A new model for agronomic nitrogen efficiency in cereals based on yield window

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Context and objective: Understanding the response of cereal yields to fertilizer nitrogen (N) is crucial in modelling food security within planetary boundaries, and in economic optimization of N use within environmental limits. Here we explore how agronomic N efficiency (N-AE, kg grain per kg N applied) in cereals depends on locally attainable yield (Ymax, highest yield observed per trial) and control yield (Y0, yield at zero fertilizer-N). Methods: Data were collected from 593 N response trials with winter wheat and spring wheat (Triticum aestivum L.), spring barley (Hordeum vulgare L.), and spring oats (Avena sativa L.) in Denmark, the Netherlands, Spain, the UK and Sweden, giving 3268 N-AE data points. We applied linear mixed effect modelling to quantify the effects of N rate (NF), Ymax, Y0, and the ‘yield window’ ΔY (the difference Ymax-Y0) on N-AE. Results and Conclusions: N-AE was largely governed by the yield window, besides the well-known effect of N rate. As such, the yield window captured effects of soil, year, cultivar and country on N-AE. Combined, ΔY and NF explained 80% (barley), 86% (winter wheat), 94% (spring wheat), and 91% (oats) of N-AE variance. Depending on crop species, a widening of ΔY by 1 t/ha raised N-AE by 4.7–6.5 kg grain per kg N, at N rates common in local practice. Substantial differences in mean N-AE at given N rate were found between data subsets by country. Economically optimal N rate (Nopt) varied widely between subsets, with averages of 175 kg N/ha (UK trials) and 215 kg N/ha (Swedish trials) at N-to-grain price ratio P=3; or 156 and 200 kg/ha, respectively, at P=6. Across trials, N-AE at Nopt responded linearly to ΔY by 5.0 (spring barley) and 3.2 (winter wheat) kg/kg per 1 t/ha increment of ΔY, independent of price ratio P. Significance: The new concept of yield window provides opportunities for N optimization in modelling and practice. A larger yield window reduces N requirement per unit grain output well beyond reductions commonly associated with changes in yield potential in food security assessments. The newly presented N-AE model enables better estimation of N requirements to meet cereal production targets. It also enables a generic expression for optimal N rate as a function of yield window. Provided that control yield can be estimated, the model may contribute to more sustainable N management in cereals.

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