reproducibility in different laboratories. In a ring trial, we assessed the reproducibility of a SOC fractionation method introduced by Zimmermann et al. (2007). The isolated fractions were linked to the model pool sizes of the Rothamsted carbon model (RothC). We found significant differences between six laboratories for all five defined fractions in three different soils with coefficients of variation ranging from 14 to 138%. During ultrasonic dispersion, the output power (energy per unit time) was identified as an important factor controlling the distribution of SOC within these five fractions, while commonly only the output energy is standardized. The amount of water used to wet-sieve dispersed soil slurry significantly influenced the amount of extracted dissolved organic carbon (DOC). We therefore suggest using a fixed amount of power for ultrasonic dispersion (20 W) and a minimum amount of water for wet sieving (2000 ml). RothC pool sizes were predicted from the measured fractions and compared with RothC equilibrium pool size distributions. This model initialization using measured SOC fractions, however, led to an over-estimation of stable RothC SOC pools when compared with pool size distributions derived from RothC equilibrium runs under a bare fallow soil model simulation. To improve the isolation of particulate organic matter from stable mineral-bound organic matter, we suggest that the density should be increased from 1.8 to 2.0 g cm −3 in the density fractionation step. We formulated a modified fractionation procedure, which aims specifically to enhance reproducibility across laboratories and to improve the match of the isolated SOC fractions with RothC's SOC pools.

Biochemical Fractionation of Soil Organic Matter after Incorporation of Organic Residues

“…Existing fractionation methods are divided into physical and chemical fractionation, which are based on different scopes regarding the mechanisms of SOC stabilization [47]. The POM is a SOM quality indicator sensi- tive to changes in C inputs [48].…”

“…However, a mechanistic explanation of a saturation level is still required for the unprotected and the biochemically protected C pools [49]. Loose terms such as "recalcitrance', the vague definitions for fractions such as particulate organic matter (POM) or mineral-associated organic matter (MOM) and poorly defined fractionation procedures have confused the discussion on SOM [47]. Although this research did not address the functionality of the operationally derived [SOL | HOLO] balance, it showed that the van Soest method modified for compost analysis can detect the changes in SOM quality and thus has the potential to assist modeling changes in biochemical C pools in soil particle-size fractions as driven by crop residues and animal wastes management.…”

Effect of ultrasonic power on soil organic carbon fractions

“…However, it is a major problem in soil fractionation, when a fraction of a defined aggregate stability is to be isolated. In a ring trial to evaluate the performance of a soil C fractionation method Poeplau et al (2013a) recently attributed deviations in different SOC fractions between laboratories to differences in applied ultrasonic power. However, systematic and quantitative evidence about the effect of varying ultrasonic power but constant energy on the distribution of SOC stocks and quality is missing.…”

Soil active carbon as an indicator of nitrous oxide loss potential

“…The active carbon pool has proven more sensitive to tillage, manuring, fertilisation, crop rotation and other interventions than TOC in its entirety (Strosser 2010 Substantial efforts have been made to quantify these microbial substrates, using either chemical, physical or incubation methods, either alone or in combination (Wang et al 2003). Physical fractionation methods assume that an association with soil particles and aggregates is the key determinant of SOC stabilisation, whereas chemical fractionation is based on the supposition that chemical composition determines the stability and turnover rate of each fraction (Poeplau et al 2013). …”

“…Therefore, physical fractions can be partitioned on the basis of varying degrees of disaggregation, dispersion, density fractionation and particle size separation (Poeplau et al 2013). …”