The potential of active and passive infrared thermography for identifying dynamics of soil moisture and microbial activity at high spatial and temporal resolution

Spatio-temporal analyses of soil properties are important for more profound insights into soil processes. Up to now, non-invasive approaches analyzing physical and biological soil properties and dynamics at the microscale are not available due to methodological, instrumental, and analytical challenges. In this study, we evaluate the use of active and passive infrared thermography (IRT), a non-invasive and non-contact technique, for the detection of surface temperature-based parameters on soil surfaces. The potential and possibilities of IRT were analyzed with a focus on the detection and calibration of soil moisture using active IRT and the determination of microbial activity using passive IRT. A pool of 51 soil samples was used to cover a wide range of chemical, physical, and biological soil properties. The samples were rewetted to 16 different moisture contents, filled into vessels, and placed in an air-proof glove box with an adjusted relative humidity of about 92% to reduce soil drying. Immediately after rewetting, the soil surface temperature was determined using active and passive IRT procedures at a high temporal resolution (1 min for passive IRT, hourly for active IRT) and a spatial resolution of 0.283 mm. Soil material was also sterilized by γ-irradiation in order to obtain sterile samples for validating the passive IRT procedure. Active IRT measurements were qualified for the detection of soil surface moisture due to changing specific heat capacity at varying water contents. The mean volumetric water contents explained up to 88% of active IRT values, which were a good approximation for relative differences in the spatial and temporal distribution of moisture contents. Passive IRT measurements are useful for the detection of microbial activity on soil sample surfaces since temperature increases by up to 0.5 K were detected on the surfaces of all non-sterile samples immediately after rewetting. In sterile samples, rewetting did not result in heat production. With regard to the commonly observed “Birch”-CO2-pulse, these results strongly suggested that the heat evolution on the surface of the non-sterile soils was associated with the rapidly increasing microbial activity from consuming dead and easily available soil biomass. In conclusion, IRT is a promising mapping tool of soil surface processes especially for undisturbed soil samples, since IRT techniques allow studying moisture and microbial activity of intact soil structures.