An inter-laboratory comparison of soil organic carbon analysis on a farm with four agricultural management systems

Abstract

Soil organic carbon (SOC) as a key soil health indicator is integral to the soil's capacity to function as a vital living ecosystem that sustains plants, animals, and humans. Accurate SOC estimation is essential to decision-making for an increasing number of stakeholders, such as farmers, industry professionals, and policymakers, to determine the environmental benefit of agricultural practices, and more recently, allocate financial rewards through carbon market initiatives. Our study examined SOC variability in soils from four different regenerative management systems on a single farm using stratification and sample compositing, and analyzed by four different laboratories using dry combustion, the recommended analytical method, but one which varied according to laboratory standard operating procedures (SOP). Results showed significant variation in SOC levels for the same soil samples at different laboratories (1.6 ± 0.2 g kg⁻¹), variation comparable to that between the distinct management systems (1.5 ± 0.4 g kg⁻¹). Our findings show that analytical variability within and between laboratories must be considered, that use of the same laboratory, and to the extent possible the same SOP for successive SOC measurements at the same location is necessary, and that rigorous stratification alongside minimal sample consolidation should be conducted to generate analytical sample numbers that cater to logistics, economics, and scientific rigor.