Introduction

There is an increased interest to utilize beneficial soil biota as a tool to enhance plant nutrition and plant productivity (Barrios 2007). The presence of beneficial soil biota can be stimulated by altering agricultural practices such as crop rotation or tillage intensity that favour particular groups of microorganisms (Altieri 1999, Köhl et al. 2014). In addition, beneficial soil biota can also be deliberately introduced into agroecosystems through inoculation or seed coating in order to add a desired function or enhance an already existing one (Berg 2009, Vessey 2003).

Within the soil microbial community, arbuscular mycorrhizal fungi (AMF) are well known for their ability to enhance plant nutrient uptake, improve plant growth and influence ecosystem functioning (Smith & Read 2008). Up to 50% of the soil microbial biomass consists of AMF (Olsson et al. 1999). AMF form a symbiosis with over 80% of the land plants including many important crops (Smith & Read 2008). AMF can provide a range of soil nutrients to plants in exchange for carbohydrates. In addition, AMF can also contribute to soil aggregate formation (Leifheit et al. 2014), protect their hosts against abiotic (Galli et al. 1994, Bothe 2012) and biotic stresses (Azcón‐Aguilar & Barea 1996), influence nutrient cycling (Bender & van der Heijden 2015, Cavagnaro et al. 2006) and soil respiration (Wamberg et al. 2003, Langley et al. 2005), and reduce the production of the greenhouse gas N 2 O (Bender et al. 2014).

Arbuscular mycorrhizal fungi are native to all terrestrial ecosystems and can be found in almost every soil (Abbott & Robson 1982, Öpik et al. 2006, Jansa et al. 2009). Several studies report reduced AMF diversity upon land use intensification (Helgason et al. 1998, Verbruggen et al. 2010). The reduction of mycorrhizal abundance and species diversity is due to factors related to intensive agricultural management such as high fertilization, intensive tillage, fallow and crop sequence with non‐host crops (Jansa et al. 2006, Koide & Peoples 2012, Säle et al. 2015). It has been shown in microcosms that this loss of fungal diversity in soil can disrupt a range of soil ecosystem services (Maherali & Klironomos 2007, van der Heijden et al. 1998, Wagg et al. 2014). Moreover, some studies indicate that intensive agriculture selects for inferior mutualists (Johnson 1993, Scullion et al. 1998).

Soil inoculation with beneficial AMF has been proposed to overcome this limitation, and contribute to more efficient nutrient use. Inoculation with beneficial AMF is increasingly considered for species‐poor and often sterile soils in nurseries (Azcón‐Aguilar & Barea 1997) and in tropical crop production where soils are low in plant available phosphorus and AMF abundance (Ceballos et al. 2013, Sieverding 1991). The hesitant application of AMF in commercial agriculture in the temperate zone might be due to high application costs, the perception that AMF are not very beneficial when P‐availability is high, and that AMF may even lead to plant growth depression in some crops (Ryan & Graham 2002). Despite these concerns, meta‐analyses have revealed that biomass production and P‐uptake can indeed be increased by inoculation of soil with AMF (McGonigle 1988, Lekberg & Koide 2005, Hoeksema et al. 2010).

One of the crucial biotic soil factors determining the success of the fungal inoculant is the indigenous mycorrhizal community. If the strain is compatible with a particular soil, it still needs to outcompete the indigenous AMF community, and AMF already established in the field may be competitively superior (priority effect) compared with the introduced ones (Verbruggen et al. 2013). Furthermore, it is thought that ecosystems can only support AMF populations to a certain quantity (carrying capacity) preventing further establishment if this carrying capacity has already been reached. Thus, it seems questionable, if inoculation can be successful in fields with high fungal abundance. Despite numerous inoculation studies, only a few attempts using molecular tools have been made to assess, if a foreign strain can successfully colonize host plants and persist in field soil despite of other AMF being present (Farmer et al. 2007, Ceccarelli et al. 2010, Pellegrino et al. 2012, Sýkorová et al. 2012). Moreover, all these studies focused on one particular field, and it has not yet sufficiently been tested whether a particular inoculant can establish in a wide range of soils. It is also still unclear, whether the same fungal isolate as it often occurs in commercial inoculum can successfully established in a broad range of field sites. Such a broad applicability is one pre‐condition for commercial AMF inocula.

In this study, we introduced Rhizoglomus irregulare to a range of agriculturally managed field soils. R. irregulare (formerly named Rhizophagus irregularis/Glomus irregulare/Glomus intraradices, Sieverding et al. 2014) is a widespread AMF present in almost any ecosystem investigated (Öpik et al. 2006), and is especially abundant in agricultural soils (Jansa et al. 2003, Oehl et al. 2010). Earlier studies with this isolate have shown that it has a positive impact on the growth and nutrition of a range of plant species, when added to sterilized soil (Scheublin et al. 2007, van der Heijden et al. 2015, Wagg et al. 2011a). Here, we specifically test whether (1) the introduced AMF can establish in a wide range of field soils; (2) the AMF is able to establish and compete with different resident AMF communities; and (3) whether AMF inoculation enhances plant productivity and nutrient uptake. In order to test this, we inoculated or mock inoculated the AMF R. irregulare into microcosms planted with a grass–clover mixture. The microcosms were filled with unsterilized field soil originating from eight agriculturally managed fields that differed strongly in soil type and chemical characteristics.