Promotion of utilization of arbuscular mycorrhiza through reduced P fertilization 1. Bioassays in a growth chamber

The study investigated the possibilities of promoting utilization of arbusc ular mycorrhiza (AM) in crop P nutrition in Northern European conditions by decreasing P fertilization. The effect of two contrasting long-term P fert ilization regimes on fungal (AMF) infectivity, on contribution of AM to cro p growth and nutrient uptake, and on P and N responses was investigated in bioassays in a growth chamber with the original field soil. A control with suppressed AM was successfully created by benomyl application. Functional p roperties of the field AMF communities were compared after back- and cross- inoculation to the irradiated field soils. The two long-term field experime nts that were utilized represented clay and loam soils and P levels from lo w to high. The results show that annual dressings of soluble inorganic P fe rtilizers, even in moderate amounts, decrease the infectivity and effective ness of AMF communities of Northern European field soils. The functional pr operties of the communities also seem to adapt to the different P regimes. Moderate P fertilization generally decreases the total AM benefit to crops and can lead to growth depression by AM. At the lower end of the P supply o f Nordic field soils, however, AM may at least transiently impose a net cos t to crops with a high P demand such as flax or with a low dependency on AM such as barley. The impact of the P history on AM was not related to plant P nutrition only. Mycorrhization can be immediately improved by omitting P application.