Differential responses of tallgrass prairie species to nitrogen loading and varying ratios of NO3– to NH4+

Abstract

Worldwide atmospheric N deposition varies in both quantity and composition of inorganic N forms (NO₃^– and NH₄⁺). Many studies designed to assess the potential consequences of N pollution, however, pay little attention to the relative abundance of N forms. We hypothesized that native species with different physiological properties will show varying responses to different forms and amounts of N deposition. Here, we assessed the responses of six herbaceous species native to tallgrass prairies in North America to increased N loading at differing ratios of NO₃^– : NH₄⁺. Individual plants of each species were grown for roughly 80 d in a sand culture under field conditions, three N levels (0.1, 1, 3 mM N), and three ratios of NO₃^– : NH₄⁺ (1:1, 4:1, 1:3). Analysis of total biomass and relative growth rate revealed a significant 3-way interaction between N level, NO₃^– : NH₄⁺ ratio, and species. We found that the C₃ grasses showed a greater relative response to increasing N addition than did C₄ grasses or forbs. We also found a strong correlation between specific root uptake capacity for N and growth response to N level, for five of the six species. Specific leaf area of green leaves and % N of senesced leaves both showed a significant N-level by ratio interaction. Cautiously interpreted, these species-specific responses in growth and tissue quality could lead to changes in community-level and ecosystem dynamics as atmospheric N deposition continues to rise. We suggest that future studies addressing the potential impacts of N loading on natural communities use inorganic N compositions that are consistent with those expected from atmospheric deposition.