Towards a Quantitative Estimate of Anthropogenic Subsoil Compaction in European Croplands Based on National Soil Surveys

Subsoil compaction can lead to lower yields and reduced ecosystem functioning due to limited root growth of crops and can affect the cycling of nutrients and water within the soil. Subsoil compaction is often assessed using soil packing density, which accounts for textural differences in bulk density, and a reference threshold. Due to the high costs of sampling bulk density, subsoil compaction is rarely assessed at regional or national scales. While some soils are naturally compact, human activities, for example agriculture, are known to further increase subsoil compactness. The assessment of the anthropogenic component of the subsoil compactness is challenged by the lack of reference soil (i.e., unaffected by anthropogenic activities) that is otherwise comparable in terms of pedogenic and climatic parameters. In this study, a data-driven reciprocal modelling approach was used to model a reference subsoil bulk density for annual croplands based on observations from permanent grasslands, as grasslands are assumed to be free of anthropogenic subsoil compaction. The data originated from soil monitoring networks in five European countries (Belgium (Flanders), Denmark, France, Germany and Ireland). Depending on the country, the subsoil surpassed the compaction threshold of 1.71 g cm⁻³ packing density for 14%–52% of sites. The highest proportion of compacted sites was found in Flanders, while Denmark had the lowest proportion. Similarly, the highest estimated anthropogenic subsoil compaction was found in Flanders (mean 0.05 g cm⁻³) while the lowest was found in France (mean 0.00 g cm⁻³). Overall, the highest estimated anthropogenic subsoil compaction was found in loamy soils and soils with the lowest organic C content, such as eastern Germany and eastern Denmark. Based on our results, between 0% (France) and 47% (Flanders) of the annual cropland sites currently surpass the packing density threshold for compacted soil due to anthropogenic activities rather than due to pedogenic drivers.