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

Background and Aims: Crop responses to elevated atmospheric CO are likely to be different in semi-arid cropping systems of Australia. This experiment aimed to investigate the interactive effects of atmospheric CO 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 under ambient CO (390 ppm) or elevated CO (eCO; 550 ppm) using free-air CO enrichment (SoilFACE). Repeated CO pulse labelling was used to quantify C transfer via plant to the soil. Destructive sampling was performed at grain filling and maturity. Results: eCO 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, and for field pea it indicated enhanced biological N fixation. However, eCO increased the C:N ratio of wheat even at the high N level. Greater C in soil of wheat grown under eCO 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 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 exceeded the N that would be removed in wheat grain, albeit with lower than expected grain yield due to dry conditions. [ABSTRACT FROM AUTHOR]