Tree processes belowground are highly complex and strongly affect the soil carbon (C). Trees assimilate C and allocate up to 20% of the C to the rhizosphere as root exudates. Abiotic factors such as drought strongly affect trees’ physiology and influence root exudation. However, little is known how drought – induced shifts in root exudates will affect long-term soil C storage.
We investigated seasonal dynamics and the effects of intensified drought—simulated through a rain-exclusion treatment – on root exudation rate and composition in Pinus, Pistacia and Quercus in a mixed Mediterranean forest. Two years of monitoring revealed exudation rates increased under seasonal drought across species, and exudate chemistry shifted in species- and season-specific patterns that mirror shallow-, intermediate-, and deep-rooting strategies. Rain-exclusion treatment showed no effect on root exudation.
In a follow-up experiment, we investigated the effect of 13C labelled “drought” and “control” metabolite mixtures – identified in our previous study – on soil C storage. ¹³C-labelled C mixtures revealed that sugars were preferentially integrated into the mineral-associated organic C (MAOM) fraction of the soil, more so than amino acids or organic acids, regardless of soil moisture conditions.
Our results corroborate that exudation amount and composition vary by species and season, and that metabolite identity steers soil C pathways. We highlight the need to consider metabolite-level interactions in forest C cycling models under climate change.
