Plant tolerance of ammonium varies between co-existing Mediterranean species

Background: Previous studies showed that the two main Mediterranean plant functional groups, summer semi-deciduous and evergreen sclerophylls, differ in soil characteristics and nitrate (NO3−) use strategies: even though summer semi-deciduous plants have higher NO3− availability than evergreen sclerophylls, NO3− reduction (i.e., nitrate reductase activity—NRA) is lower, and is not stimulated by substrate (NO3−) availability. Aims: Test if in Cistus albidus plants, a summer semi-deciduous species, ammonium (NH4+) can inhibit NRA, despite the availability of NO3− , and whether Olea europaea plants, evergreen sclerophyll, are more tolerant of NH4+ than the former. Methods: One-year-old C. albidus and wild O. europaea potted plants were supplied with both NH4+ and NO3− at increasing levels (0.1; 0.2; 0.4; 0.8 and 1.6 % N). Tolerance of NH4 + was evaluated using integrative (mortality and biomass accumulation) and plant nitrogen metabolism parameters (in vitro NRA and concentrations of NO3− and NH4+) determined in roots and leaves. Results: C. albidus plants were consistently less NH4 + tolerant than O. europaea, displaying: higher mortality; growth and NRA inhibition and NH4+ accumulation above 0.2 % NH4NO3-N in the soil. In contrast, O. europaea plants seemed to buffer the full range of tested NH4NO3 levels. Conclusions: C. albidus plants were less NH4+ tolerant than O. europaea. The ecological implications of this contrasting NH4+ tolerance are discussed.