Estimating the Contribution of Sampling, Sample Pretreatment, and Analysis in the Total Uncertainty Budget of Agricultural Soil pH and Organic Carbon Monitoring

Abstract

Monitoring of soil organic carbon (SOC) and pH is needed to manage soil protection and tackle possible degradation in support of, i.e, the upcoming European Soil Framework Directive. Harmonized monitoring procedures and protocols produced under the auspices of the International Organization for Standardization (ISO) and the European Committee for Standardization (CEN) will be recommended. The uncertainty contributions of sampling, sample pretreatment, and analysis in the monitoring of soil pH and organic carbon in agricultural parcels using these harmonized monitoring procedures have been studied.A within-laboratory comparison between the different analytical methods and sample pretreatments was made on 451 soil samples for SOC and 150 samples for soil acidity. Thereafter, a field study was performed to evaluate the contribution of the sampling method. Finally, an interlaboratory trial (including sampling) was organized to assess the overall monitoring uncertainty.The results indicate that the influence of different sample pretreatments (e.g., milling) in combination with different analytical methods (elemental combustion versus chemical oxidation) are the main contributions to the observed uncertainty in the monitoring of SOC. For the monitoring of soil acidity, a similar observation was made, showing that differences in the practical implementation of the analytical method (e.g., mechanical shaking) are the main contributions to the monitoring uncertainty. The monitoring uncertainties derived from an interlaboratory trial (including sampling) amounted to ±20% (95% confidence interval, CI) for SOC and ±0.3 pH units (95% CI) for soil acidity on an agricultural parcel.