Indirect role of soil acidification on plant diversity decline under elevated atmospheric nitrogen deposition

Abstract Soil acidification and ion dynamics are pivotal in the decline of plant diversity and alterations in community composition resulting from nitrogen (N) deposition. Nevertheless, the intricate interactions among these factors pose challenges in discerning direct causal linkages. We investigated the direct and indirect effects of soil acidification on plant community in response to N enrichment by integrating field surveys with a series of microcosm experiments conducted in a temperate grassland. Prolonged N addition led to a decrease in the species richness and above‐ground biomass of forbs. Microcosm experiments demonstrated that forb species exhibited a unimodal response in their shoot and root traits along N gradients, peaking at low N levels, whereas grass species showed a relatively broad threshold. Experimental manipulations that involved solely increasing hydrogen (H + ) ions due to soil acidification had minimal effects on the growth of both forb and grass species. In contrast, simulated acidification, accompanied by concurrent ion alterations, particularly the accumulation of toxic metals, more significantly suppressed the growth of forb species compared with grass species. These results suggest that changes in soil cations, rather than H + toxicity alone, are responsible for the decline of forb species in the community. Synthesis . These findings challenge the prevailing assumption that soil acidification is the direct cause of plant species decline in grasslands affected by N deposition. This study enhances our mechanistic understanding of ecosystem responses to N deposition and provides evidence‐based guidance for grassland restoration under global changes.