Nitrogen isotopes in the soil-to-tree continuum - Tree rings express the soil biogeochemistry of boreal forests exposed to moderate airborne emissions.

Anthropogenic N emissions represent a potential threat for forest ecosystems, and environmental indicators that provide insight into the changing forest N cycle are needed. Tree ring N isotopic ratios (δN) appear as a contentious choice for this role as the exact mechanisms behind tree-ring δN changes seldom benefit from a scrutiny of the soil-to-tree N continuum. This study integrates the results from the analysis of soil chemistry, soil microbiome genomics, and δN values of soil N compounds, roots, ectomycorrhizal (EcM) fungi and recent tree rings of thirteen white spruce trees sampled in five stands, from two regions exposed to moderate anthropogenic N emissions (3.9 to 8.1 kg/ha/y) with distinctive δN signals. Our results reveal that airborne anthropogenic N with distinct δN signals may directly modify the NO δN values in surface soils, but not the ones of NH, the preferred N form of the studied trees. Hence, the tree-ring δN values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring δN range, we report differences in: soil nutrient content and N transformation rates; δN values of NH, total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root δN values with fungal identification to infer that hydrophobic EcM fungi transfer N from the dissolved organic N (DON) pool to roots under acidic conditions, and hydrophilic EcM fungi transfer various N forms to roots, which also assimilate N directly under less acidic conditions. Despite the complexities of soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring δN averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring δN values as a suitable indicator for environmental research on forest N cycling.