Promotion of AM utilization through reduced P fertilization 2. Field studies

The hypothesis of this study was that cumulative P fertilization decreases the contribution of arbuscular mycorrhiza (AM) to crop growth and nutrient uptake in Northern European field conditions. The modes of action of P fert ilization were evaluated through effects on mycorrhization, crop dependence on AM, and AM fungal (AMF) community. Field studies were carried out withi n long-term experiments on soils with low and intermediate initial content of extractable P, where no P fertilization and 45 kg ha(-1) a(-1) P were ap plied for 20 years. AM effectiveness in terms of growth and nutrient uptake of flax, red clover and barley, percentage root length colonized by AMF, P response of flax, and spore densities and species composition of the AMF c ommunities, were assessed. In the soil with low initial P supply, cumulativ e P fertilization decreased AM contribution to crop growth and nutrient upt ake. The higher AM effectiveness in soil with no added P compensated the cu mulative P fertilization (soil P-H2O 2.5 v. 9.5 mg kg(-1)) for flax, but no t completely for clover. In contrast, barley obtained no benefit from AM at harvest and only a slight benefit from cumulated P. In the soil with inter mediate initial P supply, AM reduced growth of flax and barley, especially with no added P, and no response to AM was obtained on clover due to retard ed mycorrhization. Cumulative P fertilization reduced yield losses of flax by AM (P-H2O 18.8 v. 5.4 mg kg(-)1), because fertilization inhibited mycorr hization. In both soils, root colonization and spore density were decreased by cumulative P fertilization, but no changes in AMF species composition w ere observed.