Prokaryotic community assembly after 40 years of soda solonetz restoration by natural grassland and reclaimed farmland

Solonetz soils are typically characterized with excessive saline-alkali content, and physiochemical methods are often employed for solonetz reclamation. However, increasing lines of evidences have suggested that microbiomes play a pivotal role in sustaining soil fertility than previously appreciated, and it remains poorly understood about the taxonomic identity of microbiomes in solonetz soils and the underlying mechanisms that govern the community assembly of solonetz restoration. In this study, we show that significant increase of rare phylotypes occurred after solonetz restoration by natural grassland and reclaimed farmland, and prokaryotic community assembly was dominated by deterministic rather than stochastic processes. The exchangeable sodium decreased significantly from 27.04% in solonetz, to 4.35% in grassland and 0.52% in farmland. Soil organic carbon (SOC) increased significantly with 4.17- and 2.18- fold in grassland and farmland soils relative to solonetz, respectively. Real-time quantitative PCR indicated drastic increases in the abundance of functional genes responsible for nitrogen fixation and nitrification. High-throughput sequencing of 16S rRNA genes further revealed that 180 and 215 phylotypes could be considered as key taxa showing significant increase after solonetz restoration by grassland and farmland. Intriguingly, more than 96% of these key phylotypes were originally rare phylotypes with <0.3% relative abundance in solonetz, including uncultivable lineages such as Acidobacteria Gp6 and Gp4 and well-recognized phylotypes such as Arthrobacter and Microvirga. The advanced statistical analysis of beta-nearest taxon index indicated that deterministic selection dominated microbial community assembly which was closely associated with high soil organic carbon turnover. These results demonstrated that the conversions of solonetz to grassland and farmland lead to significant changes of soil physicochemical properties which in turn could have likely selected for ecologically and agriculturally important functional microorganisms, and rare phylotypes might play important roles than previously appreciated in reclamation of solonetz.