Carbon isotope composition for agronomic diagnostic: Predicting yield and yield response to nitrogen in wheat

Rainfed crops rely on two sources of water: stored soil water at sowing and seasonal rain. In strongly seasonal winter-rainfall environments, stored soil water at sowing is minor, and uncertain seasonal rainfall is a source of risk. In south-eastern Australia, under-fertilisation is a common outcome of nitrogen risk management with implications for wheat yield and mining of soil organic matter. Here we explore the use of carbon isotope composition (δ 13C) to capture the effects of crop water status on grain yield in a context of nitrogen top dressing. In the sampled environments, crops receive at least 50% of seasonal rainfall by stem elongation, and at least 70% of seasonal rainfall by flowering. In a sample of 1518 plots, yield varied from 0.07 to 9.96 t ha-1 and correlated with δ 13C measured with isotope ratio mass spectrometer (IRMS) at flowering (r = −0.76, p