Comparison of different methods for the determination of total organic carbon and humic substances in Brazilian soils

Raphael Bragança Alves Fernandes,Eduardo De Sá Mendonça,Emerson Silva Ribeiro Junior,Ildeu Afonso De Carvalho Junior

doi:10.1590/0034-737x201562050011

Raphael Bragança Alves Fernandes, Eduardo De Sá Mendonça + Show 2 more

Open Access

https://doi.org/10.1590/0034-737x201562050011

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Abstract

ABSTRACTAiming to compare three different methods for the determination of organic carbon (OC) in the soil and fractions of humic substances, seventeen Brazilian soil samples of different classes and textures were evaluated. Amounts of OC in the soil samples and the humic fractions were measured by the dichromate-oxidation method, with and without external heating in a digestion block at 130 °C for 30 min; by the loss-on-ignition method at 450 °C during 5 h and at 600 °C during 6 h; and by the dry combustion method. Dry combustion was used as reference in order to measure the efficiency of the other methods. Soil OC measured by the dichromate-oxidation method with external heating had the highest efficiency and the best results comparing to the reference method. When external heating was not used, the mean recovery efficiency dropped to 71%. The amount of OC was overestimated by the loss-on-ignition methods. Regression equations obtained between total OC contents of the reference method and those of the other methods showed relatively good adjustment, but all intercepts were different from zero (p < 0.01), which suggests that more accuracy can be obtained using not one single correction factor, but considering also the intercept. The Walkley-Black method underestimated the OC contents of the humic fractions, which was associated with the partial oxidation of the humin fraction. Better results were obtained when external heating was used. For the organic matter fractions, the OC in the humic and fulvic acid fractions can be determined without external heating if the reference method is not available, but the humin fraction requires the external heating.

Highlights

Nowadays there is a great interest in carbon cycling in the context of climate change
The establishment of the relationship among different methods of organic carbon determination is important for soil carbon inventories focusing “Land Use Change and Forestry” evaluations for the Kyoto Protocol and the fitting of data previously obtained by different analytical methodologies (Lettens et al, 2007)
These results indicate that the amount of organic carbon (OC) in Brazilian soils is mostly underestimated by routine laboratories, which largely use the dichromate oxidation without heating

Summary

Introduction

Nowadays there is a great interest in carbon cycling in the context of climate change. The dynamics of this element has been modified as a result of human activities, primarily due to fossil fuel use and secondarily to changes in land use (IPCC, 2013). Hutchinson et al (2007) discussed some management practices that contribute to the mitigation of CO2 emissions by trapping C in soils, indicating challenges and opportunities for policymakers, farmers, and soil scientists. It is well known the importance of soil organic matter for soil quality and plant growth. The establishment of the relationship among different methods of organic carbon determination is important for soil carbon inventories focusing “Land Use Change and Forestry” evaluations for the Kyoto Protocol and the fitting of data previously obtained by different analytical methodologies (Lettens et al, 2007)

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