Nutrient economy of red pine is affected by interactions between Pisolithus tinctorius and other forest-floor microbes

The influence of interactions between the mycorrhizal fungus, Pisolithus tinctorius, and saprotrophic organisms on nutrient transfer to host red pine (Pinus resinosa) seedlings is presented here. Red pine seedlings were grown axenically, and with P. tinctorius and forest-floor microbes (both individually and in combination), in two experiments varying in nitrogen availability. Root and shoot growth, as well as tissue nitrogen and phosphorus content, were analyzed after harvesting. At low nitrogen availability forest-floor microbes, but not P. tinctorius, significantly reduced seedling nitrogen content. Moreover, P. tinctorius did not ameliorate this negative effect. However, seedling phosphorus content increased with forest-floor microbes and P. tinctorius individually, and these combined to give an additive effect. Forest-floor microbes and P. tinctorius, individually, significantly increased seedling nitrogen and phosphorus contents at high nitrogen availability, interacting to give additive and synergistic effects on nitrogen and phosphorus content, respectively. The effect of ectomycorrhizal fungi on host-plant nutrition might depend strongly on the nutrient status of coexisting saprotrophic soil microbes. When nutrients are not limiting, their beneficial effects on host nutrition may be additive or synergistic.