Day-night root dynamics change through the growing season

Roots are difficult to measure at high temporal resolution but are important as belowground sinks and sources of CO₂. Even in an age of automated and remote estimation of many ecosystem properties, belowground plant biomass is opaque at all but the coarsest timescales. Further, root dynamics are not fully predictable from aerial biomass change. Many uncertainties in predicting whole ecosystem function derive from this lack of data belowground.

Using automated minirhizotrons, we captured images of root dynamics four times a day from February to October in a permanent temperate grassland in Germany. We processed all images collected using a trained neural network model and image analysis scripts to extract morphological traits from segmented images. We found root growth occurring continuously, even at sub-zero aboveground air temperatures. As a whole population, most of the root growth was in spring, although turnover and replacement happened at all times. Root length density and extractable root surface area increased through spring but decreased through a dry summer into autumn. Mean rooting depth increased until summer but did not decrease during the study period. Mean root diameter only increased once the dry period began.

We also examined the patterns of growth between day and night. Early in the year roots grew in both day and night time periods, but after initial rapid biomass growth, root growth was at night. We examine the  reasons for this switch relating to source and sink control of plant growth. We also consider implications for an accurate partitioning of carbon budgets in terrestrial ecosystems.