Free-air CO2 enrichment (FACE) reduces the inhibitory effect of soil nitrate on N2 fixation of Pisum sativum

Background and Aims Additional carbohydrate supply resulting from enhanced photosynthesis under predicted future elevated CO2 is likely to increase symbiotic nitrogen (N) fixation in legumes. This study examined the interactive effects of atmospheric CO2 and nitrate (NO3-) concentration on the growth, nodulation and N fixation of field pea (Pisum sativum) in a semi-arid cropping system. Methods Field pea was grown for 15 weeks in a Vertosol containing 5, 25, 50 or 90 mg NO3--N kg(-1) under either ambient CO2 (aCO(2); 390 ppm) or elevated CO2 (eCO(2); 550 ppm) using free-air CO2 enrichment (SoilFACE). Key Results Under aCO(2), field pea biomass was significantly lower at 5 mg NO3--N kg(-1) than at 90 mg NO3--N kg(-1) soil. However, increasing the soil N level significantly reduced nodulation of lateral roots but not the primary root, and nodules were significantly smaller, with 85 % less nodule mass in the 90 NO3--N kg(-1) than in the 5 mg NO3--N kg(-1) treatment, highlighting the inhibitory effects of NO3-. Field pea grown under eCO(2) had greater biomass (approx. 30 %) than those grown under aCO(2), and was not affected by N level. Overall, the inhibitory effects of NO3- on nodulation and nodule mass appeared to be reduced under eCO(2) compared with aCO(2), although the effects of CO2 on root growth were not significant. Conclusions Elevated CO2 alleviated the inhibitory effect of soil NO3- on nodulation and N-2 fixation and is likely to lead to greater total N content of field pea growing under future elevated CO2 environments.