Site, season and soil depth affect the composition of root exudates in three temperate tree species

Root exudation is a key process for plants to acquire nutrients. This process works directly, or indirectly through the microbiome priming effect. Likewise, plants release a significant amount of carbon into the soil, which stresses the importance of root exudation for carbon cycling. Nonetheless, detailed data on root exudation, especially compound-specific data from forest trees are scarce, but urgently needed. Recent studies suggest a high importance of the forest floor for nutrient acquisition, which is therefore of special interest for studying root exudates.

In this study, we sampled root exudates of Fagus sylvatica, Picea abies and Acer pseudoplatanus at four temperate forest sites with varying mean annual air temperature, annual precipitation sums and soil phosphorus (P) levels. Samplings took place in spring and autumn and in two soil depths: the forest floor (surface layer of the forest soil with ≥ 15% organic carbon) and the upper mineral soil (A5 horizon). Root exudates were collected using an in-situ cuvette-based approach. For this purpose, living tree roots were cleaned, and after a period of recovery, incubated for 24 hours in a cuvette filled with glass beads and a nutrient solution. Compounds in the retrieved solution were analysed by a mass spectrometer coupled to a gas chromatograph.

81 compounds were included in the analysis and divided into functional groups. All studied species showed higher exudation in spring compared to autumn with a higher share of amines in spring. Differences in exudation patterns between species could be detected for the two soil depths: While F. sylvatica showed a higher exudation in the forest floor, P. abies and A. pseudoplatanus exuded more in the mineral soil. This pattern is expected for F. sylvatica and A. pseudoplatanus, since F. sylvatica, which usually is associated with ectomycorrhiza (ECM), is said to follow an organic nutrient strategy and A. pseudoplatanus, which usually is associated with arbuscular mycorrhiza (AM), is said to follow an inorganic nutrient strategy. However, the behaviour of P. abies usually being associated with ECM and therefore following an organic nutrient strategy is in contrast to what literature suggests. A site-species-interaction effect was found with increased exudation on P-poor sites with low temperature for F. sylvatica, and with high temperature for P. abies and A. pseudoplatanus.

Facing a thinning of the forest floor with globally increasing temperatures, studying root exudation can indicate the forest floor’s role for tree nutrition. Clear differences in root exudation in quantity and composition between species, seasons and soil depths urge the need for further research to elucidate the effect of site conditions on exudation patterns.