Mycorrhizal fungi provide nitrogen to N-limited plants: Effect of N deposition

When N is strongly limiting to shrubs and trees, certain ectomycorrhizal and ericoid mycorrhizal fungi mine the soil for nitrogen and provide significant amounts to their host plants. When this function is active, these fungi fractionate against ¹⁵N during synthesis of amino acids for transfer to plants. The ¹⁵N-depleted amino acid is transferred to the plant while ¹⁵N-enriched compounds, such as protein and chitin, remain in fungal hyphae and fruiting bodies. Therefore, plant foliage depleted in ¹⁵N and fungal fruiting bodies enriched in ¹⁵N relative to soil N are markers of this particular fungal process. There is a carbon cost to the plant, however, as 15-20% of NPP goes to the fungi. These ¹⁵N signals are absent when N deposition is high (e.g., Germany) and plants do not need the additional N, suggesting diminished carbon flux to fungal symbionts and also suggesting that this transfer process is unimportant under such conditions. In contrast, when N deposition is very low (e.g., Arctic, boreal forest, Northwest Coast of U.S.), 70-90% of the plant’s N enters through the mycorrhizal fungi. In regions of moderate N deposition such as central Europe and the eastern U.S, ¹⁵N signals are present in some ectomycorrhizal fungi and the percent of plant N entering via mycorrhizal fungi is reduced. ¹⁵N analyses on different ectomycorrhizal morphotypes on the roots of a single tree revealed that some morphotypes continue to carry out this soil mining under moderate N deposition.