Carbon allocation to root exudates is maintained in mature temperate tree species under drought

• Published in: The New phytologist Vol. 235; no. 3; pp. 965 - 977
• Main Authors: Brunn, Melanie, Hafner, Benjamin D., Zwetsloot, Marie J., Weikl, Fabian, Pritsch, Karin, Hikino, Kyohsuke, Ruehr, Nadine K., Sayer, Emma J., Bauerle, Taryn L.
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.08.2022
• Subjects: Assimilation; belowground carbon allocation; Carbon; carbon partitioning; Drought; Ecosystem resilience; experimental drought; Exudates; Exudation; Fagus sylvatica (European beech); fine‐root exudation; Moisture effects; Picea abies (Norway spruce); Plant species; Rhizosphere; Roots; Soil; Soil moisture; Soil surfaces; temperate forest C budget; Trees; Wilting; Wilting point; Picea abies (Norway spruce); carbon partitioning; Fagus sylvatica (European beech); belowground carbon allocation; temperate forest C budget; experimental drought; fine-root exudation; rhizosphere
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.18157

Summary Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought‐stressed tree, that is, how much of net‐C assimilation is allocated to exudation at the tree level. We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above‐ and belowground C fluxes with leaf, stem and fine‐root surface area. Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine‐root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two‐ to three‐fold. Water‐limited P. abies released two‐thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one‐third. Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought‐imposed belowground C investment, which could be beneficial for ecosystem resilience.