On the significance of long-term trends in tree-ring N isotopes - The interplay of soil conditions and regional NOx emissions.

As anthropogenic nitrogen (N) emissions have been rising for decades, it is critical to develop natural archives that help understand how natural processes were modified in the past. Tree-ring δ15N values may represent such an indicator but its validity as faithful record of N cycling changes is still debated. Here we produce long-tree-ring δ15N series for five white spruce stands from two boreal regions submitted to moderate industrial N inputs. The obtained δ15N series show sharp differences among stands, even from the same region, despite the fact that they show similar increases in intrinsic water use efficiency (iWUE), a proxy for foliar strategies derived from δ13C values. The statistical modeling of these series and the basal area increment (BAI) of the trees allow to suggest that the mechanisms controlling the isotopic fractionation of N assimilated by tree rings are decoupled from the foliar strategies under the anthropogenic N emissions. The iWUE trends mainly reflect rise of pCO2 and changes in air quality. The long-term δ15N trends echo different biogeochemical processes responding to N deposition due to distinct original soil pH at the various sites. We contend that tree-ring δ15N series can record changes in the forest N cycle, but their rigorous interpretation requires laborious work, particularly an understanding of the biogeochemistry in the soil immediately around the investigated trees. "Seek simplicity and distrust it", Alfred North Whitehead.

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 (δ15N) appear as a contentious choice for this role as the exact mechanisms behind tree-ring δ15N 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 δ15N 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 δ15N signals. Our results reveal that airborne anthropogenic N with distinct δ15N signals may directly modify the NO3- δ15N values in surface soils, but not the ones of NH4+, the preferred N form of the studied trees. Hence, the tree-ring δ15N values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring δ15N range, we report differences in: soil nutrient content and N transformation rates; δ15N 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 δ15N 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 δ15N averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring δ15N values as a suitable indicator for environmental research on forest N cycling.