Using nitrification inhibitors and deep placement to tackle the trade-offs between NH 3 and N 2 O emissions in global croplands.

Ammonia (NH 3 ) and nitrous oxide (N 2 O) are two important air pollutants that have major impacts on climate change and biodiversity losses. Agriculture represents their largest source and effective mitigation measures of individual gases have been well studied. However, the interactions and trade-offs between NH 3 and N 2 O emissions remain uncertain. Here, we report the results of a two-year field experiment in a wheat-maize rotation in the North China Plain (NCP), a global hotspot of reactive N emissions. Our analysis is supported by a literature synthesis of global croplands, to understand the interactions between NH 3 and N 2 O emissions and to develop the most effective approaches to jointly mitigate NH 3 and N 2 O emissions. Field results indicated that deep placement of urea with nitrification inhibitors (NIs) reduced both emissions of NH 3 by 67% to 90% and N 2 O by 73% to 100%, respectively, in comparison with surface broadcast urea which is the common farmers' practice. But, deep placement of urea, surface broadcast urea with NIs, and application of urea with urease inhibitors probably led to trade-offs between the two gases, with a mitigation potential of -201% to 101% for NH 3 and -112% to 89% for N 2 O. The literature synthesis showed that deep placement of urea with NIs had an emission factor of 1.53%-4.02% for NH 3 and 0.22%-0.36% for N 2 O, which were much lower than other fertilization regimes and the default values recommended by IPCC guidelines. This would translate to a reduction of 3.86-5.47 Tg N yr -1 of NH 3 and 0.41-0.50 Tg N yr -1 of N 2 O emissions, respectively, when adopting deep placement of urea with NIs (relative to current practice) in global croplands. We conclude that the combination of NIs and deep placement of urea can successfully tackle the trade-offs between NH 3 and N 2 O emissions, therefore avoiding N pollution swapping in global croplands.