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 (delta N-15) appear as a contentious choice for this role as the exact mechanisms behind tree-ring delta N-15 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 delta N-15 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 delta N-15 signals. Our results reveal that airborne anthropogenic N with distinct delta N-15 signals may directly modify the NO3- delta N-15 values in surface soils, but not the ones of NH4+ , the preferred N form of the studied trees. Hence, the tree-ring delta N-15 values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring delta N-15 range, we report differences in: soil nutrient content and N transformation rates; delta N-15 values of NH4+ , total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root delta N-15 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 & nbsp;soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring delta N-15 averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring delta N-15 values as a suitable indicator for environmental research on forest N cycling. Crown Copyright (C) 2021 Published by Elsevier B.V.

Automated summary

This study analyzed tree ring nitrogen isotopic ratios of thirteen white spruce trees in two regions exposed to moderate anthropogenic nitrogen emissions. The results showed that tree-ring nitrogen isotopic ratios reflect specific soil nitrogen conditions and assimilation modes by trees.