High winter diversity of arbuscular mycorrhizal fungal communities in shallow and deep grassland soils

The identity and diversity of the arbuscular mycorrhizal fungal (AMF) symbionts have a large impact on ecosystem functioning and stability, indicating the need to assess their distribution in terrestrial environments. Four temperate grassland sites of low to medium fertility located on a common soil were sampled in winter and summer at two different depths (0-40 cm, 40-80 cm) using a spatially intensive experimental sampling design. Arbuscular mycorrhizal fungal community composition was determined via the amplification of AMF rRNA gene fragments present in fine roots combined with terminal-restriction fragment length polymorphism (T-RFLP) and sequencing analyses. For one out of three endonucleases applied, a site × depth interaction on T-RF richness (S) and Shannon's diversity index (H′) was observed, indicating differences between sites in the upper soil layer and depth effects on S and H′ for the extensively managed, low fertility site. The S and H' of the AMF taxa present in fine plant roots were marginally lower in winter than in summer (−14% and −16% for S and H′, respectively). Evenness E did not vary as a function of site, season or depth. The AMF community profiles, as determined by nonmetric multidimensional scaling, differed between sites and seasons, but not among soil depths. The intersite AMF community variations were attributed to niche differentiation based on soil phosphorus availability and pH. Seasonal shifts could not be related to variations in root densities or physico-chemical soil properties measured in this study, suggesting climate and plant regulation as major processes responsible for the variations between winter and summer AMF communities. It is concluded that diverse winter AMF communities are present in temperate grassland soils, and that plant roots colonizing subsoils harbour similar and equally diverse AMF assemblages than those present in topsoil layers.