Comparison of three carbon determination methods on naturally occurring substrates and the implication for the quantification of 'soil carbon'

Abstract

Accounting for carbon (C) in soil will require a degree of precision sufficient to permit an assessment of any trend through time. Soil can contain many chemically and physically diverse forms of organic and inorganic carbon, some of which might not meet certain definitions of ‘soil carbon’. In an attempt to assess how measurements of these diverse forms of C might vary with analytical method, we measured the C concentration of 26 substrates by three methods commonly used for soil C (Walkley–Black, Heanes, and Leco). The Heanes and Leco methods were essentially equivalent in their capture of organic C, but the Leco method captured almost all of the inorganic C (carbonates, graphite). The Heanes and Walkley–Black methods did not measure carbonates but did measure 92% and 9%, respectively, of the C in graphite. All three of the common soil test procedures measured some proportion of the charcoal and of the other burnt materials. The proportion of common organic substrates (not the carbonates, graphite, or soil) that was C by weight ranged from ~10% to 90% based on the Heanes and Leco data. The proportion of the organic fraction of those same substrates, as measured by loss-on-ignition, that was C by weight ranged from 42% to 100%. The relationship between Walkley–Black C and total C (by Heanes and Leco) showed that Walkley–Black C was a variable proportion of total C for the 26 substrates. Finally, the well-known, apparent artefact in the Cr-acid methods was investigated: dichromate digestion should contain at least 7–10 mg C in the sample or over-recovery of C might be reported. Our observation that common soil C procedures readily measure C in plant roots and shoots, and in burnt stubble, means that there will likely be intra-annual variation in soil C, because avoidance of these fresh residues is difficult. Such apparent intra-annual variation in soil C will make the detection of long-term trends problematic.

Change in Soil Organic Carbon Stocks under 12 Climate Change Projections over New South Wales, Australia

“…The variation in different laboratory methods caused by the different dates and jurisdictions of the analyses resulted in a degree of inconsistency in the test results and potential error in the predictive models. The Walkley-Black method has been reported to underestimate total SOC levels (Skjemstad, 2000) but no correction was applied, as there is much uncertainty regarding the most appropriate correction factor (Conyers et al, 2011;Bui et al, 2009). The final analysis excluded samples with less than 0.1% SOC, as these were considered unreliable, and those with greater than 18% SOC, as these are always defined as organic materials in the Australian Soil Classification (Isbell, 2002).…”

Methods of soil organic carbon determination in Brazilian savannah soils

“…The results of the Mebius method were the closest (R 2 = 0.87) to the results of the elemental analysis (Figure 2), justified by the external heating under reflux (Chatterjee et al, 2009;Gatto et al, 2009;Conyers et al, 2011) that potentialized the oxidation process. Rheinheimer et al (2008) obtained a higher coefficient (R 2 = 0.97) of linear regression for the relation between the same methods, although a smaller number of samples were used of a more homogeneous origin, from two locations only.…”

“…Furthermore, the colorimetric method showed positive correlation with the reference method (R 2 = 0.85). Other studies have demonstrated that it is possible to combine wet digestion methods with photometric determination (Conyers et al, 2011), permitting a simpler way of determining soil C instead of titration.…”

“…However, this recovery may vary depending on the soil properties such as clay content, mineralogy, aggregation and the quality of the soil organic matter. Thus, Conyers et al (2011) suggested that the SOC determined by Walkley and Black should be considered as readily oxidisable carbon and no attempt to convert to total carbon should be taken due to the large variation in soil properties.…”

“…Furthermore, the recovery efficiency of C by different methods depends on the type of substrate that comprises the organic matter (Conyers et al, 2011;Carmo and Silva, 2012) hence, there is still no concensus on considering one ideal method as standard, making the comparison of experimental results difficult, and causing problems in the evaluation of soil C stocks.…”