Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Their flexible root growth provides plants with a strong ability to adapt and develop resilience to droughts and climate change. But this adaptability is badly included in crop- and climate models. Most of them rely on a simplified representation of root growth, independent of soil moisture availability. To model plant development in changing environments, we need to include the survival strategies of plants, but data of subsurface processes and interactions, needed for model set-up and validation, are scarce.

Here we investigated soil moisture driven root growth. To this end we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (length x width x height, 45 x 7.5 x 45cm). The development of the vertical soil moisture and root growth profiles are tracked with a high spatial and temporal resolution.

The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments also suggest that vertical root growth stops when the soil moisture at the root tip drops below a threshold value. We show that even a parsimonious one-dimensional water balance model, driven by the measured water input and output fluxes, can be convincingly improved by implementing root growth driven by soil moisture availability. The model performance suggests that soil moisture is a key parameter determining root growth.