Nitrogen facilitation was maintained in sown Mediterranean forage mixtures despite drought stress conditions with concurrent general benefits upon plant aboveground water status and yield

There is an urge to optimise the sustainability and resilience of grasslands, especially in light of the extreme weather risks posed by climate change. Plant sown diversity could contribute to mitigating these risks by improving resource use efficiency as well as reducing reliance on fertilisation. We established a field experiment to determine forage swards' performance in terms of water use, nitrogen (N) use, and dry matter (DM) yield in a semi-arid Mediterranean area which faced a severe drought period. Most variables were tested over three growing seasons, whilst DM yield had an extra year of sampling in which two levels of N fertilisation were tested. Following the Generalised Diversity-Interactions modelling approach, we manipulated the composition and evenness of forage swards including a grass (Festuca arundinacea), a legume (Medicago sativa), and a non-legume forb (Cichorium intybus). Communities were grown as monocultures or mixtures of three species according to a simplex design, which allowed us to estimate separately the two components of the diversity effect: the individual species effects and that due to species interactions. Models were tested both with sown proportions (P) and realised proportions (P). The P models indicated a major beneficial effect of mixtures in contrast to the average monoculture performance for all studied variables. In contrast, P models revealed that some of the effects in P models are indeed explained by the growing proportion of M. sativa over time. However, beneficial interaction effects are still found in P models, especially concerning N fixation (with δN as proxy), which was maintained regardless of the drought stress level experienced by the swards. All things considered, mixed swards merit attention as a valuable nature-based solution that can optimise N management (by boosting N input through fixation) as well as water use. This would result in well-nurtured swards with a better ability to cope with drought stress and maintain productivity.