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Nitrogen (N) is one of the essential macronutrients for plants. Under normal growth conditions, nitrate (NO3−) and ammonium (NH4+) are the main sources of N. Most plants show a strong preference for NO3− over NH4+ ions, and others grow best if they have access to both NO3− and NH4+ (Zhou et al., 2011). However, the optimum NO3-/ NH4+ ratio changes, depending on the plant species, the environmental conditions and the stage of development (Lin Lu et al., 2009).It is known that the atmospheric CO2 concentration has increased in the last two centuries. This increase has a series of beneficial effects on plant growth and on production and quality of fruit. Moreover, it causes partial stomatal closure and increases the rate of photosynthesis, which improves the water use efficiency (Juan Li et al., 2007).The main objective of this study was to evaluate the best NO3-/ NH4+ ratio for pepper (Capsicum annuum L.) and the combined effect of nitrogen fertilization strategies and CO2 enrichment.Therefore, a study was carried out in a climatic chamber with fully-controlled environmental conditions. Pepper plants were irrigated with different N regimes: NO3-/ NH4+ proportion of 100/0 (T1), 100/0 plus urea (T2), 90/10 (T3), 50/50 (T4) and 25/75 (T5). These treatments were imposed at standard (400ppm) and elevated (800ppm) CO2 concentration. Both the anion concentrations (NO3− and Cl-) and the cation concentrations (Na+, K+, Ca2+, P+, Mn2+ and Mg2+) were determined in leaves taken at the end of the experiment. In addition, gas exchange (net CO2 assimilation (ACO2) and evapotranspiration (E)) were measured after each growth period, and these parameters were used to determine the instantaneous water-use efficiency (WUEi), as the ratio A/E.The most important results of this study indicate that the highest WUEi was observed in plant grown under elevated CO2 concentration treated with 90/10 (76.78% with respect to standard CO2 concentration and 100/0 ratio). The application of low proportions of NH4+ (90/10) decreased some leaf ion concentrations such as Ca+2 (16.97%), Mg+2 (21.32%), Na+ (98.90%) and K+ (5.35%), but increased Mn2+ (102.78%). The combined application of high CO2 and NH4+ caused a greater reduction in the following ion concentrations: Ca2+ (31.06%), Mg+2 (39.85%), Na+ (69.93%), K+ (15.62%) and NO3− (79.56%).In summary, the combination that gave the best water use efficiency was the high concentration of CO2 with a low contribution of NH4+ (90/10). However, further study is needed on how to solve the reduced uptake of certain nutrients due to an effect of both CO2 and ammonium

Effect of Different Nitrogen forms and CO2 Enrichment on the Nutrient Uptake and Water Relations of Pepper Plants (Capsicum annuun L.)