Ammonium preference and microbial nitrification inhibition drive the expansion of a poisonous plant in alpine ‐meadows

Abstract Poisonous plants are proliferating unprecedentedly across global terrestrial ecosystems, threatening ecosystem health. However, the mechanisms underlying their rapid expansion, including nitrogen (N) uptake strategies and effects on soil processes, remain poorly understood. We hypothesized that poisonous plants may possess specialized N acquisition and utilization strategies (e.g. preference for particular N forms) via rhizosphere microbiome alterations, thereby influencing ecosystem N transformations and gaining a competitive advantage. To test this idea, we investigated Ligularia virgaurea ( L. virgaurea ), a dominant poisonous plant on the Qinghai–Tibetan Plateau (QTP), which thrives in degraded alpine meadows. Our study integrated (1) long‐term multi‐site spatiotemporal surveys (2010–2020) to assess L. virgaurea population dynamics and associated vegetation and soil changes; (2) 15 N‐tracing techniques to compare N form preferences (NO 3 − vs. NH 4 + ), soil N transformations and ecological consequences between L. virgaurea and the non‐poisonous grass Elymus nutans ( E. nutans ); and (3) qPCR and amplicon sequencing techniques to analyse rhizosphere microbiome, particularly ammonia‐oxidizing bacteria (AOB) and archaea (AOA). From 2010 to 2020, L. virgaurea density increased 4.9‐fold. Higher L. virgaurea densities correlated with elevated soil organic carbon, total N, NH 4 + ‐N, available phosphorus and microbial biomass, but reduced soil NO 3 − ‐N. Compared with E. nutans , L. virgaurea demonstrated a greater N uptake efficiency with particular affinity for ammonium, suppressed AOA/AOB abundance, altered phylogenetic composition of nitrifiers, attenuated nitrification rates and reduced nitrous oxide (N 2 O) emissions. Synthesis . Ligularia virgaurea 's expansion is driven by enhanced N utilization efficiency via nitrification inhibition, likely mediated by rhizosphere microbiome shifts. This unique N acquisition and utilization strategy likely gives L. virgaurea a competitive advantage in degraded QTP grasslands, with implications for restraining poisonous plant expansion through targeted nutrient management strategies in alpine grassland ecosystem.