High nitrogen (N) uptake is one of the main reasons for invasive alien plant invasions. However, little effort has been made to compare the effects of different N forms on N uptake between invasive and native plants, especially those on N form acquisition strategies (preference and plasticity), which influence N uptake, and thus exotic plant invasions. Related studies are particularly few in barren habitats, where the effects of N deposition on invasiveness are considered to be much weaker than in fertile habitats. In this study, we grew Solanum rostratum, a noxious invader in barren habitats, and the native plants Leymus chinensis and Agropyron cristatum in both mono- and mixed cultures under nitrate and ammonium addition treatments, and analyzed the effects of the soil N availability and forms on the growth, N uptake, and N form acquisition strategies for these plants. The invader outperformed the natives in N uptake (in most cases) and growth (always) in both mono- and mixed cultures under all N treatments. N addition increased the N uptake and growth of the invader. The advantages of the invader over the natives were higher under ammonium relative to nitrate addition. The growth advantage of the invader was associated with its higher N uptake and higher N-use efficiency. Higher plasticity in N form uptake may contribute to the higher N uptake for the invader when grown in mixed cultures. Our findings indicate that N deposition, particularly in the form of ammonium, may accelerate exotic plant invasions in barren habitats.

Global changes, such as atmospheric nitrogen deposition, can facilitate alien plant invasions, which are often attributed to the increase in soil nitrogen availability. However, few studies have considered the effects of global change-driven alterations in soil nitrogen forms, especially under conditions with interspecific competition. In this study, we first determined the differences in growth, biomass allocation, and photosynthesis under different nitrogen forms and addition levels between three noxious invasive species (Xanthium strumarium, Ambrosia trifida, and Bidens frondosa) and their respective related natives grown with and without interspecific competition and then assessed the interspecific difference in nitrogen form preference using the ¹⁵N labeling technique. Interspecific competition significantly decreased the positive responses of growth to nitrogen addition for all three natives, while increasing the responses for all three invaders, particularly under nitrate addition. When grown in competition, all invaders showed significant growth advantages over their related natives in most cases, and responded more positively to the addition of nitrate relative to ammonium, while the natives responded more positively to ammonium addition. These findings indicate that the invaders prefer nitrate, while the natives prefer ammonium. Consistently, the growth advantages are more pronounced for the invaders under nitrate relative to ammonium addition, indicating that nitrate-rich habitats may be more vulnerable to the invaders. When grown in monoculture, however, the growth advantage of the invaders became smaller or even disappeared. Nitrogen form preference also disappeared in Siegesbeckia glabrescens (native) and Bidens frondosa (invasive). Interestingly, the native plant Xanthium sibiricum showed significantly higher total biomass than its invasive congener under ammonium addition in both mixed and monoculture conditions. Our ¹⁵N labeling experiment showed that all six species preferred nitrate over ammonium, although this was not significant for two natives (S. glabrescens and X. sibiricum), which is not completely consistent with the results from our nitrogen addition experiment. Our results indicate that global change-driven alterations in soil nitrogen forms, particularly the shift from ammonium to nitrate, may facilitate alien plant invasions. Planting patterns significantly affect the responses of invasive and native species to nitrogen forms and addition levels, with mixed-culture experiments providing better insights into the invasiveness of alien species.

Global change such as atmospheric nitrogen (N) deposition can facilitate alien plant invasions, which is often attributed to the increase in soil N availability. However, few studies have considered the effects of global change-driven alterations in soil N forms, especially under conditions with interspecific competition. In this study, we first determined the differences in growth, biomass allocation, and photosynthesis under different N forms and levels between three noxious invasive species and their respective related natives grown with and without interspecific competition, and then assessed the interspecific difference in N form preference using the ¹⁵N labeling technique. Interspecific competition significantly decreased the positive responses of growth to N addition for all three natives, while increased the responses for all three invaders, particularly under nitrate addition. When grown in competition, all invaders showed significant growth advantages over their related natives in most cases; and responded more positively to addition of nitrate relative to ammonium N, while the natives more positively to ammonium-N addition. These findings indicate that the invaders prefer nitrate, while the natives prefer ammonium. Consistently, the growth advantages are more pronounced for the invaders under nitrate relative to ammonium addition, indicating that nitrate-rich habitats may be more vulnerable to the invaders. When grown in monoculture, however, growth advantage of the invaders became smaller or even disappeared; and N form preference disappeared in Siegesbeckia glabrescens (native) and Bidens frondosa (invasive). Interestingly, the native plant Xanthium sibiricum showed significantly higher total biomass than its invasive congener under ammonium-N addition in both mixed and monoculture. Our ¹⁵N labeling experiment showed that all six species preferred nitrate over ammonium, although not significant for two natives (S. glabrescens and X. sibiricum), which is not completely consistent with the results from our N addition experiment. Our results indicate that global change-driven alterations in soil N forms, particularly the shift from ammonium to nitrate, may facilitate alien plant invasions. Planting patterns significantly affect the responses of invasive and native species to N forms and levels, with mixed-culture experiments providing better insights into invasiveness of alien species.

Introduction: Nitrogen enrichment can reshape above- and belowground traits, but it remains unclear whether N form and supply level alter leaf-root coordination and whether such coordination is related to biomass. We therefore tested how ammonium and nitrate addition affect leaf and root traits, whether leaf-root relationships persist after accounting for species differences, and whether individual traits are independently associated with biomass. Methods: Five perennial forage species, including two Fabaceae and three Poaceae, were grown for one field season under a control and three levels of ammonium or nitrate (6, 24, and 72 g N m ^- ²). We measured specific leaf area (SLA), leaf N concentration (LN), specific root length (SRL), root N concentration (RN), and total biomass. Trait responses were evaluated using treatment-versus-control contrasts and factorial models, and leaf-root relationships using raw, within-species centered, and N-induced response data. Species-standardized biomass responses were used as a robustness check. Results: LN and SRL generally increased with N level, whereas SLA, RN, and biomass showed stronger species-dependent responses. Significant treatment-control differences occurred mainly at intermediate and high N addition. SLA-SRL covariation occurred within Poaceae in raw and centered data, but functional-group slopes did not differ. Raw LN-RN slopes differed between functional groups, but this difference disappeared after species mean centering. N-induced SLA-SRL responses were not associated, whereas LN-RN responses were positively related within Poaceae without a significant difference in slopes between groups. Relative biomass responses increased with N level and differed between N forms, but not between functional groups. No individual trait showed a strong independent association with biomass after accounting for species, N regime, and trait covariation. Discussion: N enrichment produced trait-specific and species-dependent responses, while apparent leaf-root coordination depended on the source of variation considered. These findings highlight the importance of separating cross-species covariation from within-species responses when interpreting plant responses to N enrichment.