Towards an agronomic assessment of N₂O emissions: a case study for arable crops

Agricultural soils are the main anthropogenic source of nitrous oxide (N₂O), largely because of nitrogen (N) fertilizer use. Commonly, N₂O emissions are expressed as a function of N application rate. This suggests that smaller fertilizer applications always lead to smaller N₂O emissions. Here we argue that, because of global demand for agricultural products, agronomic conditions should be included when assessing N₂O emissions. Expressing N₂O emissions in relation to crop productivity (expressed as above-ground N uptake: ‘yield-scaled N₂O emissions') can express the N₂O efficiency of a cropping system. We show how conventional relationships between N application rate, N uptake and N₂O emissions can result in minimal yield-scaled N₂O emissions at intermediate fertilizer-N rates. Key findings of a meta-analysis on yield-scaled N₂O emissions by non-leguminous annual crops (19 independent studies and 147 data points) revealed that yield-scaled N₂O emissions were smallest (8.4 g N₂O-N kg⁻¹N uptake) at application rates of approximately 180–190 kg N ha⁻¹ and increased sharply after that (26.8 g N₂O-N kg⁻¹ N uptake at 301 kg N ha⁻¹). If the above-ground N surplus was equal to or smaller than zero, yield-scaled N₂O emissions remained stable and relatively small. At an N surplus of 90 kg N ha⁻¹ yield-scaled emissions increased threefold. Furthermore, a negative relation between N use efficiency and yield-scaled N₂O emissions was found. Therefore, we argue that agricultural management practices to reduce N₂O emissions should focus on optimizing fertilizer-N use efficiency under median rates of N input, rather than on minimizing N application rates.