Diurnal and Seasonal Dynamics of Root Exudation in Mediterranean Forest Tree Species

Roots secrete large amounts of carbon (C) into the rhizosphere, which mediate multipartite interactions in it. Yet, little is known about root exudation flux under natural settings in the forest, especially in water-limited climates. In this project we characterized the C flux of root exudates and the factors affecting their seasonal dynamics in key Mediterranean forest trees species. Root exudates of mature trees were collected in the field on a monthly temporal resolution, and the amount of total organic carbon (TOC) was determined. In parallel, we studied short-term exudation dynamics by ¹³CO₂ labelling on seedlings of the same species (Cupressus sempervirens, Pinus halepensis, Pistacia lentiscus, Quercus calliprinos, and Ceratonia siliqua). Using a novel methodology, ¹³C/¹²C ratios were measured diurnally and simultaneously in root biomass (solid C), exudation (dissolved C in liquid), and root respiration (CO₂ gas). In the forest, exudate TOC ranged from 0.5 to 66.8 µg C cm^-2 root day^-1, varying among species and increasing in the dry season. In the seedlings, ¹³C tracing showed C allocation into root respiration and exudation within 4 and 5 days, respectively. C allocation into root processes was faster and stronger in conifers vs. broadleaf tree species. Our results indicate important species and season variations in root exudation dynamics in forest trees. It is possible that the higher exudation rates in the dry season are part of a microbial priming strategy under water-related nutrient limitation. Further investigation is required to understand the observed differences in root process allocation between conifers and broadleaves.