Free air CO2 enrichment (FACE) improves water use efficiency and moderates drought effect on N 2 fixation of Pisum sativum L.

Background and aims: Legume N 2 fixation is highly sensitive to drought. Elevated [CO 2 ] (e[CO 2 ]) decreases stomatal conductance (g s ) and improves water use efficiency (WUE), which may result in soil water conservation and allow N 2 fixation to continue longer under drought. Using a Free-Air CO 2 Enrichment (FACE) approach, this study aimed to elucidate whether e[CO 2 ] improves N 2 fixation of Pisum sativum L. under drought. Methods: In a FACE system, plants were grown in ambient [CO 2 ] (~400 ppm) or e[CO 2 ] (~550 ppm) and subjected to either terminal drought or well-watered treatments. Measurements were taken of photosynthesis, soil water dynamics, water soluble carbohydrates (WSC), amino acids (AA) and N 2 fixation. Results: Lower g s under e[CO 2 ] increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated [CO 2 ] increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N 2 fixation was stimulated (+51%) by e[CO 2 ] in proportion to biomass changes. Under e[CO 2 ] a greater proportion of plant total N was derived from fixed N 2 and a smaller proportion from soil N uptake compared to a[CO 2 ]. Conclusion: This study suggests that e[CO 2 ] increased WUE and this resulted in slower soil water depletion, allowing pea plants to maintain greater nodule activity under drought and resulting in appreciable increases in N 2 fixation. Our results suggest that growth under e[CO 2 ] can mitigate drought effects on N 2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems. © 2019, Springer Nature Switzerland AG.