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 ti nctorius, 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 di d not ameliorate this negative effect. However, seedling phosphorus content increased with forest-floor microbes and P. tinctorius individually, and t hese combined to give an additive effect. Forest-floor microbes and P. tinc torius, individually, significantly increased seedling nitrogen and phospho rus contents at high nitrogen availability, interacting to give additive an d synergistic effects on nitrogen and phosphorus content, respectively. The effect of ectomycorrhizal fungi on host-plant nutrition might depend st rongly on the nutrient status of coexisting saprotrophic soil microbes. Whe n nutrients are not limiting, their beneficial effects on host nutrition ma y be additive or synergistic.