Carbon and nitrogen partitioning of wheat and field pea grown with two nitrogen levels under elevated CO2

Background and Aims Crop responses to elevated atmospheric CO2 are likely to be different in semi-arid cropping systems of Australia. This experiment aimed to investigate the interactive effects of atmospheric CO2 and nitrogen (N) fertiliser on carbon (C) and N partitioning in the soil-plant system of Wheat (Triticum aestivum L.) and field pea (Pisum sativum L.). Methods Plants were grown with 40 or 100 mg N kg(-1) under ambient CO2 (390 ppm) or elevated CO2 (eCO(2); 550 ppm) using free-air CO2 enrichment (SoilFACE). Repeated 13CO(2) pulse labelling was used to quantify C transfer via plant to the soil. Destructive sampling was performed at grain filling and maturity. Results eCO(2) increased shoot biomass of field pea (36 %) and wheat (55 %) but only increased root biomass of wheat (13.5 %) in the 25-50 cm soil layer. Total N content of both species was greater under eCO(2), and for field pea it indicated enhanced biological N-2 fixation. However, eCO(2) increased the C:N ratio of wheat even at the high N level. Greater C-13 in soil of wheat grown under eCO(2) indicated a minor increase in soil C via rhizodeposition. Conclusions Increased biomass and C:N ratio of wheat could have implications for residue decomposition. eCO(2) and low N tended to increase grain yield but the increase was highly variable and not significant. Additional N content of field pea under eCO(2) exceeded the N that would be removed in wheat grain, albeit with lower than expected grain yield due to dry conditions.