The importance of rare versus abundant diazotrophs subcommunities in driving soil biological nitrogen fixation in alpine grassland along a natural aridity gradient

Abstract

Biological nitrogen fixation (BNF) facilitated by diazotrophs, which convert N₂ to ammonia, plays a key role in nutrient supply of terrestrial ecosystems. However, the differential contributions of rare versus abundant subcommunities to nitrogen fixation dynamics remain poorly characterized, especially in alpine ecosystem. This study examined BNF changes and shifts in abundant and rare soil diazotrophic taxa along an aridity gradient (arid, semi-arid, semi-humid, and humid) across the Tibetan Plateau. We found a significantly higher N fixation rate, vegetation coverage and biomass, nifH gene abundance, and diazotroph diversity in semi-arid and arid habitats than in semi-humid and humid habitats. Rare subcommunity composition explained more of the variation in N fixation rates than did the abundant subcommunities, suggesting greater roles of diazotrophic rare taxa in supplying nitrogen availability in alpine grasslands. The main influence factors of nitrogen fixation are aridity, plant coverage and soil C:N ratio. Structural equation modeling indicated that soil factors (e.g., bulk density, C:N ratio) and climatic factors (aridity and temperature) affected the composition of rare subcommunity through altering plant coverage and biomass, consequently affecting soil nitrogen fixation. This study establishes rare diazotrophs as critical regulators of soil nitrogen fixation and deciphers their mediation in climate-altered N-cycling processes in alpine ecosystems.

Graphical abstract

Keywords

aridity / abundant and rare subcommunities / alpine ecosystem / plant community / soil microbial function

Highlight

	● We evaluated the relationship of soil biological nitrogen fixation of alpine grassland to the diazotrophic communities along a natural aridity gradient.
	● Compared to abundant communities of diazotrophs, rare communities responded sensitively to the aridity.
	● Diazotrophic rare taxa play greater roles than abundant taxa in supplying N availability in alpine grasslands.
	● Soil and climatic factors affected the diazotrophic rare composition through altering functional plant coverage and biomass.
	● Our results provide novel insights into the mechanism by which climate changes affect the alpine ecosystem’s N processes.

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