Can nitrogen fertiliser maintain wheat (Triticum aestivum) grain protein concentration in an elevated CO 2 environment? Cassandra Walker A,D , Roger Armstrong A,B , Joe Panozzo A , Debra Partington A , and Glenn Fitzgerald A,C A Department of Economic Development, Jobs, Transport and Resources, 110 Natimuk Rd, Horsham, Vic. 3400, Australia. B Department of Animal, Plant and Soil Sciences, AgriBio-Centre for AgriBioscience, La Trobe University, 5 Ring Rd, Bundoora, Vic. 3083, Australia. C Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, 4 Water St, Creswick, Vic. 3363, Australia. D Corresponding author. Email: cassandra.walker@ecodev.vic.gov.au Abstract. The effect of different nitrogen (N) management strategies (i.e. N rate; 0, 25, 50, 100 kg ha –1 , split N application, foliar N application, legume precropping) were assessed for how they may reverse the reduction of grain protein concentration (GPC) under elevated CO 2 (eCO 2 ; 550 mmol mol –1 ) of wheat (cv. Yitpi) using the Australian Grains Free Air CO 2 Enrichment facility. GPC did not increase significantly under eCO 2 for most of the N management strategies assessed when compared with ambient CO 2 (aCO 2 ; 390 mmol mol –1 ). Grain yield of cv. Yitpi under aCO 2 increased by 43% (P < 0.001) with application of 100 kg N ha –1 when compared with 0 kg N ha –1 at sowing; this response was approximately double (82%) when 100 kg N ha –1 was applied under eCO 2 conditions. Under aCO 2 conditions, by adding 100 kg N ha –1 at sowing, the GPC increased by 37% compared with the GPC at N0; whereas under eCO 2 conditions, by adding the same quantity of N fertiliser, the GPC increased by only 28%. The highest level of N applied (100 kg ha –1 ), chosen for economic and practical reasons in a low-rainfall, yield-limiting environment, was lower than that reported in other global studies (250–350 kg ha –1 ). In a low-rainfall, yield-limiting environment, it is not practical to increase GPC by applying N alone; new cultivars may be required if grain growers are to maintain grain protein (and functionality) in the future as CO 2 levels continue to increase. Additional keywords: Australian Grains Free Air CO 2 Enrichment (AGFACE), nitrogen management. Received 3 February 2017, accepted 23 May 2017, published online 13 July 2017 Introduction Atmospheric carbon dioxide (CO 2 ) levels have risen from 280 mmol mol –1 in the 1750s to an unprecedented level of over 400 mmol mol –1 in 2016 (Prentice et al. 2001; Hatfield et al. 2011; McGee 2016). In the next 35–50 years, atmospheric CO 2 levels are predicted to increase exponentially, reaching ~550 mmol mol –1 by the year 2050 (Intergovernment Panel on Climate Change (IPCC) 2013, RCP8.5 scenario). To better understand how rising CO 2 will affect plants, free air carbon dioxide enriched (FACE) studies have been established throughout the world. FACE facilities, such as the Australian Grains FACE (AGFACE) facility in Horsham (Vic., Australia; Mollah et al. 2009; Fitzgerald et al. 2016), have been used to evaluate crop responses to elevated CO 2 (eCO 2 ) conditions under field conditions typical of dryland cropping systems. Wheat cultivars typically exhibit increases in plant biomass, grain yield and grain weight, and decreases in grain protein concentration (GPC) when grown under eCO 2 conditions (Kimball et al. 1995, 2001; Blumenthal et al. 1996; Taub et al. 2008; Högy et al. 2009; Erbs et al. 2010; Fernando et al. 2012; Panozzo et al. 2014; Fitzgerald et al. 2016) but the effects on yield and its components when N fertiliser is applied under eCO 2 are less well understood in low-rainfall (dryland) conditions. In wheat and other C 3 plants, the N concentration of the leaf, plant and grain in plants grown under eCO 2 conditions generally decreases by 10–15% compared with wheat grown under current ambient CO 2 (aCO 2 ) conditions (Conroy and Hocking 1993; Taub et al. 2008; Panozzo et al. 2014). Many hypotheses have been proposed to account for the decrease in GPC, including: (1) the ‘dilution effect’ as a result of increased biomass production and C uptake; (2) a decrease in mass flow; (3) changes in root growth, architecture and functionality; (4) decreased nitrate reduction; and (5) changes in nutrient allocation and remobilisation under eCO 2 (Conroy et al. 1994; Bloom et al. 2010, 2014; Tausz-Posch et al. 2014; Broberg et al. 2017). The decrease in N concentration was thought to be due primarily to a dilution effect of the starch (Mooney and Koch 1994; Broberg Journal compilation Ó CSIRO 2017 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2017, 55, 518–523 http://dx.doi.org/10.1071/SR17049