Bimodal diurnal patterns of belowground carbon exudation and nitrogen uptake and release in beech and spruce suggest source and sink driven controls

Root exudation constitutes a major pathway by which plants exchange carbon and nitrogen with the soil, yet its temporal dynamics remain poorly understood under field conditions. In particular, it is unclear to what extent short-term variations in exudation reflect diurnal carbon assimilation patterns or are driven by transport-related processes within the plant.

We investigated diurnal patterns of root carbon exudation and nitrogen uptake and release in two temperate tree species, European beech (Fagus sylvatica) and Norway spruce (Picea abies), at high-resolution (4-hour) sampling intervals over three full diel cycles. For each species, we studied both mature canopy trees and juvenile understory individuals to assess the dependency of exudation dynamics on light availability and plant internal storage capacity. In addition, we quantified non-structural carbohydrate (NSC) pools in fine roots.

Across species and tree age, root carbon exudation exhibited a pronounced bimodal diurnal pattern, with one peak occurring during midday and a second peak during the evening. These peaks were separated by two distinct minima in the early morning and afternoon. Nitrogen was released during the day with a peak during midday, similar to the time of carbon release. In turn we found that nitrogen uptake by fine roots happened during the night, while carbon exudation was still detectable. Nighttime carbon release and nitrogen uptake was higher in mature than in understory trees.

Our results demonstrate that root exudation in forest trees follows diurnal dynamics that cannot be explained by instantaneous carbon assimilation alone. We propose that transport-related processes and internal carbon storage play an important role in regulating belowground carbon release. (Net) nitrogen uptake occurred exclusively at night, possibly to support nighttime tree growth or regeneration. These diurnal carbon and nitrogen dynamics have important implications for associated soil biogeochemical processes.