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

Abstract

Determination of soil organic carbon (SOC) is highly desirable for assessing fertility and carbon sequestration; however, numerous methods of determination warrant study of method agreement. Recently, a novel method was developed following dichromate oxidation using a microplate spectrophotometer. This novel method was compared with (a) total C by dry combustion − soil inorganic carbon (DCw/o pretreatment–pressure calcimater [Pcal]); (b) traditional Walkley–Black titration (WBTIT) and (c) loss on ignition (LOI360°C) in calcareous soils of south-central Idaho (n = 75) in conjunction with North American Proficiency Testing program soils (n = 10). A two-way ANOVA was fit with soils as a blocking factor to identify any difference between methods; means were separated using Tukey's honestly significant difference test (α =.05). Additional comparisons were made for all soils (n = 85) and for soils in the lower 75th percentile of SOC determined by WBTIT (n = 56) using regression analysis. Only the WBTIT and LOI360°C methods were statistically equivalent nevertheless there was high agreement (Lin's concordance coefficients >.90) between all methods (n = 85). Under low SOC soils (n = 56) the agreement between all methods decreased, but the Walkley–Black spectrophotometric method (WBSPEC) method fit other methods comparatively well r2=.71,.74, and.78 for LOI360°C, DCw/o pretreatment–Pcal, and WBTIT respectively. The WBSPEC method provided estimates of SOC between the methods currently used in the region while reducing hazardous waste generation over traditional WBTIT and sample handling over LOI360°C and DCw/o pretreatment–Pcal methods, positioning it as a sensible option for SOC determination in low SOC calcareous soils of south-central Idaho.