Water availability controls seasonal shifts in root growth timing

Root growth dynamics are difficult to observe both on phenological and sub-daily scales as manual destructive measurement is high effort and prone to error. Close synchrony and prescriptive links with more easily observed above ground dynamics on seasonal timescales are often assumed, affecting interpretation of greenhouse gas fluxes without a solid basis in observational whole system data. Increasingly, we now recognize that seasonal root growth can be desynchronized from leaves, causing a rethink of these relationships. However sub-daily patterns are still opaque because measuring field root dynamics remains extremely difficult, especially this frequently. This is even more true over sustained, seasonal timescales where controls and dynamics may shift. Potential drivers of diel growth include photosynthesis (carbon), cell turgor (water), environmental temperature, and intrinsic circadian rhythms. Controls may differ through time, and between organs, and are difficult to separate under natural conditions in observational studies.

We use automated minirhizotrons and neural networks for image interpretation to bypass many previous observational constraints and gather resampled root dynamics data at up to sub-daily resolution. We observing roots four times a day for two years, every day, in a temperate grassland in Germany. We observed a strong underlying cell turgor signal in these uniquely frequent observations, visible through diel oscillation of root width. Removing this signal, we found root growth generally had little diel pattern except in periods of leaf-level water stress. Here roots consistently grew during the day and not at night. We examine the reasons for this switch in diel dynamics through the lens of potential environmental, water and carbon control. We found little evidence for direct temperature limits in our system. Instantaneous C supply, which should increase as canopies develop through the season, also did not appear to impact rate of growth despite previous isotope tracer studies showing a tight temporal coupling between carbon assimilation and bulk soil CO₂ efflux. Our observations point towards water and cell turgor as the main control on root growth timing variation in contrast to the carbon-centric view of plant-soil system functioning indicated by pulse chase experiments. Underlying growth dynamics and their controls should be considered when interpreting whole system fluxes, and their sensitivity to environmental conditions in our dynamic and changing world.