Due to the fact that the AMF symbiosis is based upon nutrient limitation and availability to the plants, the effects of increasing the soil nutrient profile via fertilization is an important issue to consider as anthropogenic nutrient pollution from industry and agriculture continues to be a concern. As fertilization increases, the plant’s need for the carbon-nutrient exchange with AMF decreases. Although the nutrient benefits reaped by the plant may decrease, carbon costs may be unaffected or decline at a lesser rate, thereby altering the cost:benefit ratio of the symbiosis. We examined this idea of a symbiotic shift from mutualism to parasitism on a factorial gradient of increasing nitrogen and phosphorus fertilization using Rudbeckia hirta both inoculated with AMF and without. We individually manipulated nitrogen and phosphorus to investigate their effects individually and together at differing ratios in order to identify if either drive a shift toward parasitism more than the other, and at what levels they do so. To characterize symbiotic benefits we measured plant biomass, root:shoot ratios, and root colonization, the latter of which is still underway.
Results/Conclusions
Rudbeckia hirta grew larger with mycorrhizae at low fertility, but the benefits of this symbioses decreased as nitrogen and phosphorus increased (AMF*phosphorus p = 0.056; AMF*nitrogen p = 0.078). As a result, Rudbeckia hirta nearly doubles in growth in the presence of AMF at low fertility, but experiences no increase in growth at high levels of nitrogen or phosphorus. We also note that, surprisingly, nitrogen and phosphorus had negative effects on plant growth (p < 0.01), indicating that fertilization may have direct negative effects in addition to the negative effects of disrupting the plant-mycorrhizal symbiosis. In addition to changes in total biomass, we observed reduced root:shoot ratios with both nitrogen and phosphorus addition (p<0.01), and we are currently examining root colonization by mycorrhizae to determine whether there is also reduced investment in the mycorrhizal symbiosis. Overall, we observe reduced investment belowground with increasing fertilization, and we see a shift of the mycorrhizal symbiosis from mutualism to no effect as fertility levels increase. The implication of these results is that nutrient pollution in natural systems may lead to the breakdown of the AMF mutualism with negative consequences for both ecological health and biodiversity.