Aim
Soil microbes can significantly influence restoration outcomes via interaction with plant community assembly processes, yet knowledge about variation in soil microbial communities – in particular functional variation – during oldfield succession is limited.

Methods
We divided a well-dated successional chronosequence on the Tibetan Plateau into five stages: stage 1 (continued arable land), stage 2 (arable abandoned for 2 years), stage 3 (arable abandoned for 10 years), stage 4 (arable abandoned for 20 years), and natural grassland. We investigated the changes in taxonomic and functional composition of bacterial and fungal communities in these successional stages.

Results
The richness of bacterial and fungal communities had a unimodal relationship with successional age, as the both were initially low and decreased again in natural grasslands. These changes were more correlated to soil properties. For both bacterial and fungal communities, taxonomic similarity to natural grasslands increased monotonously with successional age. The functional composition of bacterial communities shifted with successional age towards increased importance of strains involved in the C cycle rather than the N cycle, due to higher plant richness. For fungal communities, saprotrophs showed an increasing trend with successional age although low relative abundance in natural grasslands, which was regulated by belowground biomass. Symbiotrophs did not change during succession, but pathotrophic fungal relative abundance decreased rapidly after agricultural abandonment because of increased plant richness.

Conclusions
Overall, twenty years of oldfield succession did not appear to restore richness of soil bacterial and fungal communities to the levels of natural grasslands. Community taxonomic and functional composition in successional stages up to 20 y old were also different from natural grasslands. Our results suggest more important role of plant community than microbial community to soil nutrient cycling during restoration in oldfields.