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Resumen:
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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.
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