From root to leaf: Multi-sensor monitoring of the soil-plant-atmosphere continuum

The soil-plant-atmosphere continuum (SPAC) is the interconnected water pathway between soil, plants, and atmosphere, and plays a pivotal role in distribution of water and nutrients in terrestrial ecosystems. In order to understand and predict the dynamics between its components, especially in the context of advancing climate change, it is essential to investigate both the above- and below-ground part of the SPAC with high temporal resolution. However, while methods to observe the above-ground part of the plant are frequently employed, due to its inaccessibility, in-situ measurements of root system activity are still scarce.

In this study, we employed a novel combination of sensors at the plot scale to obtain a more complete picture of the dynamics between root water uptake, plant photosynthesis and transpiration, and atmospheric conditions. During the growth season of 2023, we studied the rhizosphere beneath maize plots using spectral electrical impedance tomography, a method which has been shown to be sensitive to soil water content dynamics and root structure and activity. Water transport through the plant stem was monitored via sap flow sensors, while photosynthetic activity and atmospheric conditions were measured continuously using a sun-induced fluorescence sensor and a weather station, respectively. Time series data were analyzed across multiple time windows, focusing on environmental events such as precipitation, prolonged dry periods, and variations in cloud cover.

Our results demonstrate we achieved consistently high-quality electrical impedance data throughout the monitoring period. The electrical imaging results exhibit spatially and temporally well resolved diurnal variations in the subsurface polarization behaviour, suggesting a sensitivity to root ion uptake processes. In particular, variability in polarization signatures was more pronounced near the surface early in the season, and shifted to deeper layers later in the season. We attribute this behaviour to the seasonal shift in water availability towards deeper layers, causing a deeper active root water uptake zone. Additionally, rain events promote polarization variability in shallow soil layers. Above-ground data showed cyclical variations both for sap flow and fluorescence measurements and revealed a clear connection to meteorological conditions such as cloud cover or precipitation, confirming the coupling of above-ground plant activity to the atmosphere. Together, the below- and above-ground observations provide a holistic view of the processes within the SPAC, and allow analysis of the complex relations between transpiration, photosynthesis, and root water uptake. To conclude, this study contributes to a deeper understanding of water uptake and plant activity dynamics in crop systems and may inform the breeding of adapted plant varieties, the optimization of agricultural management practices, and the calibration of physiological models describing the SPAC.