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 yield and soil mining. Here we explore the use of carbon isotope composition ({delta} 13C) to capture the effects of water in the prediction of yield and guide nitrogen management. In the sampled environment, 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 {delta} 13C measured with isotope ratio mass spectrometer (IRMS) at flowering; this is consistent with the rainfall pattern and the physiology of the crop featuring a critical period for yield from 300 {degrees}Cd before to 100 {degrees}Cd after anthesis. In a sample of 135 plots, yield varied from 1.2 to 8.4 t ha-1 and correlated with {delta} 13C measured with IRMS at stem elongation. Yield response to nitrogen, defined as the difference between yield in fertilised crops (50 to 200 kg N ha-1) and unfertilised controls, correlated with {delta} 13C measured with IRMS at stem elongation, except for late-sown crops. Mid-infrared spectroscopy (MIR) returned estimates of {delta} 13C that agreed with {delta} 13C measured with IRMS (calibration: R2 = 0.82, RMSE = 0.53{per thousand}, n = 833; validation: R2 = 0.70, RMSE = 0.75{per thousand}, n = 364). We conclude that a MIR based, high-throughput, affordable measurement of {delta} 13C could be scaled to guide nitrogen management of wheat in winter-rainfall environments.