Stoichiometric N:P flexibility and mycorrhizal symbiosis favor plant resistance against drought

1. Drought induces changes in the nitrogen (N) and phosphorus (P) cycle but most plant species have limited flexibility to take up nutrients under such variable or unbalanced N and P availability. Both the degree of flexibility in plant N:P ratio and of root symbiosis with arbuscular mycorrhizal (AM) fungi might control plant resistance to drought-induced changes in nutrient availability, but this has not been directly tested.
2. Here, we examined the role of plant N:P stoichiometric status and mycorrhizal symbiosis in the drought-resistance of dominant and subordinate species in a semi-natural grassland.
3. We reduced water availability using rainout shelters (control vs drought) and measured how plant biomass responded for the dominant and subordinate species. We then selected a dominant (Paspalum dilatatum) and a subordinate species (Cynodon dactylon), for which we investigated the N:P stoichiometric status, mycorrhizal root colonization and water-use efficiency.
4. The biomass of all dominant plant species, but not subordinate species, decreased under drought. Drought increased soil available nitrogen, and thus increased soil N:P ratio, due to decreasing plant N uptake. The dominant Paspalum dilatatum showed a high degree of plant N:P homeostasis and a considerable reduction in biomass under drought. At the opposite, the more flexible subordinate species Cynodon dactylon increased its N uptake and water-use efficiency, apparently due to stronger symbiosis with mycorrhizae, and maintained its biomass.
5. Synthesis. We conclude that the maintenance of N:P homeostasis in dominant species, possibly because of a large root nutrient foraging capacity, becomes inefficient when water stress limits N mobility in the soil. By contrast, we demonstrate that higher stoichiometric N:P flexibility coupled with stronger mutualistic association with mycorrhizae allow subordinate species to better withstand drought perturbations. Using a stoichiometric approach in a field experiment, our study provides for the first time clear and novel understandings of the mechanisms involved in drought-resistance within the plant-mycorrhizae-soil system.