Evaluation of a microplate spectrophotometer for soil organic carbon determination in south-central Idaho

Authors: BIERER, ANDREW - Orise Fellow; Leytem, April; Dungan, Robert - Rob; Rogers, Christopher

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2020
Publication Date: 9/3/2020
Citation: Bierer, A., Leytem, A.B., Dungan, R.S., Rogers, C.W. 2020. Evaluation of a microplate spectrophotometer for soil organic carbon determination in south-central Idaho. Soil Science Society of America Journal. https://doi.org/10.1002/saj2.20165.
DOI: https://doi.org/10.1002/saj2.20165

Interpretive Summary: The amount of organic carbon in soil (SOC) is influential to soil function and therefore measured in agricultural systems. The most common method of measuring SOC does not work in Idaho because they contain carbonates. There are other methods of measuring SOC in Idaho but these methods require substantial handling of samples or produce a large amount of hazardous waste. The new Walker-Black spectrophotometric (WBSPEC)method was recently developed which reduces sample handing and waste generation compared with other methods but has not been fully evaluated or tested in this region. Therefore, the new WBSPEC method was compared with standard methods of SOC determination in Idaho soils. After comparing all methods it was found that the WBSPEC method produced similar results to the other methods while reducing waste generation and sample handling. As a result, the WBSPEC method can be used for SOC determination in soils of south central Idaho and in regions with similar soils.

Technical Abstract: Soil organic carbon (SOC) is traditionally measured through dry combustion of soil but is inaccurate in soils recently limed or containing carbonates. Soils of south central Idaho contain carbonates therefore 3 alternative methods are typically used. Walkley-Black titration (WBTIT) has an extensive history but generates a large volume of hazardous waste for each sample analyzed. Low temperature loss on ignition (LOI360°C) may be utilized but requires frequent sample manipulation and is therefore prone to human error. A pressure calcimeter (Pcal) may be used, however the sample container may leak leading to inaccurate results. Therefore, a new method of SOC determination (WBSPEC) utilizing a microplate spectrophotometer was compared to LOI360°C, Pcal, and WBTIT in 75 south central Idaho soils and 10 standard soils. First, it was confirmed that soils of south central Idaho contain carbonates leading to inaccurate SOC determination by dry combustion. During the alternative method comparison, the WBSPEC method reduced waste production over the traditional WBTIT method by 89% while reducing sample handling over LOI360°C. The LOI360°C and WBTIT methods were most similar, however, the WBSPEC method performed adequately; the Pcal method often overestimated SOC compared to each other method. As the low SOC soils of south central Idaho were of particular interest, the methods were compared a second time on low SOC (<13.11 g kg-1) soils. Here, SOC determination was challenging however the WBSPEC method followed other methods well. It was determined that WBSPEC allows for accurate SOC determination in low SOC soils containing carbonates while reducing hazardous waste production and sample handling.