Assessment of arbuscular mycorrhiza inoculum potential

Arbuscular mycorrhizae (AM) are essential for plant growth in soils of low fertility, but mycorrhiza formation under natural conditions can be constrained by limited inoculum. Assessment of mycorrhiza inoculum potential (MIP), defined as the capacity of AM fungi existing at a site to form sufficient mycorrhizae to influence host plant growth, is crucial to understanding the potential importance of mycorrhizae in the field.AM fungal propagules that comprise inocula in field soils include spores, mycorrhizal fragments, and intact hyphal networks attached to living roots. Techniques for assessment of MIP are based on soil extraction and manipulation which fragments roots and excludes hyphal networks. Extractive bioassay measures of MIP cannot provide information on mycorrhiza formation and function under field conditions. This dissertation reports experiments conducted to examine (1) effects of manipulating mycorrhizal fragments on their efficacy as inoculum, (2) the extent to which extractive bioassays agree with one another and can predict plant performance in the field, and (3) host plant effects on infectivity of hyphal networks.Mycorrhizal fragments of 100 cm total length cut into 0.5 cm, 1.0 cm, 2.0 cm and 4.0 cm lengths produced different amounts of initial colonization in Psidium guajava seedlings roots. Colonization was maximized by the 1.0 cm and 2.0 cm fragments. Freshly collected root inoculum added to pots containing sterile soil and planted with Abutilon theophrasti seedlings at 0, 3, 6, 9, and 18 days after inoculation of pots showed a decline in infectivity after 6 days. These results suggest that manipulation of root fragments in extractive bioassays may affect their contribution to MIP.The extent to which extractive bioassays agree with each other was determined in a field experiment on ten adjacent 6m x 6m plots established in a grassy field. I mowed five of the plots over a three month period to produce differences in the occurrence of AM fungi. I counted spores and sporocarps, performed extractive bioassays on soil samples, and examined colonization in root samples from the plots. Spore and sporocarp counts were not correlated with Most Probable Number estimates and direct bioassay measures. Extractive bioassays, however, were significantly correlated with each other. No MIP measure was correlated with growth of P. guajava seedlings transplanted to the plots and grown for ten weeks. Inocula in the plots may have exceeded the amount necessary to stimulate seedling growth, so that apparent variation in MIP was irrelevant. Host plants might encounter high MIP in the field because of the potential to attach to intact hyphal networks.I examined the inoculum potential of hyphal networks by removing shoots of mycorrhizal donor P. guajava plants growing in pots attached to established hyphal networks and assessing-consequent effects on mycorrhiza formation by recipient P. guajava seedlings. Mycorhiza formation was improved initially by donor decapitation, but growth of recipient seedlings was suppressed by the presence of just two donor shoots.These studies suggest that MIP measures by extractive bioassay may not be reliable for management purposes because of potentially altered infectivity of propagules, and lack of correlation with plant performance under field conditions. Because mycorrhiza formation and effect on host plants can be difficult to predict in the field, the use of field bioassays to assess MIP is essential for management purposes