Mechanisms underlying the effects of alpine wetland vegetation succession on the community assembly and functional potential of nirK-type denitrifier

Denitrification in alpine wetlands critically regulates regional nitrogen budgets and N 2 O emissions, and nirK -harboring denitrifiers represent an important functional group involved in nitrite reduction during denitrification. Spatial heterogeneity across distinct plant communities can modulate soil microenvironments, thereby shaping N 2 O production, consumption, and emission dynamics. Here, using a space-for-time substitution approach, we investigated four successional stages—early (BS), transitional (CR), middle (EN), and mature (LS)—in the alpine wetlands of Qinghai Lake. High-throughput sequencing combined with a null model framework was applied to characterize the coupling between microbial community patterns and their functional potential. Vegetation succession stimulated microhabitat divergence characterized by shifts in total nitrogen (TN), total carbon (TC), and the carbon-to-nitrogen ratio (CNR). This environmental filtering induced a directional microbial succession, culminating in the highest diversity at the LS stage. The CNR emerged as the primary driver accounting for community variance, mediating a microhabitat niche separation that selectively enriched key heterotrophic denitrifiers (such as Sinorhizobium and Paracoccus ). The βNTI null model revealed that deterministic processes governed community assembly; notably, the underlying assembly rules underwent a fundamental shift as succession advanced, becoming dominated by heterogeneous selection at the LS stage. Co-occurrence networks peaked in complexity at the EN stage but reorganized into a more consolidated, modular, and highly competitive topology at the LS stage. Furthermore, functional profiling indicated that while the BS stage was functionally oriented toward nitrate respiration and nitrogen fixation, the LS stage exhibited a distinct advantage in possessing predicted denitrification-related functional profiles. In summary, vegetation succession reshaped nirK -type denitrifier communities via environmental filtering, steering the assembly mechanisms from stochasticity toward determinism; meanwhile, the “insurance effect” of the diverse community structure sustained stable functional output amid nutrient fluctuations. This study clarifies that vegetation succession is a core driving force of the nitrite-reducing potential of nirK -type denitrifiers, providing micro-scale mechanistic insights for the restoration of degraded alpine wetlands, vegetation reconstruction, and the maintenance of regional nitrogen homeostasis under changing climates.