Key enzymes of the urea cycle and 15N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota) roots and Glomus intraradices in two-compartment petri dishes and three ammonium levels were supplied to the compartment containing the extraradical mycelium (ERM), but no roots. Time courses of specific enzyme activity were obtained for glutamine synthetase, argininosuccinate synthetase, arginase, and urease in the ERM and AM roots. 15NH4 + was used to follow the dynamics of nitrogen incorporation into and turnover of Arg. Both the absence of external nitrogen and the presence of l-norvaline, an inhibitor of Arg synthesis, prevented the synthesis of Arg in the ERM and resulted in decreased activity of arginase and urease in the AM root. The catabolic activity of the urea cycle in the roots therefore depends on Arg translocation from the ERM. 15N labeling of Arg in the ERM was very fast and analysis of its time course and isotopomer pattern allowed estimation of the translocation rate of Arg along the mycelium as 0.13 μg Arg mg−1 fresh weight h−1. The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle. This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium.

Summary: 1. Atmospheric ammonia (NH₃) is one of the main drivers for ecosystem changes world‐wide, including biodiversity loss. Modelling its deposition to evaluate its impact on ecosystems has been the focus of many studies. For that, universal indicators are needed to determine and compare the early effects of NH₃ across ecosystems.2. We evaluate the effects of atmospheric NH₃ in ecosystems using lichens, which are one of the most sensitive communities at the ecosystem level. Rather than measuring total diversity, we use a functional diversity approach because this is potentially a more universal tool.3. We evaluated the spatial and temporal patterns of atmospheric NH₃ concentrations ([NH₃]_atm) emitted from a point‐source over a 1‐year period in a cork oak Mediterranean woodland. We observed a temporal pattern of [NH₃]_atm, with maximum concentrations during autumn.4. The distribution of lichen species was c. 90% explained by [NH₃]_atm. The tolerance of lichen species to atmospheric NH₃, based on expert knowledge from literature, was tested for the first time against direct measurements of atmospheric NH₃. Most species were well classified, with the exception of Lecanora albella and Chrysothrix candelaris, which were more tolerant than expected. Our updated lichen classification can be used to establish lichen functional groups that respond to atmospheric NH₃, and these can be used in other Mediterranean countries.5. Increasing [NH₃]_atm led to a complete replacement of oligotrophic by nitrophytic species within 65 m of the NH₃ source. The geostatistical analysis of functional diversity variables yielded a spatial model with low non‐spatial variance, indicating that these variables can cope robustly with high spatial variation in NH₃.6. Synthesis and applications. Our results support the use of functional diversity variables, such as a lichen diversity value, as accurate and robust indicators of the effects of atmospheric NH₃ on ecosystems. The spatial modelling of these indicators can provide information with high spatial resolution about the effects of atmospheric NH₃ around point‐ and diffuse sources. As this methodology is based on functional groups, it can be applied to monitor both the impact of atmospheric NH₃ and the success of mitigation strategies.