Soil fungi promote nitrogen transfer among plants involved in long-lasting facilitative interactions

Plant facilitative interactions may persist in the long term when there are benefits for the interacting adult plants. Whereas persistent benefits for adult nurse plants have been demonstrated, the long-term benefits derived by adult facilitated plants have been largely unexplored. We hypothesize that common mycorrhizal networks (CMNs) can provide a pathway through which nurse species can benefit adult facilitated plants persistently. We specifically test whether nitrogen can be transferred from nurse plants to their adult facilitated plants, and evaluate to which extent CMNs mediate the transfer. We selected 32 adult individuals of 6 facilitated plant species growing in 15 vegetation patches in a Mexican desert. We treated some vegetation patches with fungicide and left others as controls. Then, we labeled the nurse plants with 15N-enriched urea and quantified the amount of 15N transferred to their adult facilitated plants. We expected a greater 15N transfer to facilitated individuals growing in vegetation patches with intact CMNs than in those treated with fungicide. Facilitated plants growing in patches with intact CMNs showed on average a greater increment in their foliar δ15N (i.e. difference between post-labeling-pre-labeling) than those in patches treated with fungicide. Our results provide evidence that CMNs enhance nitrogen transfer among adult plants, thus providing a potential mechanism contributing to the long-term persistence of plant facilitative associations.