The researchers used a fast method called Vis-NIR spectroscopy to track changes in soil carbon over time without having to do time-consuming lab tests. They looked at soil samples from different parts of Denmark in 1986 and 2009. Their results showed that soil carbon levels decreased slightly in Denmark over this period, especially in the topsoil. In some regions, the Vis-NIR predictions matched well with the lab measurements, suggesting it's a good way to monitor soil carbon changes, especially on the surface. However, in areas with low soil carbon deeper down, the method didn't work as well, so more research is needed to improve its accuracy for all types of soils.