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Summary: Recent droughts have strongly impacted forest ecosystems and are projected to increase in frequency, intensity, and duration in the future together with continued warming. While evidence suggests that tree diversity can regulate drought impacts in natural forests, few studies examine whether mixed tree plantations are more resistant to the impacts of severe droughts.Using natural variations in leaf carbon (C) and nitrogen (N) isotopic ratios, that is δ13C and δ15N, as proxies for drought response, we analyzed the effects of tree species richness on the functional responses of tree plantations to the pan‐European 2018 summer drought in seven European tree diversity experiments.We found that leaf δ13C decreased with increasing tree species richness, indicating less drought stress. This effect was not related to drought intensity, nor desiccation tolerance of the tree species. Leaf δ15N increased with drought intensity, indicating a shift toward more open N cycling as water availability diminishes. Additionally, drought intensity was observed to alter the influence of tree species richness on leaf δ15N from weakly negative under low drought intensity to weakly positive under high drought intensity.Overall, our findings suggest that dual leaf isotope analysis helps understand the interaction between drought, nutrients, and species richness. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) is essential for nutrient cycling and soil carbon (C) accumulation, both of which are heavily influenced by the quality and quantity of plant litter. Since SOM dynamics in relation to plant diversity are poorly understood, we investigated the effects of willow variety and mixture, and site on the soil C stocks, SOM chemical composition and thermal stability. Using pyrolysis-field ionization mass spectrometry (Py-FIMS), a method of stepwise thermal degradation in ultrahigh vacuum combined with soft ionization in a high electric field, followed by mass-spectrometric separation and detection of molecular ions, we analyzed SOM in the top 10 cm of soil from two 7-year-old experimental sites in Germany and Sweden. Monocultures and mixtures of two willow varieties (Salix spp.) belonging to different species were grown at the experimental plots. Overall, site had the strongest effect on SOM quality. The results showed significant variability across sites for willow identity and mixture effects on C accumulation and SOM chemistry. In the German site (Rostock), yearly soil C accumulation was higher (p < 0.05) for variety 'Loden' (1.0 Mg C ha−1 year−1) compared to 'Tora' (0.5 Mg C ha−1 year−1), whilst in the Swedish site (Uppsala), both varieties exhibited similar soil C accumulation rates of around 0.6 Mg C ha−1 year−1. Willow variety identity significantly affected SOM quality at both sites, while mixing had minor effects. Our findings emphasize the significance of site-specific context and variety and species identity in shaping soil C accumulation in willow plantations. [ABSTRACT FROM AUTHOR]

Phosphorus is one of the most important nutrients required for plant growth and development. However, owing to its low availability in the soil, phosphorus is also one of the most difficult elements for plants to acquire. Phosphorus released into the soil from bedrock quickly becomes unavailable to plants, forming poorly soluble complexes. Phosphate-solubilizing bacteria (PSB) can solubilize unavailable phosphorus-containing compounds into forms in which phosphorus is readily available, thus promoting plant growth. In this study, two willow species, Salix dasyclados cv. Loden and Salix schwerinii × Salix viminalis cv. Tora, were inoculated with two selected bacterial strains, Pantoea agglomerans and Paenibacillus spp., to evaluate the plant growth parameters and changes in gene expression in the presence of different concentrations of tricalcium phosphate: 0mM(NP), 1mM(LP), and 2mM(HP). Inoculation with PSB increased root, shoot and leaf biomass, and for the HP treatment, significant changes in growth patterns were observed. However, the growth responses to plant treatments tested depended on the willow species. Analysis of the leaf transcriptomes of the phosphate-solubilizing bacterium-inoculated plants showed a large variation in gene expression between the two willow species. For the Tora willow species, upregulation of genes was observed, particularly for those involved in pathways related to photosynthesis, and this effect was strongly influenced by bacterial phosphate solubilization. The Loden willow species was characterized by a general downregulation of genes involved in pathway activity that included ion transport, transcription regulation and chromosomes. The results obtained in this study provide an improved understanding of the dynamics of Salix growth and gene expression under the influence of PSB, contributing to an increase in yield and phosphorus-use efficiency. [ABSTRACT FROM AUTHOR]