Comparison of soil exchangeable calcium estimated using five extractants and near-infrared spectroscopy

• Published in: Geoderma Regional Vol. 40; p. e00911
• Main Authors: Wang, Zhufeng, Wang, Yugang, Peng, Fei, Xing, Dengchun, Lin, Litao, Feng, Wenting
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2025
• Subjects: Arid regions; carbon sequestration; Cation exchange capacity; climate; EDTA (chelating agent); Exchangeable calcium; humidity; Near-infrared spectroscopy; Partial least squares regression; plant growth; potassium chloride; rapid methods; reflectance; sodium acetate; sodium sulfate; soil pH; soil structure; support vector machines; Exchangeable calcium; Cation exchange capacity; Partial least squares regression; Arid regions; Near-infrared spectroscopy
• ISSN: 2352-0094; 2352-0094
• DOI: 10.1016/j.geodrs.2024.e00911

Soil exchangeable Ca (Caex) regulates multiple functions, including nutrient provision for plant growth, soil structure maintenance, and carbon sequestration. However, various chemical extraction methods are currently used worldwide to determine soil Caex, each with different mechanisms and purposes. The inconsistency in assessment methods complicates comparisons and predictions regarding soil Caex functions across different environments. To address this issue, we developed equations to convert soil Caex concentrations obtained from five different methods in arid regions. Furthermore, we evaluated the accuracy of predicting five soil Caex proxies and cation exchange capacity (CEC) using near-infrared reflectance (NIR) spectroscopy. The chemical extractants (i.e., sodium acetate (NaOAc), potassium chloride (KCl), sodium sulfate (Na2SO4), ethylenediaminetetraacetic acid (EDTA), and copper chloride (CuCl2)) were used on 308 soil samples representing a wide range of soil Caex, climate, and edaphic properties in arid regions. Our findings reveal that compared to the commonly used NaOAc extraction, Na2SO4 extracted a similar amount of soil Caex. In contrast, KCl extraction underestimated soil Caex, whereas EDTA and CuCl2 extractions overestimated it. Such discrepancies suggest that EDTA and CuCl2 are inappropriate for soils with Ca-bearing minerals, such as Ca carbonate. Moreover, the quantitative correlation among soil Caex values obtained through five different extractants highlights the need for careful selection based on specific environmental factors (i.e., soil pH and humidity). Notably, NIR-driven partial least squares regression outperformed support vector machine models for the estimation of soil Caex measured by five extractants (0.78 < R2 < 0.90) and CEC (R2 = 0.96). This capability positions NIR spectroscopy as an efficient and reliable tool for rapid assessment of soil Caex and CEC, particularly in arid regions. In conclusion, the quantitative relationships derived from various extraction methods provide a robust framework for comparing soil Caex values and enhancing our ability to predict changes in its functions under differing environmental conditions. [Display omitted]