Fine root exudation rate increases in drier soils, but tree level carbon exudation does not change under drought in mature Fagus sylvatica - Picea abies trees

Drought is a severe natural risk that increases drying-rewetting frequencies of soils. Yet, it remains largely unknown how forest ecosystems respond to dry-wet cycles, hampering our ability to evaluate the overall sink and source functionality for this large carbon pool. Recent investigations suggest that the release of soluble carbon via root exudation increases under drought, influencing soil carbon stabilization and mineralization. However, an integration of root exudation into the carbon allocation dynamics of drought stressed trees is missing. We hypothesized that roots in dry soil layers have a higher exudation rate than roots in more moist layers across different soil depths. Further, we tested if higher exudation rates under drought are attenuated by reduced root abundance in dry soils and if the fraction of root exudation from total carbon allocation increases with decreasing photosynthesis rates under drought. At the KROOF experimental site in southern Germany, where mature beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.) trees were exposed to artificial drought stress for five consecutive growing seasons, we show that at the root level root exudation rate increases in drier soils. Especially roots in the upper soil profile and roots of spruce trees increased root exudation under drought. When scaled to whole tree level, we did not find differences in total exudation between drought stressed and control trees, indicating sustained root exudation at the tree level under drought. As photosynthesis rates and therefore total carbon assimilation was substantially reduced under drought (by 50 % in beech and almost 70 % in spruce), the fraction of root exudation from total assimilation slightly increased for drought stressed trees. Our results demonstrate that stimulation of root exudation rates with drought exists in natural temperate forest ecosystems but might be mitigated by reduced fine root abundance under drought. Nevertheless, increased exudation per root surface area will have localized impacts on rhizosphere microbial composition and activity especially in the topsoil exposed to more extreme dry-wet cycles. Finally, also the exudate composition can help to determine how priming of soil organic matter relates to belowground carbon allocation dynamics and to disclose processes of complementary species interaction and should be emphasised in future studies.