## 4 Discussion
### FDR Response to Soil Properties
They report negligible effects of decomposition, residual water and temperature on the calibration parameters, but a strong effect of shrinkage of the organic material both influencing \(\theta_{\rm v}\) and TDR reflection times, which may cause errors up to 0.02 m\({}^{3}\) m\({}^{-3}\) if not accounted for. We expect the same effects with FDR instruments. More research of FDR performance in forest floors and organo-mineral soils is definitely needed. It is clear from Table 10 and Figure A1 that standard functions are inadequate for predicting \(\theta_{\rm v}\) in forest floors but also in organo-mineral toposils (Ah layers or M01). Problems start when taking the calibration samples for sensors located in forest floors of variable thickness (5-9 cm in our study) if the steel cylinders are already 5 cm in height. Impossible to avoid sampling of mineral soil material. Similarly, sampling M01 without collecting forest floor material is also tentative. In our study we applied a \(\theta_{\rm b}\) threshold to discriminate real forest floor samples from those contaminated with mineral soil, but this is a surrogate solution. Anyway, it helped us to lower RMSPE from 0.22 to 0.05 m\({}^{3}\) m\({}^{-3}\) which is surprisingly similar to the RMSPE of 0.06 m\({}^{3}\) m\({}^{-3}\) found by [10] for organic layers, using the calibration function of [15]: \(\theta_{\rm FDR}=-0.283+0.0182 PA+0.0002 PA^{2}\). [PERSON] et al. [10] concluded that for the organic samples, FDR sensor outputs are in general lower than for mineral soils. This is expected due to the low \(\theta_{\rm b}\), and high porosity of organic materials. This statement is confirmed by the increasing slope in Figure 8b compared to all samples (including mineral). Accurate sensor readings depend on the absence of air gaps between the CS616 rods and soil [13]. Their study showed that 50% of the sensed volume is concentrated in the first 6 mm around and between the rods. So, hence heterogeneity of materials or air gaps should be avoided. However, since forest soils are full of life, bioturbation may cause tunnels or gaps around sensor rods affecting the readings. In addition, cracking or shrinking processes in clayey soils, such as in GON, might have disturbed sensor readings. Campbell Ltd. [20] indicates that soils exhibiting bulk electrical conductivity (BEC) > 500 \(\mu\)S cm\({}^{-1}\) will require a specific calibration. All soils in this study have an EC value well below 500\(\mu\)S/cm (Table 1), so this was no issue in our study. Metallic soil components such as ironstone, or sesquioxide rich Bs horizons may affect FDR sensor readings [11] but we found no studies confirming that. Similarly, large knowledge gaps exist for possible effects of soil biota (roots, mycorrhiza) affecting FDR measurements, since these are transporting water. What if roots align with or cross the rods of the CS616, which makes sense if these sensors are installed for decades. [PERSON] et al. [35] studied plant root growth effects in horticultural substrates. They found that various root size indicators (root fresh weight, root dry weight, and root water content) all had a negative effect on the estimation of \(\theta_{\rm v}\) by FDR sensors and that this effect should be considered in research applications. Several authors recommended considering soil temperature in the calibration process of the CS616, since they found that diurnal fluctuations in soil temperature influenced the sensor readings [3; 4]. [PERSON] et al. [4] suggested making sensor readings during periods that the soil temperature is similar (for example, every early in the morning). Our calibration dataset was compiled with observations just before or after noon, so a possible diurnal effect could not be observed in our study. However, the recalibrated functions are applied to diurnal sensor outputs and could therefore induce extra prediction error for data at night. Presumably this possible error will be greater in the toposoil than in the subsoil due to temperature damping with increasing depth.