Abstract: Numerous studies have shown that elevated nitrogen (N) availability has facilitated plant invasion across many ecosystems. However, the impact of N forms on the invasion process remains unclear, particularly under varying N availability and competitive conditions. We grew five congeneric pairs of invasive and native species that are common in wetland and farmland ecosystems in the Southern part of China, under two levels of N availability (low vs. high) with different forms (ammonium [NH₄⁺] vs. nitrate [NO₃^-] vs. organic [glycine]) that were fully crossed with competition (alone vs. interspecific competition). The results showed that under non-competitive conditions, neither invasive nor native species showed significant differences in performance among the different N forms. However, under competitive conditions combined with high N availability, invasive plants derived a specific advantage from NH₄⁺-N, achieving greater growth than when supplied with NO₃^--N or glycine. In contrast, the performance of native species did not differ significantly across N forms under the same conditions. Under NO₃^--N supply, invasive species allocated more biomass to roots than native species did. This enhanced root allocation may improve invasive performance, particularly under high N conditions. Under low N availability, N form had no significant effect on biomass allocation for either invasive or native plants. The findings suggested that invasive species outperformed native species under both high NH₄⁺-N conditions and competitive conditions. Results indicated that the risk of invasion may increase under scenarios of nutrient eutrophication and competitive interactions, particularly due to the substantial use of NH₄⁺-N fertilizers in soil management in farmland or wetland ecosystems.

Key words: nitrogen forms, nitrogen availability, competition, plant invasion, biomass allocation