Water availability moderates N fixation benefit from elevated [CO2]: A 2-year FACE study on lentil (Lens culinaris MEDIK.) in a water limited agro-ecosystem

Increased biomass and yield of plants grown under elevated [CO2 ] (e [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 e [CO2 ], provided N2 -fixation is stimulated by e [CO2 ] in line with growth and yield. In Mediterranean type agro-ecosystems, N2 -fixation may be impaired by drought and it is unclear whether e [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 (FACE) 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 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 e [CO2 ], as a consequence of greater post-flowering N2 fixation. Our results suggest that the benefit for N2 -fixation from e [CO2 ] is high as long as there is enough soil water to continue N2 -fixation during grain filling.