Depleted 15N in hydrolysable-N of arctic soils and its implication for mycorrhizal fungi-plant interaction

Uptake of nitrogen (N) via root-mycorrhizal associations accounts for a significant portion of total N supply to many vascular plants. Using stable isotope ratios (delta N-15) and the mass balance among N pools of plants, fungal tissues, and soils, a number of efforts have been made in recent years to quantify the flux of N from mycorrhizal fungi to host plants. Current estimates of this flux for arctic tundra ecosystems rely on the untested assumption that the delta N-15 of labile organic N taken up by the fungi is approximately the same as the delta N-15 of bulk soil. We report here hydrolysable amino acids are more depleted in N-15 relative to hydrolysable ammonium and amino sugars in arctic tundra soils near Toolik Lake, Alaska, USA. We demonstrate, using a case study, that recognizing the depletion in N-15 for hydrolysable amino acids (delta N-15 = -5.6aEuro degrees on average) would alter recent estimates of N flux between mycorrhizal fungi and host plants in an arctic tundra ecosystem.