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

Background and aimsLegume N-2 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 N-2 fixation to continue longer under drought. Using a Free-Air CO2 Enrichment (FACE) approach, this study aimed to elucidate whether e[CO2] improves N-2 fixation of Pisum sativum L. under drought.MethodsIn a FACE system, plants were grown in ambient [CO2] (similar to 400ppm) or e[CO2] (similar to 550ppm) 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.ResultsLower 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. N-2 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 N-2 and a smaller proportion from soil N uptake compared to a[CO2].ConclusionThis 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 N-2 fixation. Our results suggest that growth under e[CO2] can mitigate drought effects on N-2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems.