Impacts of simulated N deposition on plants and mycorrhizae from Spanish semiarid Mediterranean shrublands

Nitrogen (N) deposition threatens European Mediterranean ecosystems but investigation and understanding of impacts are limited. We report plant responses from an ongoing field N fertilization experiment conducted in a kermes oak shrubland, where NH4NO3 has been added for 1.5 years at four rates (0, 10, 20, and 50 kg N ha-1 y-1). Two annual plants (Asterolinon linum-stellatum and Limonium echiodes) were negatively affected by N fertilization in terms of density and growth. However, responses were only evident when accounting for between-plot differences in soil NO3 --N and NH4 +-N. Responses of A. linum-stellatum to simulated N deposition were also dependent on microhabitat, with the most negative effects found in the interspaces between rosemary shrubs. Negative effects were attributed either to increased soil NH4 +-N or to a nutritional (N to P) imbalance. Mycorrhizal infection rates were not altered by N addition in the case of L. echioides, whereas mycorrhizal colonization of A. linum-stellatum roots increased with N in those individuals growing under shrub protection. Living cover of rosemary shrubs was also reduced by simulated N deposition as a consequence of a reduced interannual twig growth. Contrary to annual plants, tissue N content and C:N ratios in rosemary were not affected by simulated N deposition. Overall, our data suggest a high sensitivity of plant communities from moderately polluted semiarid Mediterranean shrublands to N deposition, highlight the role of different forms of inorganic N on plant response to N deposition, and support the importance of conducting similar experiments in other Mediterranean areas spanning a wide range of climatic, soil, and background N deposition conditions.