Data from: 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.

1. 干旱会引发氮（N）与磷（P）循环的改变，但多数植物物种在氮磷有效性多变或失衡的环境下，对养分的吸收能力存在显著局限。植物N:P比值的弹性程度，以及与丛枝菌根（AM）真菌的根系共生关系，均可能调控植物应对干旱引发的养分有效性变化的抗性，但该假说尚未得到直接验证。
2. 本研究以半天然草地中的优势物种与从属物种为研究对象，探究了植物N:P化学计量状态与菌根共生关系在其抗旱性中的调控作用。
3. 我们通过避雨棚（rainout shelters）设置水分梯度控制（对照组与干旱组），测定了优势与从属物种的生物量响应特征。随后选取优势种宽叶雀稗（Paspalum dilatatum）与从属种狗牙根（Cynodon dactylon），对其N:P化学计量状态、菌根根系定殖率与水分利用效率开展了检测分析。
4. 干旱胁迫下，所有优势物种的生物量均出现显著下降，而从属物种未出现该变化。由于植物氮吸收量降低，干旱提升了土壤有效氮含量，进而提高了土壤N:P比值。优势种宽叶雀稗表现出极高的N:P稳态性，且在干旱胁迫下生物量大幅降低。与之相反，养分弹性更强的从属种狗牙根提升了自身的氮吸收能力与水分利用效率，这一现象显然源于其与菌根真菌形成了更强的共生关系，最终维持了生物量稳定。
5. 综合讨论：本研究结果表明，当水分胁迫限制了土壤中氮的移动性时，优势物种维持N:P稳态的能力（该能力可能源于其较强的根系养分搜寻能力）会失效。与之相反，本研究证实，更高的N:P化学计量弹性，加上与菌根真菌更强的互利共生关系，能够让从属物种更好地抵御干旱扰动。本研究通过野外试验结合化学计量学方法，首次为植物-菌根-土壤系统内的抗旱机制提供了清晰且新颖的认知。