Interactions between saprotrophic microbes and ectomycorrhizal fungi in the nitrogen nutrition of red pine (Pinus resinosa Ait.)

Nitrogen is a major limiting resource for plant growth in many temperate forests. Plants usually obtain ammonium and nitrate as their N source. However, some ectomycorrhizal fungi have been reported to obtain N from protein under laboratory conditions. Thus, researchers have proposed that ectomycorrhizal fungi significantly affect the N economy of their hosts by short-circuiting the N mineralization pathway. Although protein exists in high concentrations in the soils of many temperate forests, most of it is bound into polyphenolic complexes. Therefore, whether temperate forest trees have access to organic N via ectomycorrhizal fungi is not clear. The objective of this study is to understand the interaction between ectomycorrhizal fungi and microbes responsible for mineralization with respect to the N economy of red pine.; By applying N fertilizer, we determined that N limits red pine growth in the forest. Red pine seedlings were incapable of directly absorbing organic N under sterile conditions. Six of six species of the ectomycorrhizal fungi could obtain N from free protein, but not from the polyphenol-protein complex. Five of eight isolates of saprotrophic fungi obtained N from free protein as well as from the polyphenol-protein complex. Nine of nine isolates of saprotrophic bacteria could not obtain N from the polyphenol-protein complex. After saprotrophic microbes pre-treated the polyphenol-protein complex, ectomycorrhizal fungi could benefit from the N mobilized and/or mineralized from the complex. Therefore, it would appear that mobilization and/or mineralization by saprotrophic fungi is an important step for most ectomycorrhizal fungi, which gives them access to otherwise inaccessible organic N.; Laboratory and field in situ incubation further indicated that ectomycorrhizas proliferate at the forest floor where net N ammonification is high. Field litter and F-layer mesh bag incubation experiment showed a negative interaction between ectomycorrhizas and decomposition, confirming the “Gadgil effect”. However, the ectomycorrhizas compete with saprotrophs for water could be the explanation for the suppressive effects on decomposer activity. Litter and F-layer mesh bags were perceived differently by ectomycorrhizal morphotypes, indicating the partitioning of the forest floor by ectomycorrhizas.