Nitrogen addition determines whether invasive plant-derived biochar facilitates or suppresses Flaveria bidentis invasion

Invasive plant-derived biochar offers a promising strategy for converting invasive biomass into valuable soil amendments. However, its effects on plant invasion under increasing nitrogen deposition remain uncertain. Here, we established coastal saline–alkaline wetland microcosms containing native plant communities invaded by Flaveria bidentis. We evaluated biochars derived from two invasive plants, Amaranthus palmeri (AP) and Spartina alterniflora (SA), applied at 1%, 3%, and 5% (w/w), with or without nitrogen addition. We quantified invasive plant performance, native community responses, invasibility, invasion resistance, niche characteristics, and phylogenetic structure. Biochar effects were strongly dependent on feedstock, application rate, and nitrogen availability. In the absence of nitrogen addition, applying AP-biochar or SA-biochar at either 3% or 5% increased the relative invasion risk by enhancing the performance of Flaveria bidentis and weakening the resistance of the native community. Nitrogen addition altered these outcomes but did not consistently suppress relative invasion. AP-biochar with nitrogen generally reduced invasive plant dominance and enhanced community resistance. The 3% AP-biochar + nitrogen most effectively suppressed F. bidentis growth, whereas 5% AP-biochar + nitrogen maximized invasion resistance and diversity recovery. In contrast, 1% and 3% SA-biochar + nitrogen frequently promoted Flaveria bidentis , while 5% SA-biochar showed limited suppression. These findings reveal that the ecological effects of invasive plant-derived biochar depend on feedstock, application rate, and nitrogen availability. Careful feedstock selection and rate optimization are therefore essential for developing biochar-based strategies to restore invasion-resistant coastal wetland ecosystems.