Arbuscular mycorrhizae in a long-term field trial comparing low-input (organic, biological) and high-input (conventional) farming systems in a crop rotation

Arbuscular mycorrhizal (AM) root colonization was studied in a long-term fi eld trial in which four farming systems currently in use in Switzerland wer e continuously applied to a randomized set of plots at a single field site from 1978 till 1993. There were two low-input farming systems (organic and bio-dynamic) and two high-input (conventional) farming systems (according t o Swiss guidelines of integrated plant production with and without farmyard manure). The systems had an identical 7-year crop rotation and tillage sch eme and differed essentially only in the amount and type of fertilizer supp lied and in plant protection management. The percentage of root colonizatio n by AM fungi was determined in field samples 2-3 times over the growing se ason in crops in the rotation, namely in winter wheat (Triticum aestivum L. cv. Sardona), vetch-rye and grass-clover. We found the percentage of root length colonized by AM fungi to be 30-60% higher (P less than or equal to 0 .05) in the plants grown in soils from the low-input farming systems than i n those grown in conventionally farmed soils. Approximately 50% of the vari ation of AM root colonization was explained by chemical properties of the s oils (pH, soluble P and K, exchangeable Mg), the effect of soluble soil P b eing most pronounced. The potential of the field soils from the differently managed plots to cause symbiosis with AM fungi was tested in a glasshouse experiment, using wheat as a host plant. Soils from the low-input farming s ystems had a greatly enhanced capacity to initiate AM symbiosis. The relati ve differences in this capacity remained similar when propagules of the AM fungus Glomus mosseae were experimentally added to the soils, although over all root colonization by AM fungi was 2.8 times higher.