N2 Fixation and Related Traits of Two Lentil Genotypes Grown Under Free Air CO2 Enrichment (FACE) Facilities in a Semi-Arid Environment.

Rising atmospheric CO₂ concentration ([CO₂]) stimulates plant growth and yields but lowers grain nitrogen concentration ([N]), raising concerns about grain quality in cereals. In theory, legumes can avoid such N reductions through biological nitrogen (N₂) fixation. This was confirmed for studies conducted in high rainfall agro-ecosystems suggested that legumes grown under elevated [CO₂] (e[CO₂]) can overcome grain N limitation, but it is not clear whether and to what extent this can be applied to low rainfall, dryland Mediterranean regions. This study investigated N₂ fixation and grain [N] of two lentil genotypes (PBA Ace and 05H010L-07HS3010, shortened HS3010 from here onwards) in the Australian Grains Free Air CO₂Enrichment (AGFACE) facility, Horsham, Victoria, Australia during a dry (127 mm rainfall in 2015) and a wet (334 mm rainfall in 2016) growing season.

Stimulation of photosynthesis by e[CO₂] was greater in the wet than dry season (1.3 to 1.6 fold), but stimulation of biomass accumulation by e[CO₂] was similar in both seasons at flowering. Percentage of N derived from atmosphere (%Ndfa) was greater under e[CO₂]. N₂ fixation was stimulated by e[CO₂] due to greater number and mass of nodules but decreased uptake of soil N. PBA Ace fixed greater atmospheric N compared to HS3010. Elevated [CO₂] increased grain yield by 64 % and 16 % in the wet and dry season, respectively. Grain [N] was slightly depressed by e[CO₂] in the dry season but increased it (by 2 – 4 %) in the wet season and this was different between the two genotypes. These results suggest that e[CO₂] driven reduction of grain [N]/ protein concentration can be mitigated only in high rainfall regions but not in low rainfall dryland environment.

Keywords: climate change, grain protein, dryland cropping, biological N₂ fixation