Introducing their work, Butterly et al. (2016) write that rising atmospheric CO₂ concentrations are projected to increase the productivity of agricultural cropping systems in the future, primarily via enhanced photosynthesis and reduced evapotranspiration when water and nutrients are not limiting. One field crop that is economically important in many semi-arid locations is the common pea plant (Pisum sativum); yet according to Butterly et al., “few studies have examined the effects of elevated CO₂ on field pea.” Therefore, in an attempt to rectify this situation, the team of four Australian researchers set out to examine the interactive effects of elevated CO₂ and soil nitrate (NO₃^-) concentration on the growth, nodulation, and nitrogen (N₂) fixation of pea plants. Nodules house bacteria that “fix” atmospheric nitrogen into ammonia, which serves as plant food.

The study was conducted in a semi-arid location at the SoilFACE facility of the Department of Economic Development, Jobs, Transport and Resources Plant Breeding Centre in Horsham, Victoria, Australia. There, pea plants were grown for a period of 15 weeks in Vertisol soils containing either 5, 25, 50 or 90 mg NO₃^--N kg^-1 under either ambient (390 ppm) or elevated (550 ppm) carbon dioxide concentrations maintained using free-air CO₂ enrichment (SoilFACE). It was the hypothesis of the researchers that “nodule establishment (nodule number), development (nodule mass) and function (nitrogenase activity, N derived from the atmosphere) would be progressively inhibited with increasing NO₃^- (nitrate) concentration, but these effects would be reduced under elevated CO₂ via enhanced N demand due to greater photosynthetic activity and plant biomass accumulation.”

The results of their analysis confirmed the inhibitory effects of soil nitrate concentration on field pea plants growing under ambient CO₂. In the elevated CO₂ treatment, however, field pea plants had approximately 30 percent more biomass and were not affected by N level (see figure below). What is more, Butterly et al. report that “elevated CO₂ alleviated the inhibitory effect of soil NO₃^- on nodulation and N₂ fixation,” which impressive finding they say “is likely to lead to greater total N content of field pea growing under future elevated CO₂ environments.” And the end result of these findings, they add, “indicate that field pea may perform well in semiarid agricultural systems under future CO₂ concentrations irrespective of soil N status, and subsequent gains in N input via enhanced N₂ fixation will be important for maintaining the N fertility of cropping systems.”

Now that is good news worth reporting!

Figure 1. Shoot (Panel A) and root (Panel B) biomass of field pea grown for 15 weeks under either an ambient (aCO₂) or elevated (eCO₂) carbon dioxide concentration and with 5, 25, 50 or 90 mg NO3^--N kg^-1 soil.

Reference

Butterly, C.R., Armstrong, R., Chen, D. and Tang, C. 2016. Free-air CO₂ enrichment (FACE) reduces the inhibitory effect of soil nitrate on N₂ fixation of Pisum sativum. Annals of Botany 117: 177-185.