Alpine meadow degradation depresses soil nitrogen fixation by regulating plant functional groups and diazotrophic community composition

Aims Biological nitrogen fixation (BNF), a function performed by diazotrophic microbes, plays an essential role in nitrogen (N) bioavailability in terrestrial ecosystems. However, little is known about the effects of degradation on soil BNF and diazotrophic communities in alpine meadow. Methods We investigated the changes in soil BNF and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau (non-degraded, lightly degraded, moderately degraded, and severely degraded meadows) using real-time quantitative PCR and amplicon sequencing. Results Soil BNF rates decreased significantly along the meadow degradation gradient with a range of 17.34–79.84 nmol C2H4 g− 1 dry soil d− 1 across all sites. The highest BNF was observed in the non-degraded meadow and was 1.5–4.6-fold higher than that in degraded meadows. Meadow degradation significantly reduced the gene abundance of nifH and the Shannon and Chao1 diversity indices of diazotrophs, accompanied by a decrease in plant biomass, soil moisture, and nutrient content (C, N component). Soil BNF potential was closely correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, Rhizobiales, and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, and both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributed simultaneously to the BNF. The plant functional groups, especially the sedges family, were the primary drivers for soil BNF rates via mediating soil moisture, nutrient level (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Conclusions Our results reveal the underlying mechanism of changes in soil BNF during alpine meadow degradation, emphasize the importance of plant functional groups in shaping the diazotrophic community and BNF potential, and provide insights for the restoration of degraded meadow ecosystems.

Fragmentation and percolation thresholds in the degradation process of alpine meadow in the Three-River Headwaters region of Qinghai-Tibetan Plateau, China

Understanding the process and mechanisms of alpine meadow degradation is crucial for restoration and management in the Three-River Headwaters region, Qinghai-Tibetan Plateau, China. However, little is known about this complex and controversial problem because identification and quantification of the underlying causes is difficult. This research aimed to build a spatiotemporal dynamical model for alpine meadow degradation, capturing the natural process of erosion at the interface of barren patches and undamaged meadow. The model clarified the role of barren patches and meadow connectivity in degradation, and identified the ecological mechanisms and processes accounting for the spatial and temporal pattern of degradation. A fragmentation and percolation threshold exists in the process of meadow degradation, independent of spatial scale. An impulsive differential equation was used to investigate the consequence of periodic restoration of degraded meadow. Both the level of meadow degradation and the restoration period play crucial roles in determining whether the meadow can be successfully restored. This research has demonstrated theoretically that the effectiveness of meadow restoration by periodic effort depends on the degree of degradation.

Response of soil microbial communities to alpine meadow degradation severity levels in the Qinghai-Tibet Plateau

Soil microbial community structure is an effective indicator to reflect changes in soil quality. Little is known about the effect of alpine meadow degradation on the soil bacterial and fungal community. In this study, we used the Illumina MiSeq sequencing method to analyze the microbial community structure of alpine meadow soil in five different degradation levels (i.e., non-degraded (ND), slightly degraded (LD), moderately degraded (MD), severely degraded (SD), and very severely degraded (VD)) in the Qinghai-Tibet Plateau. Proteobacteria, Actinobacteria, and Acidobacteria were the mainly bacterial phyla in meadow soil across all five degradation levels investigated. Basidiomycota was the mainly fungal phylum in ND; however, we found a shift from Basidiomycota to Ascomycota with an increase (severity) in degradation level. The overall proportion of Cortinariaceae exhibited high fungal variability, and reads were highest in ND (62.80%). Heatmaps of bacterial genera and fungal families showed a two-cluster sample division on a genus/family level: (1) an ND and LD group and (2) an SD, VD, and MD group. Redundancy analysis (RDA) showed that 79.7%and 71.3% of the variance in bacterial and fungal composition, respectively, could be explained by soil nutrient conditions (soil organic carbon, total nitrogen, and moisture) and plant properties (below-ground biomass). Our results indicate that meadow degradation affects both plant and soil properties and consequently drives changes in soil microbial community structure.