Phosphorus and intraspecific density alter plant responses to arbuscular mycorrhizas

We investigated how phosphorus availability, intraspecific density, and their interaction affect plant responses to arbuscular mycorrhizas. Four facultatively mycotrophic species: chile, cilantro, tomato, and corn were examined separately in pot experiments that employed a tropical phosphorus-immobilizing soil. Each experiment comprised nine soluble phosphorus additions, two levels of intraspecific plant density, and inoculation with arbuscular mycorrhizal fungi or not. High phosphorus significantly diminished mycorrhizal colonization of corn, cilantro, and tomato, but not chile, which was highly variably colonized. Corn roots were colonized by other root-inhabiting fungi, and mycorrhizas significantly reduced colonization by these potential root parasites. High phosphorus significantly increased relative growth rates (RGR) of all species, and high density significantly reduced RGR of cilantro, tomato, and corn. Chile showed little growth at any but the highest phosphorus additions, and consequently had no RGR response to density or mycorrhizas. Mycorrhiza inoculation caused transient depression of corn growth during the first month, but mycorrhizas increased corn RGR during the second month of growth. Both RGR and dry weights at harvest, cilantro, tomato, and corn benefited from mycorrhizas at low phosphorus availability, but this benefit diminished or changed to disadvantage as phosphorus availability increased. At low phosphorus availability, high density increased the dry weight of mycorrhizal cilantro and thereby amplified the benefit of mycorrhizas. At high phosphorus availability, increased density diminished the effects of mycorrhizas on dry weight, reducing mycorrhiza benefit to tomato and chile and reducing mycorrhiza detriment to cilantro. This study demonstrates that for three of the four plant species examined, phosphorus availability, intraspecific density, and their interaction significantly modify plant responses to arbuscular mycorrhizas.