Cooperation of earthworm and arbuscular mycorrhizae enhanced plant N uptake by balancing absorption and supply of ammonia

Abstract

Earthworms and arbuscular mycorrhizal fungi (AMF) interact to regulate plant nitrogen (N) supply, but the mechanisms through which they affect plant N uptake are unclear. We hypothesized that earthworms, plants and the associated AMF exhibit different preferences for different forms of inorganic N (NH4+ and NO3−), which could regulate the effect of earthworms and AMF interaction on plant N acquisition. We outlined three independent 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 of China, which have their understory dominated by the fern Dicranopteris dichotoma. By employing an excised root 15N incubation experiment and a field in situ15N experiment, we found that the fern prefers to use NH4+ rather than NO3−. Then we did a 2 × 2 factorial microcosm experiment using AMF (Rhizophagus intraradices) and earthworms (P. corethrurus). The exotic earthworm increased soil NH4+ concentration but did not affect soil NO3− concentration, while the AMF decreased soil NH4+ concentration but had no effect on soil NO3− concentration. The increase in soil NH4+ induced by the earthworms was efficiently utilized by the AMF, and significantly increased the total N uptake by the fern. In contrast, the AMF alone increased the N concentration of leaves 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 benefit from the earthworm-induced increase in soil NH4+, suggesting that the root of the fern cannot access the ‘NH4+ hotspots’ created by the earthworms. Our work suggests that successful cooperation of earthworms and AMF on plant N uptake depends on the correct match in N-form.