Linking soil bacterial and fungal communities to vegetation succession following agricultural abandonment

Globally, the rate and extent of cropland abandonment increase greatly since 1950s. Knowledge of soil bacteria and fungi succession during long-term vegetation development is particularly limited for the abandoned croplands. In order to test the effects of agricultural abandonment on soil biota succession, we studied the soil bacterial and fungal composition and diversity in abandoned farmlands across a century of secondary vegetation successional gradient in China's Qinling Mountains. Using high-throughput sequencing technologies, the soil fungal and bacterial communities were studied in 22 abandoned farmlands, as well as 7 adjacent arable fields representing non-abandoned references. The stand age, i.e., years since agricultural abandonment, affected the soil bacterial and fungal composition and explained 8.7 and 31.6% variations of bacterial and fungal communities (at order level), repectively. The Proteobacteria, dominated by chemoorganotrophic bacteria, kept its absolute dominance status (38.66% - 40.77%) constantly during succession even though the vegetation changed obviously from crop to grass, shrub, and forest. The relative abundances of Acidobacteria, Planctomycetes, Verrucomicrobia, Nitrospirae, and Spirochaetes increased significantly with stand age (i.e., years since abandonment), while the Firmicutes, Actinobacteria, Gemmatimonadetes, Cyanobacteria, and Armatimonadetes showed an opposite trend. A distinct shift in fungal communities from Ascomycota -dominant in young stands to Basidiomycota -dominant in older stands was observed, which could be attributed to the increase of vegetation coverage and soil moisture during succession. The soil bacterial richness and diversity increased logarithmically with increasing stand age and gradually reached equilibrium in late-successional stage. Soil fungal diversity tended to increase in the early successional stages and then followed by a decreasing trend. The soil pH was the most important environmental factor predicating the soil fungal alpha-diversity measurements. Both the soil bacterial and fungal communities displayed successional trends along with vegetation succession. The soil bacteria and fungi exhibited marked differences in successional pattern during secondary succession following agricultural abandonment.