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 assessing the effectiveness of field arbuscular mycorrhizal (AM) communities in a bioassay, in terms of plant growth and P uptake. The methods compared were benomyl 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 ability of the methods to differentiate AM effectiveness. For each soil type, two pot experiments were conducted in field soil, one to investigate the effects of the methods on soil nutrient status, and the other to study the effects on mycorrhization and plant response. The test plants, flax (Linum usitatissimum) 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 bioassay 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 experiments was obtained. The bioassay accurately represented the situation in the field and the use of benomyl appeared to be the most appropriate method currently available. The advantages were the ability to use a test plant responsive to AM, the use of less elevated nutrient concentrations than with irradiation, and thus the possibility to use untreated soil as the mycorrhizal treatment. The pea mutants proved unresponsive to AM, and reinoculation to irradiated soil resulted in only half the colonization rate in untreated soil. Benomyl may, however, lead to an underestimation of AM effectiveness because the control is not totally non-mycorrhizal. Its use also carries with it health and environmental risks.