From Barren Rock to Thriving Life: How Nitrogen Fuels Microbial Carbon Fixation in Deglaciated Landscapes.

Rapidly expanding nascent ecosystems at glacier forefields under climate warming dramatically enhance the terrestrial carbon (C) sink. Microbial C fixation and degradation, closely implicated in nitrogen (N) transformation and plant-soil-microbe interactions, significantly regulate soil C accumulation. However, how shifts in microbial functional potential impact soil C sequestration during vegetation succession remains unclear. Here, we synchronized microbial C and N cycling genes in the rhizosphere and bulk soils across an ∼130-year chronosequence at the Hailuogou Glacier in the eastern Tibetan Plateau. Carbon fixation dominated microbial C cycling throughout the chronosequence, contributing to 74% of C-cycling gene abundances and increasing 3-6 times at the intermediate stage relative to the initial stage. Microbes favored energy-efficient and carbonate utilization pathways, such as the Wood-Ljungdahl and 3-hydroxypropionate cycles, to support high C-fixation potential. Ammonification, primarily driven by the ureC gene (>50% of N-cycling gene abundances), dictated N supply for plants and microbes. This enhanced soil N availability likely stimulated microbial biomass, diversity, and specific taxa, thereby optimizing C use efficiency. However, the ammonification-driven C fixation was contingent upon specific plant species at different succession stages. Our findings highlight the pivotal role of microbial N mineralization in shaping microbial communities and driving soil C accumulation in deglaciated landscapes.