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

Brunn, Melanie and Hafner, Benjamin D. and Zwetsloot, Marie J. and Weikl, Fabian and Pritsch, Karin and Hikino, Kyohsuke and Ruehr, Nadine K. and Sayer, Emma and Bauerle, Taryn L. (2022) Carbon allocation to root exudates is maintained in mature temperate tree species under drought. New Phytologist, 235 (3). pp. 965-977. ISSN 0028-646X

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Abstract

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, i.e. 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 ~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 threefold. Water-limited P. abies released two-thirds of its exudate-C into the surface soil, whereas it was only one-third in droughted F. sylvatica. 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.