Ultimate soil nitrogen microbial function evolution pathway fixation–comammox–nitrate reduction in long–term arid

Timeframe is a critical factor in understanding the impact of climate change on the evolution of microbial community structure and ecological memory. Here we demonstrate how bacterial functions have left lasting imprints through their inherent gene-regulatory network traits under alternating wet and dry climatic conditions over geological time scales. The extent and time scale of microbial functional evolution across the paleoclimate of loess and paleosol in Luochuan, China, provide an unprecedented case for examining the effects of climate change on microbial communities. The unique and extreme conditions of the paleoclimate environment can be reflected in the microbial community composition. Aridity has facilitated shifts in bacterial phyla community composition and exhibited strong evolutionary and legacy effects. In the drier loess soil, Gammaproteobacteria possess symbiotic nitrogen-fixing genes that promote microbial structural and functional succession over the ten-thousand-year scale, and support nitrogen fixation-comammox-nitrate reduction cycles and nitrogen memory. The legacy effect of soil total nitrogen appears to influence the abundance and evolutionary functioning of bacterial communities, but did not significantly enhance fungal communities. Our findings offer novel insights into the timescales and pathways of microbial functional evolution induced by long-term aridity, highlighting the crucial role of specific traits of Gammaproteobacteria as driving forces for stabilizing microbial systems in arid soils. Gammaproteobacteria mediated functional short-circuiting of nitrogen cycle in extremely arid environments may be a in long-term evolutionary strategy to stabilize microbial metabolism and cope with drought-induced nitrogen restriction.