Free air CO2 enrichment (FACE) improves water use efficiency and moderates drought effect on N2 fixation of Pisum sativum L.

Background and aims: Legume N₂ fixation is highly sensitive to drought. Elevated [CO₂] (e[CO₂]) decreases stomatal conductance (g_s) and improves water use efficiency (WUE), which may result in soil water conservation and allow N₂ fixation to continue longer under drought. Using a Free-Air CO₂ Enrichment (FACE) approach, this study aimed to elucidate whether e[CO₂] improves N₂ fixation of Pisum sativum L. under drought.Methods: In a FACE system, plants were grown in ambient [CO₂] (~400 ppm) or e[CO₂] (~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 N₂ fixation.Results: Lower g_s under e[CO₂] increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated [CO₂] increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N₂ fixation was stimulated (+51%) by e[CO₂] in proportion to biomass changes. Under e[CO₂] a greater proportion of plant total N was derived from fixed N₂ and a smaller proportion from soil N uptake compared to a[CO₂].Conclusion: This study suggests that e[CO₂] 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 N₂ fixation. Our results suggest that growth under e[CO₂] can mitigate drought effects on N₂ fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems. [ABSTRACT FROM AUTHOR]