Phylogenetic relatedness predicts plant-plant nitrogen transfer better than the duration of water scarcity periods

Intermittent water availability is a significant stress factor for plants, particularly in arid and semi-arid ecosystems. Plant nutrient demands often do not align with precipitation pulses that trigger nutrient mobilization and availability, but biotic interactions like plant facilitation (e.g. through nitrogen transfer among distant relatives) and mycorrhizal symbiosis may mitigate this asynchrony, enabling nutrient access despite temporal disparities. We conducted a field experiment with 324 plant individuals to test two hypotheses: (1) greater mycorrhizal fungi abundance increases the amount of ¹⁵N transferred between plants, particularly under conditions of fluctuating water availability, and (2) the amount of ¹⁵N transferred is affected by the phylogenetic relatedness between donor and receiver plants. We show that ¹⁵N transfer is prevalent in the studied semi-arid communities, occurring between all species pairs in 68% of the trials. Interestingly, we observed an increase in ¹⁵N transfer between distantly related species, and this phylogenetic pattern remained consistent across fungicide and water regime treatments, which did not affect ¹⁵N transfer. Elucidating the drivers of N transfer between plants under different environmental conditions can improve our predictions on how plant communities will respond to future climate challenges, especially prolonged droughts in Mediterranean ecosystems.