Altered rhizoctonia assemblages in grasslands on ex-arable land support germination of mycorrhizal generalist, not specialist orchids.

Species rich semi-natural grasslands in Central Europe suffered a dramatic loss of biodiversity due to conversion to arable land, but vast areas are being restored. Orchids, which depend on mycorrhizal fungi for germination, recover, but only slowly. We hypothesised that ploughing and fertilisation caused shifts in orchid mycorrhizal communities in soil and restricts orchid germination. We examined edaphic conditions in 60 restored and semi-natural grasslands, and germination success in 10 restored grasslands. Using a newly designed primer, we screened composition of rhizoctonias in soil, seedlings and roots of seven orchid species. Semi-natural and restored grasslands differed significantly in nutrient amounts and rhizoctonia assemblages in soil. While Serendipitaceae prevailed in semi-natural grasslands with a higher organic matter content, Ceratobasidiaceae were more frequent in phosphorus-rich restored grasslands with increased abundance on younger restored sites. Tulasnellaceae displayed no preference. Germination success in restored grasslands differed significantly between orchid species; two mycorrhizal generalist species germinated with a broad range of rhizoctonias at most restored grasslands, while germination success of specialists was low. Past agricultural practices have a long-lasting effect on soil conditions and orchid mycorrhizal communities. Altered mycorrhizal availability may be the main reason for low germination success of specialist orchid species.