Comparison of sampling with a spade and gouge auger for topsoil monitoring at the continental scale

The sampling method is a key aspect when designing a soil monitoring network. The determination of any physical and chemical property can be subject to uncertainties because of the sampling method. In this study, we compared the efficiency of sampling with a spade and a gouge auger for the physicochemical characterization of topsoil samples from 150 mineral soils under various land cover (LC) classes in Switzerland taken within the LUCAS 2015 Survey. The sampling methods differed in their scheme, accuracy of litter removal and control of sampling depth, which were more rigorous with the gouge auger than the spade method. Values of root mean square error of properties ranged between 1/2 and 1/30 of their mean values. Lin's concordance correlation coefficient showed that the spade and gouge auger methods produced similar results for all properties (LCCC ≥0.73), with a better relation for arable land than other LC classes. A poor relation was observed for potassium (LCCC = 0.35) in coniferous forest because of its shallow distribution in depth. We concluded that the simpler and cheaper spade method is an accurate method for topsoil sampling at the continental scale. From this study, it is clear that some improvements in the control of sampling depth and the accuracy of litter removal are needed, especially when monitoring forest soils and properties that change rapidly with depth. Spade sampling can help to expand the implementation of soil monitoring surveys at the continental scale at relatively low sampling cost.