Enhanced root exudation of broadleaf and conifer trees in the Mediterranean forest during the dry season

Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. The flux of C is believed to have profound effects on C and nutrient cycling in ecosystems. Since root exudates of mature trees are difficult to collect in field conditions, very little is known about their flux in water-limited ecosystems, like the Mediterranean maquis.

We qutify the flux of root exudates throughout the year, and its relation to natural environmental factors. We collected root exudates in-situ for two mature Mediterranean woody species: Cupressus sempervirens and Pistacia lentiscus, by 48-hour incubations on a monthly resolution. In parallel, we measured leaf assimilation and respiration in these trees together with abiotic parameters.

Root exudation rates varied significantly along the year, from 6 µg carbon cm^-2 root day^-1 in both species in the wet season, to 4-fold and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. Step-wise linear mixed-effects model showed that soil temperature, soil water content and potential were the most influential parameters on exudation rates. There was a significant negative correlation of exudation rate with leaf assimilation and respiration.

The enhanced exudation flux during the dry season showed an opposite trend from the times of peak tree carbon uptake and respiration, occurring in the wet season. This suggests that exudation dynamics in the field are less sensitive to leaf assimilation capacity. Instead, they might respond to seasonal changes in the rhizosphere, and particularly water and nutrient shortages.