Which method to choose for measurement of oranic and inorganic carbon content in carbonate-rich soils? Advantages and disadvantages of dry and wet chemistry

Abstract

The pedosphere is an essential reservoir of carbon represented by organic (SOC) and inorganic (SIC) forms. Various methods are used worldwide to measure SOC and SIC. The trend towards globalization of data on soils requires having the accumulated information harmonized. Global soil databases are crucial for inventory and mapping, modeling and forecasting, monitoring and rational use of soil resources.This study compared the six most common methods for measuring carbon in soils on calcareous rocks. Standard soil samples and soils of the Polar Urals with a variation in CaCO3 (0–100%) and SOC (0–40%) were selected as research targets.The loss-on-ignition (LOI) method (T = 550 °C) overestimates SOC. It is explained by the fact that the SOC to SOM conversion factor of 1.724 seems inadequate. The soils under study demand a coefficient of 2.326. The second reason is the presence of thermolabile mineral components in soils, as well as adsorbed and chemically bound water. For soils with a carbonate content of 0–100% and SOC < 8.7%, harmonized results of SOC measurements may be obtained on the analyzer and by the dichromatometric method (Walkley-Black) only with conversion factors applied (1.3). At a higher SOC (8.7–40%) content, soils in the area under study require a reduced coefficient of 1.18.The measurement results of SIC within the range of 3.2–12% (CaCO3 27–100%) obtained by LOI methods (T = 800 °C) and with a calcimeter are identical with SOC < 16%. The presence of CaCO3 does not inhibit the use of the dichromatometric method (Walkley-Black, Tyurin) to measure SOC.The reliability of carbon content measurement methods for soils was determined both by the specific composition of the soil and by the accuracy of the method itself. To facilitate future application of the methodology, guidelines were proposed to use methods for quantitative assessment of SOC and SIC.