The importance of sampling support for explaining change in soil organic carbon

Soil monitoring networks are used to detect change and ascribe cause to better understand the soil organic carbon (SOC) cycle in terrestrial ecosystems. In particular for SOC, its typically large spatial, and small temporal variability relative to its content may prevent the detection of change because of the complex interactions between the major factors driving change in SOC that cause variation within and between fields. The reduction of estimated SOC spatial variance is readily achieved within e.g., a given land use, by visiting as many fields as resources allow to reduce the estimate of between-field variance. Within-field variance is poorly estimated by sampling within a support (here called a plot), which is a small proportion of the field. We briefly summarise dispersion variances and describe an approximation which makes their calculation readily accessible. Using an example we demonstrate the proportion of total SOC dispersion variance that is omitted and which will ultimately increase noise in the explanation of SOC change. We conclude that when the purpose of monitoring is to explain SOC change we suggest that bulking soil samples from the within-field soil type unit is at least as important as revisiting the same location.