Occurrence of arbuscular mycorrhizal fungi in a phosphorus-poor wetland and mycorrhizal response to phosphorus fertilization

The presence of arbuscular mycorrhizas in fens has received little attentio n, but because fen plants are often phosphorus limited, the plant-fungus in teraction could be an important factor in plant competition for phosphorus. In this field study, we determined mycorrhizal colonization rates for 18 f en plant species. Also in the field, we examined the effect of four differe nt forms of phosphorus on the percentage colonization for one fen plant spe cies, Solidago patula. We found that in a species-rich, phosphorus-poor wet land both mycorrhizal and nonmycorrhizal species were common. Nine of ten d icotyledonous species examined formed arbuscular mycorrhizas. while all mon ocotyledonous species were at most very weakly mycorrhizal. A morphological explanation for this pattern is that the monocots in our study have more e xtensive aerenchyma, especially in coarse roots. Therefore, monocots are ab le to transport oxygen to their roots more effectively than dicots. In the organic wetland soil, additional oxygen in the rhizosphere promotes phospho r-us mineralization and availability. Two of the monocot species (Typha lat ifolia and Carex lasiocarpa), which have been described previously as mycor rhizal in other wetland types. are surprisingly nonmycorrhizal in our phosp horus-poor study site, suggesting that a mycorrhizal association would not offer improved phosphorus nutrition to these species. In contrast, our fiel d phosphorus addition decreased mycorrhizal colonization in S. patula, sugg esting that one benefit to S. patula of the mycorrhizas is phosphorus uptak e.