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

Abstract

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: 'yieldscaled 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.

Biological sources and sinks of nitrous oxide and strategies to mitigate emissions

“…The pervasive effect of pH on the product stoichiometry of denitrification lies within the pH range 5-7, that of most agricultural soils. A recent paper points out the importance of assessing emissions according to the unit of product [75]. It shows very clearly the rapid increase in N 2 O emissions when N fertilizer is added in excess of crop requirements.…”

Intensification of grassland and forage use: driving forces and constraints

“…4), but this picture is often reversed when emissions are expressed per unit of product. For example, Van Groenigen et al (2010) showed that yield-scaled N 2 O emissions are lowest around optimal N fertilisation levels, where crop yields are near maximum attainable yields. Both suboptimal and over-optimal N fertilisation leads to higher N 2 O emissions per unit of crop produced.…”

Greenhouse gas (N2O and CH4) fluxes under nitrogen-fertilised dryland wheat and barley on subtropical Vertosols: risk, rainfall and alternatives

“…Simply aiming to minimise N 2 O emissions with no regard to continued profitable grain production will produce strategies that are unlikely to be adopted by grain growers. Instead, using a yield-scaled emissions approach such as EI should ensure that proposed N 2 O-mitigation strategies are both agronomically efficient and economically profitable (Van Groenigen et al 2010). Substantially more data from direct field measurements of N 2 O emissions are needed so that EFs and EIs can be applied that are at least more specific to region, crop type and soil (Ogle et al 2014).…”