Genetic Potential for N₂O Metabolism in Tree Tissues: Insights From Nitrogen Cycling Gene Prevalence and nosZ Diversity Across Tree Species

Nitrous oxide (N2O) is a potent greenhouse gas, and microorganisms play a crucial role in its metabolism. While N2O cycling among soil microorganisms is well studied, there is a major knowledge gap regarding the distribution and diversity of these microorganisms within tree ecosystems. In this study, we aimed to comprehensively assess the potential for nitrogen (N) cycling and the diversity of N2O-reducing microorganisms in shoots (leaves and terminal branches) and wood cores of four tree species — European beech (Fagus sylvatica), European hornbeam (Carpinus betulus), birch (Betula pendula and Betula pubescens) and Norway spruce (Picea abies). We assessed N2O exchange through shoot incubation experiments and measured internal N2O concentrations in stem wood. Inorganic N species were studied as indicators of microbial transformation, and a targeted metagenomic approach was used to determine the relative abundance of N-cycling genes and nosZ clade I and II diversity. Our study revealed that hornbeam shoots showed potential N2O emissions (0.002–0.007 ng N2O g⁻¹ FW h⁻¹), while beech shoots indicated N2O consumption (-0.001 to -0.017 ng N2O g⁻¹ FW h⁻¹). Birch had internal stem wood N2O concentration of + 150.39 ppb, and beech − 9.74 ppb when compared to the ambient concentration. Targeted metagenomic analysis revealed the presence of key nitrification and denitrification genes in both tissue types. In particular, nosZ genes were detected in shoots (0 to 26.48 per 100,000 reads) and in wood cores (0 to 31.95 per 100,000 reads), with clade I dominating over clade II and Rhizobiales prevalent within clade I. Overall, our findings show that internal tree tissues harbour distinct N‑cycling microbial assemblages dominated by nosZ clade I, suggesting that trees may function as localized N2O sinks or sources depending on tissue type and microbial composition.