Infectivity and symbiotic efficiency of native arbuscular mycorrhizal fungi from high-input arable soils

Background Arbuscular mycorrhizal (AM) fungi are ubiquitous plant symbionts and an important biotic component of natural and agricultural soils. Yet we have only limited knowledge about the symbiotic functioning of native AM fungal communities in soils from high-input agricultural systems, where mycorrhiza can be suppressed by over-fertilization, tillage and other practices. Aims and Methods We therefore conducted a greenhouse bioassay to examine the functioning of mycorrhizas established by native AM fungal communities from 28 conventionally managed arable soils. Their infectivity and potential to promote plant growth and nutrient uptake were evaluated in comparison to non-mycorrhizal controls and to a highly infective reference isolate, using leek ( Allium porrum ) as indicator plant. Mycorrhizal effects on soil water-stable aggregation (WSA) were determined as a proxy for an ecosystem benefit of mycorrhizas. Results Root colonization by AM fungi as well as their effect on plant performance were negatively related to P availability as the most influential factor across the analysed gradients of soil conditions. Significant positive plant growth response to mycorrhiza was found only in a small subset of the soils, while positive effects on P uptake were more frequent and more pronounced. Root colonization and mycorrhizal growth response were higher after inoculation with the reference isolate than with the native AM fungal communities. Mycorrhiza-induced changes in WSA were significantly related to the plant mycorrhizal growth response. Conclusions The results suggest that native AM fungal communities may improve plant growth only in a small subset of conventionally managed arable soils, whereby their effect can be limited by suboptimal colonization potential.