Decoding nitrogen resilience in <i>Sphagnum palustre</i> : Microbial symbiosis and metabolic reprogramming under elevated nitrogen deposition

Abstract Sphagnum palustre, a dominant peat-forming moss, plays a pivotal role in maintaining peatland carbon and nitrogen balance. However, increasing atmospheric nitrogen (N) deposition threatens its ecological stability. To elucidate the adaptive responses of S. palustre to nitrogen-enriched conditions, especially under moderate N input, we applied physiological, microbial, metabolomic, and transcriptomic analyses to peatland across a nitrogen gradient (0, 3, 6, and 12 g N m−2 a−1). Sphagnum palustre displayed the greatest growth stability under moderate nitrogen addition (6 g N m−2 a−1), contrasting with the observed continuous increase in vascular plant biomass. Mechanistically, nitrogen enrichment markedly suppressed microbial nitrogen fixation, consistent with host-mediated downregulation of external N inputs. Simultaneously, S. palustre exhibited substantial metabolic reprogramming; the activity and expression levels of nitrogen assimilation enzymes (nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase) declined, while amino acid synthesis and nitrogen-rich metabolite accumulation were both enhanced. This dual regulatory strategy—reducing external N acquisition and reallocating internal N metabolism—represents a key adaptive mechanism that both maintains nitrogen homeostasis and mitigates toxicity under enrichment. Our findings highlight the mechanisms by which S. palustre achieves nitrogen resilience through coordinated microbial and metabolic regulation, providing new insight into the ecological stability of moss-dominated peatlands under the expected future nitrogen deposition conditions.