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 arbuscular mycorrhiza (AM) in crop P nutrition in Northern European conditions by decreasing P fertilization. The effect of two contrasting long-term P fertilization regimes on fungal (AMF) infectivity, on contribution of AM to crop 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 properties of the field AMF communities were compared after back- and cross-inoculation to the irradiated field soils. The two long-term field experiments that were utilized represented clay and loam soils and P levels from low to high. The results show that annual dressings of soluble inorganic P fertilizers, even in moderate amounts, decrease the infectivity and effectiveness of AMF communities of Northern European field soils. The functional properties 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 of Nordic field soils, however, AM may at least transiently impose a net cost 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.