Plants associating with mutualistic ectomycorrhizal (ECM) fungi may directly obtain nitrogen (N) bound in soil organic matter (N-SOM). However, the contribution of N-SOM to plant growth under field conditions remains poorly constrained. We tested the hypothesis that turnover in ECM communities along soil inorganic N gradients mediates a functional transition from plant reliance on N-SOM in low inorganic N soils, to primarily inorganic N uptake in inorganic N rich condition soils. We quantified the d¹⁵N of Q. rubra foliage and roots, organic and inorganic soil N pools, and used molecular sequencing to characterize ECM communities, morpho-traits associated with N-foraging, and a community aggregated sporocarp d¹⁵N. In support of our hypothesis, we document the progressive enrichment of root and foliar d¹⁵N with increasing soil inorganic N supply; green leaves ranged from -5.95‰ to 0.16‰ as the supply of inorganic N increased. ECM communities inhabiting low inorganic N soils were dominated by the genus Cortinarius, and other fungi forming hyphal morphologies putatively involved in N-SOM acquisition; sporocarp estimates from these communities were enriched (+4‰), further supporting fungal N-SOM acquisition. In contrast, trees occurring in high inorganic N soils hosted distinct communities with morpho-traits associated with inorganic N acquisition and depleted sporocarps (+0.5‰). Together, our results are consistent with apparent tradeoffs in the foraging cost and contribution of N-SOM to plant growth, and demonstrate linkages between ECM community composition, fungal N-foraging potential and foliar d¹⁵N. The functional characteristics of ECM communities represents a mechanistic basis for flexibility in plant nutrient foraging strategies. We conclude that the contribution of N-SOM to plant growth is likely contingent on ECM community composition and local soil nutrient availability.

This represents raw isotopic values for Quercus rubra foliage, roots, soil, and inorganic N pools. Details regarding processing and instrumentation are fully described in the main text.

Three files are uploaded. 1). Soil_Foliage contains N15 isotope values for Quercus rubra green leaves, Oi horizon leaves, and bulk SOM underneath each individual tree. N concentrations ug/mg are also presented for each measured component. 2). Diffusions file contains N15 isotope values for Ammonium, Nitrate, and Dissolved orgnaic nitrogen, as described and processed in the main text. Total N yield is also presented, and recommendations suggest that samples with less than 20ug total nitrogen may represent poor quality measures 3). Roots contains three replicate measures of N15 at the 'stand' level because individual tree sampling was not possible. The coding as designated, is Stand-Tree, whereas for the Roots dataset, only Stand is presented with replicate number listed. A total of 12 stands were studied, with 5 individual trees in each stand.