Paper Reviewed
Parvin, S., Uddin, S., Bourgault, M., Roessner, U., Tausz-Posch, S., Armstrong, R., O'Leary, G., Fitzgerald, G. and Tausz, M. 2018. 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. Plant, Cell & Environment 41: 2418-2434.

Symbiotic nitrogen-fixing bacteria exist within specialized nodules associated with leguminous plant roots. These bacteria use the enzyme nitrogenase to convert atmospheric nitrogen (N2), which plants cannot directly use, into ammonium (NH4⁺), which is readily utilized by plants. Nitrogen acquired in this manner can then be incorporated into larger N-containing compounds that are translocated into various plant organs to assist in their development and growth.

Working with two lentil (Lens culinaris) genotypes (PBA Ace and 05H010L-07HS3010), Parvin et al. (2018) recently investigated the impact of plant nitrogen acquisition on this legume species that is widely cultivated as a grain food source throughout the world. Their experiment was conducted at the AGFACE free-air CO2 enrichment facility near Horsham, Victoria, Australia. There, over two full growing seasons the researchers exposed the two lentil cultivars to either ambient (~400 ppm) or elevated (~550 ppm) levels of atmospheric CO2 during daylight hours only, collecting a series of measurements pertaining to their growth response, nodule metabolites, tissue N concentration and N2 fixation, soil N uptake, and the allocation, partitioning and remobilization of N in plant organs. Rainfall during the first growing season (2015) was very low (145.6 mm below the long-term average), whereas in the following year (2016) it was relatively high at 110 mm above the long-term mean. Consequently an additional 96 mm of irrigation water was added during the 2015 growing season to avoid drought-induced crop failure.

In terms of plant growth and yield, the results of the two-year experiment revealed that elevated CO2 increased the total biomass and grain yields of cultivar PBA Ace by 23.8% and 14.9% during 2015 and by 27.1% and 55.4% during 2016. Similarly, elevated CO2 enhanced the total biomass and grain yields of cultivar 05H010L-07HS3010 by 21.5% and 22.9% in 2015 and 31.4% and 55.8% in 2016.

Parvin et al. also found that elevated CO2 increased the number of nodules (+27%), the nodule mass (18%) and nodule fixation activity (+17% in each of the two lentil cultivars, which combination of changes resulted in an overall CO2-induced stimulation of N2 fixation that was greater in the wet growing season of 2016 versus the dry growing season of 2015. The researchers also report that the stimulation of N2 fixation under elevated CO2 during the wet year of 2016 "was more than sufficient to meet the increased N demand of stimulated biomass growth, so that soil N uptake decreased [in] absolute terms." In contrast, they say that "although N2 fixation was still sufficiently increased to meet (lower) additional demand [in the dry growing season], soil N uptake remained unaffected."

Lastly, Parvin et al. further observed that "drought also changed the effect of elevated CO2 on N allocation patterns within the plants, and most importantly, to the grains, so that grain N concentration decreased under elevated CO2 in the dry year but not in the higher rainfall year" (i.e., there was a 4% decline in grain N concentration in 2015 versus a 3% increase in 2016).

In commenting on their several findings, Parvin et al. conclude that "climate-adapted management options that maintain soil water later into the growing season in a legume system [such as irrigation] would maximize N2 fixation and contribute to maintaining grain protein as well as add more N into crop rotation systems." Consequently, the results of this study indicate that, under adequate soil water conditions, elevated CO2 will increase both the quantity (grain yield) and quality (grain N concentration) of these two lentil cultivars.