Warming combined with drought removes the beneficial effects of arbuscular mycorrhizal fungi on legume (Medicago sativa) nodulation

Background: Most legumes form symbioses with both rhizobial bacteria and arbuscular mycorrhizal (AM) fungi, which supply nitrogen (N) and phosphorus (P) respectively. These two symbioses are established through a partly shared signalling pathway in the host root and are functionally linked through plant N:P balance. Because both partners draw on host photosynthate, the balance of benefits between them may shift under environmental stress, yet how warming and drought alter the mycorrhizal contribution to nodulation remains unclear. Methods: In a greenhouse experiment with alfalfa (Medicago sativa), we examined the effects of drought, warming, and AM fungi across eight combinations. After four months, we quantified root nodule biomass and number as primary nodulation metrics, and further measured plant nutrient content and nitrogenase activity to clarify whether changes in nodulation arose from functional impairment or translated into nutritional consequences for the host. Results: Under control conditions, AM fungi increased root biomass allocation to nodules and fresh nodule biomass, and mycorrhizal colonization was positively associated with nodule biomass. Nodule biomass was negatively correlated with plant P concentration, consistent with nodulation imposing a substantial P cost. Under combined warming and drought, however, these positive effects on nodulation were no longer detected; instead, AM fungi increased shoot P concentration and content, an effect absent under control conditions. Together, these results suggest that compound climate stress redirects the mycorrhizal contribution away from supporting energy-intensive nodulation and towards maintaining host shoot P status. Conclusions: The beneficial interactions between rhizobia and AM fungi are essential for nutrient acquisition, but warming and drought can weaken these advantages. This emphasizes the urgent need for strategies that enhance legume-microbe partnerships amid climate variability for sustainable agriculture.