In situ soil moisture sensing in organic soils: what works?

Soil moisture is a key regulator of greenhouse gas emission and other biogeochemical processes as well as of land management options in peatlands and other organic soils. Water table depth is often used as a proxy for soil moisture in these systems. However, it shows no consistent relationship with volumetric water content (VWC) or water-filled pore space (WFPS).

Non-destructive measurements of soil moisture with a high temporal resolution can be obtained using an electromagnetic sensor to determine the relative dielectric permittivity (ε), which is then converted to VWC through calibration functions. As the relationship between ε and soil moisture is exponential, precise measurements of ε are essential, particularly for peat-specific calibrations where VWC may exceed 80%. While the accurate determination of ε has been extensively studied within the range typical of mineral soils (< 40), sensor performance at high ε values (> 40), which are characteristic of organic soils, has hardly been investigated. Reliable soil moisture sensors are, however, crucial for accurately quantifying the effects of VWC and WFPS on greenhouse gas exchange, as well as for assessing the trafficability of organic soils.

Our investigation aimed to examine the suitability of various commercially available soil moisture probes with five different measurement methods across the entire ε range. To this end, we tested 14 different probes, each with three sensor replicates. The experiment was conducted under laboratory conditions using different reference solutions with defined values between 1 < ε < 80. No soil was used here. In addition, the influence of different electrical conductivities (0 to 800 μS cm^-1) on the measurement accuracy of the sensors was also investigated.

Although the 14 probes operated with different measurement methods, no differences in overall sensor performance could be attributed to this. The results showed that four of the sensors tested measure very reliably and accurately between 40 > ε < 80 and are therefore recommended for use in organic soils. Five further sensors are conditionally usable, and the rest are not suitable for accurately measuring soil moisture in organic soils. Intersensory variability was found to be highest for the latter probes. Additionally, about half of the 14 sensors tested showed increasing uncertainties at elevated electrical conductivities up to 800 μS cm^-1. Soil-specific calibrations are still required to ensure reliable measurements. However, our results offer guidance for evaluating sensors based on their accuracy and performance and to choose suitable sensors for soil-specific calibration.