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

Agricultural soils are the main anthropogenic source of nitrous oxide (N 2 O), largely because of nitrogen (N) fertilizer use. Commonly, N 2 O emissions are expressed as a function of N application rate. This suggests that smaller fertilizer applications always lead to smaller N 2 O emissions. Here we argue that, because of global demand for agricultural products, agronomic conditions should be included when assessing N 2 O emissions. Expressing N 2 O emissions in relation to crop productivity (expressed as above‐ground N uptake: ‘yield‐scaled N 2 O emissions') can express the N 2 O efficiency of a cropping system. We show how conventional relationships between N application rate, N uptake and N 2 O emissions can result in minimal yield‐scaled N 2 O emissions at intermediate fertilizer‐N rates. Key findings of a meta‐analysis on yield‐scaled N 2 O emissions by non‐leguminous annual crops (19 independent studies and 147 data points) revealed that yield‐scaled N 2 O emissions were smallest (8.4 g N 2 O‐N kg −1 N uptake) at application rates of approximately 180–190 kg N ha −1 and increased sharply after that (26.8 g N 2 O‐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 N 2 O 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 N 2 O emissions was found. Therefore, we argue that agricultural management practices to reduce N 2 O emissions should focus on optimizing fertilizer‐N use efficiency under median rates of N input, rather than on minimizing N application rates.