Earthworms and arbuscular mycorrhizal fungi (AMF) interact to regulate plant nitrogen (N) supply, but themechanisms through which they affffect plant N uptake are unclear. We hypothesized that earthworms, plants andthe associated AMF exhibit difffferent preferences for difffferent forms of inorganic N (NH4+ and NO3−), whichcould regulate the effffect of earthworms and AMF interaction on plant N acquisition. We outlined three in dependent but complementary experiments to test this hypothesis in the context of exotic earthworm,Pontoscolex corethrurus. The earthworm is dominating the plantation forests in subtropical and tropical regions ofChina, which have their understory dominated by the fern Dicranopteris dichotoma. By employing an excised root 15N incubation experiment and a fifield in situ 15N experiment, we found that the fern prefers to use NH4+ ratherthan NO3−. Then we did a 2 × 2 factorial microcosm experiment using AMF (Rhizophagus intraradices) andearthworms (P. corethrurus). The exotic earthworm increased soil NH4+ concentration but did not affffect soilNO3− concentration, while the AMF decreased soil NH4+ concentration but had no effffect on soil NO3− con centration. The increase in soil NH4+ induced by the earthworms was effiffifficiently utilized by the AMF, andsignifificantly increased the total N uptake by the fern. In contrast, the AMF alone increased the N concentration ofleaves and coarse roots, but not the total plant N uptake, primarily due to the lower levels of available NH4+compared with the earthworm treatments. The uninoculated fern did not benefifit from the earthworm-inducedincrease in soil NH4+, suggesting that the root of the fern cannot access the ‘NH4+ hotspots’ created by theearthworms. Our work suggests that successful cooperation of earthworms and AMF on plant N uptake dependson the correct match in N-form.