Altitudinal Distribution of Ammonia-Oxidizing Archaea and Bacteria in Alpine Grassland Soils Along the South-Facing Slope of Nyqentangula Mountains, Central Tibetan Plateau

Abstract

<div><p>Nitrogen is a major limiting nutrient for the net primary production of terrestrial ecosystems, especially on sentinel alpine ecosystem. Ammonia oxidation is the first and rate-limiting step on nitrification process and is thus crucial to nitrogen cycle. To decipher climatic influence on ammonia oxidizers, their communities were characterized by qPCR and clone sequencing by targeting <i>amoA</i> genes (encoding the alpha subunit of ammonia mono-oxygenase) in soils from 7 sites over an 800 m elevation transect (4400–5200 m a.s.l.), based on “space-to-time substitution” strategy, on a steppe-meadow ecosystem located on the central Tibetan Plateau (TP). Archaeal <i>amoA</i> abundance outnumbered bacterial <i>amoA</i> abundance at lower altitude (<4800 m a.s.l.), but bacterial <i>amoA</i> abundance was greater in surface soils at higher altitude (≥4800 m a.s.l.). Archaeal <i>amoA</i> abundance decreased with altitude in surface soil, while its abundance stayed relatively stable and was mostly greater than bacterial <i>amoA</i> abundance in subsurface soils. Conversely, bacterial <i>amoA</i> abundance gradually increased with altitude at all three soil depths. Statistical analysis indicated that altitude-dependent factors, in particular pH and precipitation, had a profound effect on the abundance and community of ammonia-oxidizing bacteria, but only on the community composition of ammonia-oxidizing archaea along the altitudinal gradient. These findings imply that the shifts in the relative abundance and/or community structure of ammonia-oxidizing bacteria and archaea may result from the precipitation variation along the altitudinal gradient. Thus, we speculate that altitude-related factors (mainly precipitation variation combing changed pH), would play a vital role in affecting nitrification process on this alpine grassland ecosystem located at semi-arid area on TP.</p></div