Simultaneous measurement of soil organic and

Simultaneous measurement of soil organic and inorganic carbon: evaluation of a thermal gradient analysis Truong Xuan Vuong, Felix Heitkamp, Hermann F. Jungkunst, Andreas Reimer & Gerhard Gerold Journal of Soils and Sediments ISSN 1439-0108 Volume 13 Number 7 J Soils Sediments (2013) 13:1133-1140 DOI 10.1007/s11368-013-0715-1

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Author's personal copy J Soils Sediments (2013) 13:1133–1140 DOI 10.1007/s11368-013-0715-1

SOILS, SEC 1 • SOIL ORGANIC MATTER DYNAMICS AND NUTRIENT CYCLING • RESEARCH ARTICLE

Simultaneous measurement of soil organic and inorganic carbon: evaluation of a thermal gradient analysis Truong Xuan Vuong & Felix Heitkamp & Hermann F. Jungkunst & Andreas Reimer & Gerhard Gerold

Received: 6 November 2012 / Accepted: 29 April 2013 / Published online: 15 May 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract Purpose The best method for determining soil organic carbon (SOC) in carbonate-containing samples is still open to debate. The objective of this work was to evaluate a thermal gradient method (ThG), which can determine simultaneously inorganic carbon (SIC) and SOC in a wide range of soil samples. Materials and methods The determination of SOC by ThG (SOCThG) was compared to the following widespread standard methods: (1) acidification (ACI) as pretreatment and subsequent dry combustion (SOCACI) and (2) volumetric quantification of SIC by a calcimeter (CALC) and subtraction of the total carbon content as determined by dry combustion (SOCCALC). Precision (F test) and bias (t test) were tested on a subset of seven samples (n=3). Comparison of the ThG and CALC methods was performed by regression analysis (n=76) on samples representing a wide range of SOC (5.5 to 212.0 g kg−1) and SIC (0 to 59.2 g kg−1) contents. Results and discussion Tests on the replicated subset showed that the precision of ThG was not significantly different from ACI or CALC (F values6) the bias (a=3.95±1.41) was not significantly different from that of the whole data set (t value=0.50; df=119; critical t value=1.98). It is likely that parts of SOC evolved in gaseous form (as CO2 or volatile organic C) due to the acid treatment (Bisutti et al. 2004). Any evolved gas will be measured as a carbonate equivalent by the CALC method and will lead to an underestimation of SOCCALC contents.

4 Conclusions Precision of all tested methods confirmed their potential to be used as standard methods. However, the use of the thermal gradient method systematically resulted in higher estimates of SOC contents. Thus, the suspicion that acid treatments lead to an underestimation of soil organic carbon contents was further corroborated by this study. When relative results are the focus of the study, e.g. management “A” results in higher C stocks compared to management “B”, all methods are suitable, provided that the carbonate contents are in the same magnitude. Care has to be taken when comparing SOC contents in soil samples with markedly different carbonate contents. In this case, the thermal gradient method is more reliable. The chosen method of SOC or SIC determination has to be taken into account in inter-study comparisons, and values from different studies should be recalculated by suitable functions. Acidification of samples is still a required pretreatment in mass spectrometry when the isotopic signal of SOC is of interest. Coupling an MCD with an isotope ratio mass spectrometer in order to assign isotopic signatures to SIC and SOC may be a valuable future advancement of the thermal gradient method. Acknowledgments Excellent technical assistance from Petra Voigt and Anja Södje is greatly acknowledged. We are thankful to Dr. Jürgen Grotheer for the fruitful discussion. Truong Xuan Vuong was supported by the German Academic Exchange Service (DAAD) and project 322 of the Vietnamese Ministry of Training and Education. The comments of an anonymous reviewer significantly improved the content of the manuscript, and Dr. Alphonce Guzha improved the language.

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