Elevated atmospheric carbon dioxide and chronic atmospheric nitrogen deposition change nitrogen dynamics associated with two Mediterranean climate evergreen oaks

Effects of long-term elevated CO2 versus chronic high nitrogen deposition on carbon and nitrogen dynamics were determined on two Mediterranean evergreen oak tree species and their associated soils. Quercus ilex L. was studied in central Italy at a natural CO 2 spring in a mixed-oak Mediterranean forest, where a gradient of CO 2 concentration occurs. Q. agrifolia Nee was studied in the Mediterranean climate region of southern California across a N deposition gradient. These natural experiments have occurred longer than any controlled experiments on intact mature ecosystems with well-established soil microbial and fungal communities.; I anticipated nitrogen would be limiting under elevated CO2, and carbon would be limiting under high N deposition. More integrative measurements of stable N and C isotopic ratios of plants and soils were compared with shorter-term measurements of total and available soil N, net N mineralization and nitrification rates, and various microbial and fungal measures.; Leaf and soil delta15N under high N deposition were lower than under low N deposition and differed from those of more mesic systems in eastern U.S. and northern Europe. During the drier periods, leaf delta 15N values dropped along with soil moisture, while net nitrification rates and nitrate contents increased. 15N-depletion from nitrification plus nitric acid vapor from vehicle exhaust likely contributed. Leaf delta 15N of a co-occurring arbuscular mycorrhizal plant species was similar to that of ectomycorrhizal Q. agrifolia under high N deposition, only. Therefore, Q. agrifolia used primarily nitrate under high N deposition.; In contrast, Q. ilex leaf delta15N was higher at elevated CO2 sites, while net nitrification rates, nitrate concentrations, and water availability decreased. Leaf delta 15N was more similar between an arbuscular mycorrhizal tree species and ectomycorrhizal Q. ilex under elevated CO2. Therefore, Q. ilex used primarily ammonium under elevated CO2. Reduced nitrification conserves limited N. As a result, Q. ilex maintained leaf N concentrations, which may explain previously observed enhanced photosynthesis rates under elevated CO2.; Mediterranean climate regions characteristically experience seasonal drought. Adaptations of evergreen oaks and the soil microbial and fungal community to limited water may have contributed to these oak species' adjustments to long-term excesses in carbon versus nitrogen.