Lower grain nitrogen content of wheat at elevated CO2 can be improved through post-anthesis NH4+ supplement

We test the hypothesis that reduction in grain N concentration under elevated CO2 concentration (e[CO2]) is associated with N types (NH4+ and NO3−) and their ratios. Wheat (Triticum aestivum L. cv. H45) was grown in a glasshouse under two CO2 concentrations (389 μmol mol−1 and 700 μmol mol−1), supplied with equal amount of N with different ratios of NH4+ and NO3−: (i) 100% NO3−-N; (ii) 50% NO3−-N and 50% NH4+-N; and (iii) 25% NO3−-N and 75% NH4+-N. Plant growth, N uptake and partitioning were measured during plant development. Plant biomass and grain yield was increased at e[CO2] when N was supplied as an equal proportion of NO3− and NH4+. Despite the yield increment, grain N concentration was not affected by e[CO2], in 50% NO3−-N treatment. In contrast, grain N concentration decreased in 100% NO3−-N and 25% NO3−-N treatments. In 50% NO3−-N treatment, N uptake during post-anthesis stage (from 69 to 141 days after planting) was significantly stimulated under e[CO2] compared to 100% NO3−-N and 25% NO3−-N treatments. We concluded that supplement of N in an equal proportion of NO3− and NH4+ which increases post-anthesis N uptake, avoid the reduction of grain N concentration under e[CO2].