Arbuscular mycorrhizal fungi (AMF; Mucoromyceta) are an important component of fertile soils, as for instance they improve plant growth and health, stabilize soil aggregation and retain macro- and micro nutrients in the upper soil layers. Currently, > 300 species are known worldwide. This number is steadily increasing, since many detected species have not been described, while others have so far remained undetected. During the last 20 years, the diversity of these fungi were studied in a series of natural or agricultural soils in Central Europe, or elsewere, under different climatic conditions, and subjected to different conventional or organic farming and soil tillage practices. It was found that multiple AMF species as well as the overall AMF community structure can be used as powerful indicators in various aspects such as agricultural sustainability, soil fertility, water and nutrient budgets, or soil degradation due to over uses or climate change. About 2-60 species were detected in natural and agricultural soils depending - among other factors - on climate, land use type and intensity, plant species diversity and specific soil parameters. In arid soils, only Glomeraceae, Diversisporaceae or Paraglomeraceae species were detected. Under semi-arid to humid conditions, also others such as Acaulosporaceae, Gigasporaceae and Racocetraceae species occur, but they are generally more sensitive to high fertilization levels than Glomeraceae species, and often they are less common in neutral to alkaline soils. In a wide range of soils types, e.g. Tschernosems, Luvisols, Cambisols and Ferralsols, a high AMF diversity can be found even under intensive agricultural production, as long as the majority of the fungi have suitable living conditions during the vegetation periods. Also herbicide and even repeated fungicide applications might then be only minor factors affecting the overall AMF communities in the soils. Nevertheless, organic farming systems generally show a higher AMF diversity than conventional farming systems. AMF diversity can, however, be dramatically decreased, especially when the plant root and hyphal network is periodically disturbed by harsh tillage practices, or when plants suffer due to increasing drought events. For Central Europe, e.g. Acaulospora paulinae and A. sieverdingii are useful microbial indicator species for fertile soils and sustainable land use in siliceous soils, while Rhizoglomus invermaium, Dominikia aurea and Do. bernensis can be used as indicators also in calcareous soils, as observed in several long-term field experiments from Germany or Switzerland.
