Water availability moderates N2 fixation benefit from elevated [CO2]: A 2‐year free‐air CO2 enrichment study on lentil (Lens culinaris MEDIK.) in a water limited agroecosystem

Increased biomass and yield of plants grown under elevated [CO2] often corresponds to decreased grain N concentration ([N]), diminishing nutritional quality of crops. Legumes through their symbiotic N2 fixation may be better able to maintain biomass [N] and grain [N] under elevated [CO2], provided N2 fixation is stimulated by elevated [CO2] in line with growth and yield. In Mediterranean‐type agroecosystems, N2 fixation may be impaired by drought, and it is unclear whether elevated [CO2] stimulation of N2 fixation can overcome this impact in dry years. To address this question, we grew lentil under two [CO2] (ambient ~400 ppm and elevated ~550 ppm) levels in a free‐air CO2 enrichment facility over two growing seasons sharply contrasting in rainfall. Elevated [CO2] stimulated N2 fixation through greater nodule number (+27%), mass (+18%), and specific fixation activity (+17%), and this stimulation was greater in the high than in the low rainfall/dry season. Elevated [CO2] depressed grain [N] (−4%) in the dry season. In contrast, grain [N] increased (+3%) in the high rainfall season under elevated [CO2], as a consequence of greater post‐flowering N2 fixation. Our results suggest that the benefit for N2 fixation from elevated [CO2] is high as long as there is enough soil water to continue N2 fixation during grain filling. Using a free‐air CO2 enrichment facility, this study found that elevated [CO2] stimulated N2 fixation in lentil through increasing number, mass, and specific activity of root nodules, but the effect was different between a low and high rainfall year. Only in the high rainfall year, N2 fixation continued until late in the season, and grain N concentration was maintained under elevated [CO2]. These findings suggest that elevated [CO2] stimulation of N2 fixation can optimize N supply to legume grains, but only if sufficient water maintains symbiotic fixation activity during the grain filling period.