Carbon sink strength differences are examined here between ectomycorrhizal fungi in interaction with additions of ammonium and apatite (a phosphorus- and calcium-containing mineral). Pinus muricata associated with Paxillus involutus and four suilloid isolates (Suillus pungens and members of three Rhizopogon section Amylopogon species groups) were used in microcosm nutrient addition experiments. The associations differed in ectomycorrhizal biomass, mycelial growth rate, biomass and respiration. P. involutus produced the lowest biomass of ectomycorrhizal connections to P. muricata, but it consumed proportionally more carbon per connection and transferred more than twice as much ammonium to the host per unit mycorrhizal biomass. Paxillus also colonized the soil more rapidly and intensely than the other fungi, but its mycelial respiration was lowest. Ammonium and apatite addition resulted in a marked increase in respiration and mycelial biomass, respectively, by the suilloid fungi. The high carbon cost of ammonium uptake is suggested as one explanation for reduced sporocarp production and mycelial growth by ectomycorrhizal fungi commonly found after high levels of nitrogen addition.