Number of Replications during Monitoring of the Soil Organic Carbon Content in Forest

Abstract

The estimation of the required number of soil samples to assess the content of soil organic carbon (SOC) in forest biogeocenosis during monitoring studies is considered using the example of data given in an article by E.A. Dmitriev et al. Primary data on the SOC content were obtained in a spruce forest at 166 sites in layers of 0–10, 10–20, and 20–30 cm after removal of the litter. The sampling was performed at points of a regular grid formed by equilateral triangles with 1-m sides within a regular hexagon with a side of 7 m. The SOC content was determined by the method by Tyurin. The original article presents statistics for three zones: near-trunk, under-crown, and inter-crown. The spatial variation of carbon content in all the zones and at all depths is high, the coefficients of variation are about 50%. It is shown that the number of replications required to estimate the mean SOC content at a 95% confidence level is hundreds of samples in the 0- to 10-cm layer and decreases to tens of samples in the 20- to 30-cm layer. Since the number of replications for testing hypotheses about the equality of means depends not only on the confidence level, but also on the power of the criterion used, the required number of replications increases several times. Testing by samples taken from the entire vertical 0- to 30-cm-thick layer with the formation of composite samples reduces the number of required replications. However, careful observation of sample preparation, including primary mixing of samples, is required.