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Linket til højre henviser til rapporten i trykt format til download. Dokumentet over linket er selve leveringen til ministeriet med følgebrev.Traffic-induced soil compaction occurs when mechanical stresses from machinery exceeds the mechanical strength of the soil. During field traffic, significant stresses are transmitted to the subsoil. Danish arable fields are generally very dense in the subsoil. Compaction significantly affects soil functions and ecosystem services including crop yields. Compaction-induced reduction in soil water conductivity may increase surface runoff and loss of nutrients and soil sediments to the aquatic environment. Compaction of the subsoil may increase the risk of by-pass water flow, hence decreasing soils’ filter function for contaminants. Subsoil compaction is long-term or effectively permanent. Mechanical loosening of compacted subsoils is not a solution among others because of a severe risk of soil recompaction. There is lack of knowledge on biological tillage as an effective mitigation measure for severely compacted subsoils. The subsoil is increasingly at risk of compaction because modern farming includes a range of field operations with heavy machinery. Subsoil compaction is accumulating, persistent, and not directly visible. In addition, short-term cost-benefit analyses do not provide an incentive for management changes. This calls for public intervention. We recommend consideration of a general requirement for financial EU-support. Farmers should report their planned field traffic one year in advance. In order to increase farmers’ focus on the compaction threat, we recommend as a first step only a documented planning, while a later step might include a request of modifying the planned traffic in case the evaluation indicates significant compaction damage. We further recommend eleven potential measures that could be used for voluntary action (eco-schemes) to minimize soil compaction.

Assessments of changes in soil organic carbon (SOC) stocks depend heavily on reliable values of SOC content obtained by automated high‐temperature C analysers. However, historical as well as current research often relies on indirect SOC estimates such as loss‐on‐ignition (LOI). In this study, we revisit the conversion of LOI to SOC using soil from two long‐term agricultural field experiments and one arable field with different contents of SOC, clay and particles −1 soil, converting LOI to SOC by the conventional factor 0.58 overestimated the SOC stock by 45 Mg C ha−1. The error increased with increasing contents of clay and Fines20. Converting LOI to SOC by a regression model underestimated the SOC stock by 5 Mg C ha−1 at small clay and Fines20 contents and overestimated the SOC stock by 8 Mg C ha−1 at large contents. This was due to losses of structural water from clay minerals. The best model to convert LOI to SOC incorporated clay content. Evaluating this model against an independent dataset gave a root mean square error and mean error of 0.295 and 0.125 g C 100 g−1, respectively. To avoid misleading accounts of SOC stocks in agricultural soils, we recommend re‐analysis of archived soil samples for SOC using high‐temperature dry combustion methods. Where archived samples are not available, accounting for clay content improves conversion of LOI to SOC considerably. The use of the conventional conversion factor 0.58 is antiquated and provides misleading estimates of SOC stocks.