Secondary succession of soil, plants, and bacteria following the recovery of abandoned croplands in two semi-arid steppes

In many parts of the world, former agricultural sites have been abandoned particularly when productivity is marginal or policies are implemented to develop ecosystem recovery. Understanding the recovery trajectory of soil, plants, and microbes is critical for developing restoration plans and the most effective policies. Here, we evaluated the changes in soil properties, plants, and the bacterial community along a chronosequence of agricultural abandonment (5, 15, and 20 years) in two different types of steppes (desert and typical steppes), respectively, in Inner Mongolia, China. Active farmland and natural grassland were selected as reference sites. In both study sites, soil water content and soil organic carbon content increased, while bulk density and nitrogen decreased across the chronosequence, all becoming comparable to that of natural grassland. Plant diversity, above- and below-ground biomass increased, while perennial graminoids and forbs replaced annuals as the dominant functional groups with the elongation of abandonment. Bacterial diversity increased along the chronosequence in the drier desert steppe, but not in the wetter typical steppe. Over the chronosequence, Acidobacteria, a phyla tending to live in lower carbon conditions, were replaced by Proteobacteria and Actinobacteria, which favor abundant carbon environments. Redundancy analysis shows that soil organic carbon, below-ground plant biomass, and nitrate were the main factors that driving bacterial community composition. Our results demonstrated that spontaneous recovery without any human disturbance was an effective way for the restoration of arid and semi-arid grassland ecosystems in Inner Mongolia, and emphasized the importance of soil and plant restoration for the recovery of bacterial community.