Root distribution shifts at both seasonal and daily scales following precipitation events in a temperate grassland

Roots are fundamental plant organs mediating water and nutrient uptake, among other functions. The amount of soil water available to roots fluctuates over time. With increasing climatic variability and extended periods of drought, it is important to understand how roots respond to fluctuations in available soil water. Furthermore, soil-vegetation-atmosphere transfer models need a precise characterization of the spatial and temporal organization of root systems for more accurate predictions of water fluxes mediated by vegetation.

It has been shown that root systems dynamically adapt to seasonal changes in soil moisture by shifting their growth allocation from the upper soil to deeper depths as a dry period progresses. In previous work we explored the phenomenon of “Hydromatching” in young individual maize plants, which involves the daily-timescale promotion of root growth in a newly wetted soil layer accompanied by a decline in root growth in drier layers. Here we report results from a 1.5-year-long field study in Luxembourg, where we investigated if the results of Hydromatching can also be observed at a community scale in a temperate grassland.

Near a well-instrumented weather station, we installed 12 minirhizotrons enabling us to obtain images of roots growing down to a soil depth of 115 cm. We imaged the tubes every two weeks, with increased sampling frequency shortly after major precipitation events during the growing season. We calculated local root growth rates at different depths and related them to local soil moisture and temperature variations measured by four sensors located at depths of 10, 20, 40 and 60 cm.

We found that, even under strong variations in temperature, soil moisture remained a more important predictor of root growth at 10, 20 and 40 cm depth, despite the site being more energy than water-limited. Following rain events, root growth distribution shifted from the deeper soil to the shallow soil within 1-5 days, demonstrating the potential effect of Hydromatching at community scale. Following a renewed dryness, root allocation shifted again to the deeper soil within 7-8 days from the rain event, showing a remarkably dynamic nature of the root systems in the grassland. The 2023 spring-summer transition saw a much larger change in soil moisture compared to the 2022 transition. Nonetheless, during the seasonal change both years exhibited a significant and similar growth promotion in the deeper soil coupled with a decline in root length at shallower depths. These results suggest that daily root distribution shifts following rewetting events are likely regulated by environmental variables while seasonal shifts seem to be dictated by phenological factors. Regardless, both daily and seasonal shifts appear to reflect an optimization strategy, consisting of the promotion of root growth in moist areas while discarding roots where moisture is less accessible. Such strategy might have evolved to cope with soil water heterogeneity while efficiently managing carbon budgeting.