Phosphorus availability and plant density alter facultatively mycotrophic plant species responses to arbuscular mycorrhizas

Although phosphorus availability and plant density independently have been demonstrated to alter plant growth, little is understood regarding how these factors interact to alter plant responses to arbuscular mycorrhizas (AM). In this study, I examined the effects of AM, phosphorus availability, and density on growth responses of five crop species (Capsicum annuum, Coriandrum sativum, Cucurbita pepo, Lycopersicon esculentum, and Zea mays). I grew these facultatively mycotrophic species (capable of growth without AM in fertile soil) in pots using tropical soils low in available phosphorus. I also quantified root morphological and phosphorus uptake responses to AM, phosphorus fertilization, and intraspecific density for C. annuum, C. pepo, and Z. mays. Lastly, I examined AM effects on intra- and interspecific interactions among the latter three species with a complete-additive design experiment.Despite investigating unrelated host species and different suites of AM fungus species, I consistently found increased phosphorus and intraspecific density diminished plant responses to AM. Although AM increased specific root lengths and root branching of Z. mays and C. annuum , reduced Z. mays fine root diameters, and increased C. annuum and C. pepo shoot phosphorus concentrations, increased phosphorus and density diminished these effects. Specific results, however, depended on phosphorus availability and each plant species' ability to acquire phosphorus on its own. At high phosphorus, increased density diminished AM effects on plant growth, reducing AM benefit to C. annuum and L. esculentum and reducing AM detriment to C. sativum. In contrast, for a second C. annuum variety, increased density diminished AM growth benefits at low phosphorus. I attribute these density-dependent effects of AM to increased root density causing increased overlap of root and hypha phosphorus depletion zones, which may reduce phosphorus uptake benefits of AM at low phosphorus, but may enhance uptake of phosphorus by AM at high phosphorus. Lastly, I found AM reduced both intra- and interspecific competition when AM caused growth detriment to C. pepo and Z. mays. These results suggest that similar AM effects on plant species that recruit densely may be important determinants of plant competition with consequences for community composition and plant species diversity.