Abstract
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 meadow degradation on soil BNF and diazotrophic communities in alpine meadows. We investigated 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. 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 rate in the non-degraded meadow was 1.5–4.6-fold higher than that in the degraded meadows. The abundance and diversity of diazotrophs measured by nifH abundance and Shannon diversity was also decreased in the degraded meadows, accompanied by decreases in plant biomass, soil moisture, and nutrient content (C, N). Soil BNF rate was correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, and unidentified genera within the Rhizobiales and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, with both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributing significantly to BNF. The plant functional groups, particularly the sedge family, were the primary drivers of soil BNF rates via mediating soil moisture, nutrient content (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Our results reveal the main drivers of decreased BNF during alpine meadow degradation and emphasize the importance of plant functional groups in shaping the diazotrophic community and regulating the BNF rate. This information can be applied to the restoration of degraded meadow ecosystems.
Highlights
Alpine meadows are widely distributed in the Tibetan Plateau—covering a total area of 700000 km2—and play numerous roles in soil and water conservation, carbon (C) sequestration, climate regulation, and habitat conservation (Che et al 2019; Wang et al 2020b)
We investigated the changes in soil Biological nitrogen fixation (BNF) and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau using real-time quantitative PCR and amplicon sequencing
Soil BNF potential was closely correlated with plant biomass, soil nutrient content, and diazotrophic abundance
Summary
Alpine meadows are widely distributed in the Tibetan Plateau—covering a total area of 700000 km2—and play numerous roles in soil and water conservation, carbon (C) sequestration, climate regulation, and habitat conservation (Che et al 2019; Wang et al 2020b). In the past 30 years, large areas of alpine meadows have experienced severe degradation due to severe climate change and anthropogenic activities (such as overgrazing and tramping), which poses an increasing threat to the resistance and sustainability of alpine ecosystems (Fu et al 2012; Luo et al 2018; Wen et al 2010). Meadow degradation can accelerate the loss of soil N, further increasing N limitation in alpine ecosystems (Kou et al 2019). The dynamics of N in the pedosphere have been widely studied, our understanding of how meadow degradation affects N input is still largely lacking, which may hinder the modeling and prediction of N dynamics in alpine ecosystems
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