Grassland is one of the most important terrestrial ecosystems, however, in recent years, degradation and desertification of grassland ecosystem becomes more and more serious due to intensive human activities, such as overgrazing, mowing, and conversion of grassland to cropland. Effective countermeasures, such as pasture enclosure, are suggested to maintain the grassland productivity and sustainability. Most studies on ecological restoration of degraded grassland focused on recovery of plant communities, while little information is available as for the parallel succession of belowground microbial communities, especially for symbiotic microbes associated with plants.
Abuscular mycorrhizal (AM) fungi are ubiquitous symbionts for higher plants in most natural and agricultural ecosystems. It has been widely accepted that these symbiotic fungi play important roles in stimulating biodiversity and productivity of plant communities. To reveal the impacts of grassland enclosure on recovery of the degraded grassland ecosystem, especially for the recovery of AMF communities, we conducted an investigation based on a long-term field experiment where experimental plots under different grazing intensities (heavily, moderately, slightly grazed and the ungrazed control) have been enclosed for 14 years. Plant coverage, height and species richness were recorded in situ, while soil samples were collected for analysis of soil chemo-physical properties and AM fungal parameters. As a newly developed molecular tool, the second-generation sequencing technology, 454 pyrosequencing, was applied for predicting AMF community composition and biodiversity.
The experimental results indicated that, after enclosure for 14 years, the coverage, diversity and evenness index of plant communities on different experimental plots did not show significant difference; The soil organic matter, available N, total N and total C contents tended to be higher in lightly and moderately grazed plots compared with heavily grazed plots, but statistically there were no significant differences among different plots. Available soil P was lowest in the heavily grazed plots (1.00 mg/kg), which was significantly lower than that in the lightly grazed plots (2.25 mg/kg). The 454 pyrosequencing of AM fungi from all soil samples yielded a total of 59,382 Glomeromycota sequences, assigned to 87 virtual taxa (VT) in the MaarjAM database, belonging to 7 genera, namely Diversispora, Otospora, Scutellospora, Glomeraceae Glomus, Rhizophagus, Paraglomus and Archaeospora. Similar to previous reports, Glomus was the dominant genera on the grassland, as 83.9% of the 87 VTs belonged to Glomeraceae Glomus; while only 13 sequences (1 VT) were identified as Archaeospora, which was undoubtedly the rarest genus in the research area. In contrast with plant communities, the AM fungal communities had not equally recovered in different experimental plots. Although there were common VTs for all plots, but each plot clearly exhibited some specific VTs, and most specific VTs were recorded in CK plot. Furthermore, the diversity index and evenness index were lowest in CK plot, lower than any other plots, and significantly lower than that in the moderately grazed plot.
This investigation suggested that grassland enclosure after overgrazing is essentially important for the recovery of plant communities, soil chemo-physical properties and also soil microbial communities. However, recovery of AM fungal communities was out of synch with plant communities. Further research is still necessary to reveal the interactions between plant and functional soil microbial communities during the ecological restoration of degraded grassland.