Relationship of compound-specific root exudation with nitrogen and water status of temperate tree species

The rhizosphere is a hotspot of biological activity, representing a zone of interaction between plants and microbial communities. Root exudates are a key factor shaping this unique environment by significantly influencing belowground processes, such as carbon (C) and nutrient cycling. Despite this importance, main drivers of root exudation are still unknown. In this study, we investigated how the composition of root exudation in temperate forests relates to nitrogen (N) concentrations and δ¹³C isotopic signatures in different tree tissues. Enriched δ¹³C values can hereby serve as an indicator for drought stress.

Root exudates were sampled at four temperate forest sites in Germany in sycamore maple (Acer pseudoplatanus), European beech (Fagus sylvatica) and Norway spruce (Picea abies). We used an in-situ approach, where cleaned roots were incubated in cuvettes with glass beads and a diluted nutrient solution for 24h. Compound-specific root exudation rates from four sampling events in late spring and late summer of two consecutive years (2023 & 2024) were analysed by gas chromatography-mass spectrometry.
Tree tissues were sampled in late summer 2023 and in late spring 2024, including roots, branch bark, branch wood and leaves from the sun-lit tree crowns and analysed by isotope ratio mass spectrometry to determine C and N concentrations and the isotopic signature δ¹³C.

In maple, a higher N status in leaves, bark and wood went along with an elevated exudation of hydrocarbons, including fatty acids and sugars. In contrast, the exudation of N-containing compounds, namely amino acids, was reduced under higher tree N concentrations. Therefore, the exudation of hydrocarbons could be a mechanism to scavenge for N, while the loss of N through exudation is reduced. A reduced water availability indicated by more enriched δ¹³C values led to compound-specific reactions in the exudates of maple. While the exudation of hydrocarbons was reduced under more enriched δ¹³C values in leaves, bark and wood, N-containing compounds were exuded in higher rates, even though not significantly higher. This suggests a targeted exudation of specific compounds under reduced water availability.
In spruce, we observed significant tissue dependent correlations between tree N status and exudation. In contrast to maple, higher tree N concentrations in needles, bark and roots generally went along with reduced exudation. Also contrasting maple, spruce tended to decrease N exudation and increase the exudation of other compounds, when tree tissues were more enriched in ¹³C. This effect was especially strong and significant for roots, indicating an elevated investment into roots and root exudates under drier conditions. 
In beech, no significant correlations between N concentration or δ¹³C and exudation could be observed.

Our results indicate that the interaction of N and water status in tree tissues with root exudation strongly depends on the tree species, which could partially explain contrasting results reported in literature. While differing abiotic conditions are often held responsible for inconsistencies, our results suggest species identity as an important factor.