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

<p dir="ltr">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 (<i>Triticum aestivum</i> L.) and field pea (<i>Pisum sativum</i> L.).</p><p dir="ltr">Methods: Plants were grown with 40 or 100 mg N kg⁻¹ under ambient CO₂ (390 ppm) or elevated CO₂ (eCO₂; 550 ppm) using free-air CO₂ enrichment (SoilFACE). Repeated ¹³CO₂ pulse labelling was used to quantify C transfer via plant to the soil. Destructive sampling was performed at grain filling and maturity.</p><p dir="ltr">Results: eCO₂ 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 eCO₂, and for field pea it indicated enhanced biological N₂ fixation. However, eCO₂ increased the C:N ratio of wheat even at the high N level. Greater ¹³C in soil of wheat grown under eCO₂ indicated a minor increase in soil C via rhizodeposition.</p><p dir="ltr">Conclusions: Increased biomass and C:N ratio of wheat could have implications for residue decomposition. eCO₂ and low N tended to increase grain yield but the increase was highly variable and not significant. Additional N content of field pea under eCO₂ exceeded the N that would be removed in wheat grain, albeit with lower than expected grain yield due to dry conditions.</p>