Effects of Different Factors on the Assessment of Total Alkalinity of Soils on Calcareous Rocks

Abstract

The dynamism of carbon pools and flows in landscapes requires special attention to the methods and tools for measuring all carbon-containing soil components. An important component of the carbon budget in carbonate geosystems is the dissolved inorganic carbon, part of which is carried out by surface and lateral flows into water bodies, which requires attention to assessing the total alkalinity (TA) of soils on calcareous rocks. Analysis of soils and parent rocks in the Polar Urals with a CaCO₃ content of 0 to 100% revealed the factors that influence the value of their TA, namely, (a) the method used for preparing soil samples and water extraction (the ratio of soil weight to the volume of distilled water, m_s : \({{V}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\), and the quality of separation of solid and liquid phases); (b) the moment of recording the acid titration endpoint for extracts; and (c) the presence of organic acids with pK_a < 4.4. The latter reduces the content of bicarbonate ion by converting it into carbonic acid, not measurable by titration. This mechanism is confirmed by (a) analysis of the model solutions of sodium bicarbonate and formic (pK_a = 3.75), tartaric (pK_a1 = 3.04 and pK_a2 = 4.37), and malic (pK_a1 = 3.46) acids; (b) cation–anion balance of soil water extracts; and (c) a negative shift in the titration TA values (the sum of carbonate and organic alkalinities) relative to the amount of equivalents of dissolved inorganic carbon determined by high-temperature catalytic oxidation at the same m_s : \({{V}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) ratios in both methods. It is possible to compare the TA of soils measured by different laboratories only strictly performing all experimental conditions. It is recommended to use a centrifuge to separate the solid and liquid phases of carbonate soils and a pH-meter or a titrator to record the endpoint of titration.