Exploring interactions between saprotrophic microbes and ectomycorrhizal fungi using a protein–tannin complex as an N source by red pine ( Pinus resinosa )

Summary•  Recent studies suggest that some plants may circumvent N mineralization carried out by saprotrophs because their ectomycorrhizal fungi have the capacity to hydrolyse protein. When complexed by tannins, however, proteins may be unavailable to some ectomycorrhizal fungi. •  Here we tested the hypothesis that when protein–tannin complex is the N source, Pisolithus tinctorius will promote N uptake into red pine ( Pinus resinosa ) only in the presence of saprotrophs. •  The model protein–tannin complex was stable at field pH. P. tinctorius could not obtain N from it, but saprotrophs could. Pre‐treatment of the complex by saprotrophs did make its N available to ectomycorrhizal fungi. However, when the protein–tannin complex was the major N source, P. tinctorius increased shoot P but not N content, even in the presence of saprotrophs. •  Interactions between saprotrophs and ectomycorrhizal fungi may be different for N and P because of immobilization of N by ectomycorrhizal fungi, or by the more rapid diffusion of ammonium than phosphate, rendering the absorptive surface area of ectomycorrhizal fungi superfluous for uptake of N but not for P.