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

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 (i) total C by dry
combustion - soil inorganic carbon (DCw/o pretreatment - Pcal); (ii)
traditional Walkley-Black titration (WBTIT) and (iii) 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 HSD
(α=0.05). 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 >0.90) between all methods (n=85). Under
low SOC soils (n=56) the agreement between all methods decreased, but the
WBSPEC method fit other methods comparatively well r2= 0.71, 0.74, and
0.78 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.