This work aimed to investigate the structure and function of arbuscular mycorrhiza (AM) in a phosphate-polluted field site. Despite exceedingly high phosphorus (P)-availability, plants were strongly mycorrhizal, although the fungi should not play an essential role in the P-nutrition of hosts, usually their main function. Molecular analyses revealed that fungal diversity was reduced; however, no species were exclusive – and thus, maybe adapted – to this site. Instead, enriching the nitrogen (N)-level in the N-poor area led to reduced mycorrhization intensities of roots, implying that plants are mycorrhizal due to N-deficiency. Reduced mycorrhization following fungicide treatments led to as well reduced N-concentrations in some plant species, indicating that in fact, AM is able to improve N-nutrition of its hosts at this field site. However, not all species benefited from this mycorrhiza-mediated N-uptake and accordingly, -distribution: even negative mycorrhizal effects could be detected. This is discussed with regard to mycorrhizal responsiveness of different plant species and possible implications of mycorrhizal networks.