Long-term record and analysis of soil temperatures and soil heat fluxes in a grassland area, The Netherlands

A 27-year soil temperature record at five depths and soil heat flux record at one depth were analyzed for a grassland area in The Netherlands. The annual mean soil temperature of the last 23 years of soil measurements (no data gaps) showed a statistically significant increase of about 1.0 °C, consistent with the observed air temperature increase of about 1.3 °C for the same period. This positive trend correlates well with global brightening of 5.3 W m-2 per decade. The 10-day mean soil temperature varied smoothly throughout the year with relatively small inter-annual variability. The deeper the measurement depth, the smoother the annual cycle and the smaller the variability. In February and at the end of the year the variability appears to be somewhat larger. A Fast Fourier Transform was applied to the measurements and revealed an annual and daily damping depth of 1.80 m and 0.10 m, respectively. An example of the usefulness of this data is provided for agriculture, where an aggressive root-knot nematode is affected by the mean soil temperature increase. It appears that the 600 degree day life-cycle threshold is reached 16 days earlier, which may lead to a potential serious increase in agriculture crop damage.