Towards An Agronomic Assessment of N2O Emissions.

Agricultural soils are the main anthropogenic source of the greenhouse gas (GHG) nitrous oxide (N₂O). These emissions stem to a large extent from the application of inorganic and organic nitrogen (N) fertilizers. Commonly, N₂O emissions are expressed as a function of N application rate. This suggests that higher fertilizer applications are always less desirable from an N₂O point of view. Here, we argue that agronomic parameters should be included in such an assessment. In particular, expressing N₂O emission as a function of crop productivity (expressed per unit aboveground N uptake: ‘yield-scaled’) and N use efficiency (NUE) provides a better basis to express the N₂O efficiency of a cropping system. Highly productive systems with moderate N₂O emissions may be more desirable than marginally productive systems with lower N₂O emissions. First, we show how conventional relations between N application rate, N uptake and N₂O emissions can result in minimal yield-scaled N₂O emissions at intermediate fertilizer levels. Second, we report the key findings of a meta-analysis of 19 field studies encompassing a total of 147 combined measurements of aboveground N uptake and N₂O emissions by annual crops. Yield-scaled N₂O emissions were lowest (8.4 g N₂O-N kg^-1 N uptake) at an application rate of 187 kg N ha^-1. Yield-scaled N₂O emissions were identical in situations of N deficit and balanced N fertilization, but increased strongly at a N surplus. There was a clear negative relation between NUE and yield-scaled N₂O emissions, with  14.8 vs 6.8 g N₂O-N kg^-1 N uptake at 8 vs 85% NUE. We conclude that agricultural management decisions to reduce N₂O emissions should focus on optimizing yield and N-fertilizer use efficiency under median rates of fertilizer-N input.