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SUMMARYRoot colonization by vesicular-arbuscular (VA) mycorrhizal fungi is often suppressed by phosphorus (P) additions; however, nitrogen (N) additions have been reported to both stimulate and suppress root colonization. Tbe objective of tbis researcb was to examine colonization of Alliutn cepa L. roots at a range of P and N concentrations in order to clarify tbe effects of tbese nutrients on root colonization. In tbe first experiment, conducted under N-limiting conditions, P addition bad no effect on root colonization by Glomus etunicatum Becker & Gerdemann and increased root colonization by Gigaspora margarita Gerdemann and Trappe. In tbe second experiment, conducted under N-sufficient conditions, P addition suppressed colonization by G. etunicatum. In tbe tbird experiment, P and N additions were sucb tbat tissue P:N ratios ranged from 0-03 (P limiting) to 0-17 (N limiting). No relationsbip was found between P:N ratio and root colonization for two isolates of G. etunicatum. Root colonization was not affected by P addition wben plants were deficient in N but, wben N was sufficient, P addition suppressed root colonization. Tbese results demonstrate tbat plant N stress affects tbe resistance of tbe bost root to colonization by VA mycorrbizal fungi.

Chinese brake fern (Pteris vittata L.) is a hyperaccumulator of arsenic (As) that grows naturally on soils in the southern United States. It is reasonable to expect that mycorrhizal symbiosis may be involved in As uptake by this fern. This is because arbuscular mycorrhizal (AM) fungi have a well-documented role in increasing plant phosphorus (P) uptake, P and As have similar chemical properties, and ferns are known to be colonized by AM fungi. We conducted a factorial greenhouse experiment with three levels of As (0, 50, and 100 mg kg(-1)) and P (0, 25, and 50 mg kg(-1)) and with and without Chinese brake fern colonized by a community of AM fungi from an As-contaminated site. We found that the AM fungi not only tolerated As amendment, but their presence increased frond dry mass at the highest As application rate. Furthermore, the AM fungi increased As uptake across a range of P levels, while P uptake was generally increased only when there was no As amendment. These data indicate that AM fungi have an important role in arsenic accumulation by Chinese brake fern. Therefore, to effectively phytoremediate As-contaminated soils, the mycorrhizal status of ferns needs to be taken into account.