Creation of a non-mycorrhizal control for a bioassay of AM effectiveness 1. Comparison of methods

The study aimed to create a control with suppressed mycorrhiza for assessin g the effectiveness of field arbuscular mycorrhizal (AM) communities in a b ioassay, in terms of plant growth and P uptake. The methods compared were b enomyl incorporation into soil, gamma-irradiation of soil by 10 and 3 kGy, and the use of a myc(-) mutant. The methods were examined On clay and loam. Two management histories were included with both soils to study the abilit y of the methods to differentiate AM effectiveness. For each soil type, two pot experiments were conducted in field soil, one to investigate the effec ts of the methods on soil nutrient status, and the other to study the effec ts on mycorrhization and plant response. The test plants, flax (Linum usita tissimum) and pea (Pisum sativum) myc(+) and myc(-) mutants, were grown in 1-1 pots for 4 weeks in a growth chamber. To test the ability of the bioass ay to reflect differences in AM effectiveness in the field, the mutants and benomyl were also studied in the field from which the loam for the pot exp eriments was obtained. The bioassay accurately represented the situation in the field and the use of benomyl appeared to be the most appropriate metho d currently available. The advantages were the ability to use a test plant responsive to AM, the use of less elevated nutrient concentrations than wit h irradiation, and thus the possibility to use untreated soil as the mycorr hizal treatment. The pea mutants proved unresponsive to AM, and reinoculati on to irradiated soil resulted in only half the colonization rate in untrea ted soil. Benomyl may, however, lead to an underestimation of AM effectiven ess because the control is not totally non-mycorrhizal. Its use also carrie s with it health and environmental risks.