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1. Arbuscular mycorrhizal (AM) fungi and ammonia oxidizers (AO) represent key soil microbial groups regulating nitrogen (N) cycling in terrestrial ecosystems. Both utilize soil ammonium-N reserves for N assimilation, whilst the latter, through autotrophic nitrification, drive ammonia oxidation to highly mobile nitrate-N. 2. An incompatible interaction between root symbiotic AM fungi and AO was hypothesized and evaluated in plant -species-rich, N-limited Mediterranean grassland soils. Such an outcome would be manifested in a negative relationship between plant mycotrophy and local soil potential nitrification rates (PNR), a standard functional measure of ammonia-oxidizing activity in soils. 3. In three independent mesocosm experiments, grassland soils that supported monocultures of mycotrophic, as opposed to weakly and non-mycotrophic, plants exhibited significantly lower PNR. Under field conditions in a fourth experiment, we verified that soils from stands of weakly mycotrophic Agrostis capillaris sustained higher PNR than counterparts supporting highly mycorrhizal Prunella vulgaris and Fragaria vesca. 4. Discussion of mycotrophy-related modulation of AO activity centres on whether the observed relationships highlight evidence for either direct competition or a functionally important example of plant -microbial allelopathy. 5. Synthesis. Substantial evidence has been presented confirming (i) plant species identity-related regulation of PNR and (ii) negative relationships between plant mycotrophy and plant species-mediated impact on PNR in N-limited Mediterranean grassland soils. Likely mechanisms (i.e. competition and/or allelopathy) that underpin this functionally significant plant “microbe “soil relationship controlling the fate of ammonium-N require urgent elucidation in N-deficient ecosystems.