Using nitrification inhibitors to mitigate agricultural N2 O emission: a double-edged sword?

Nitrification inhibitors show promise in decreasing nitrous oxide (N2 O) emission from agricultural systems worldwide, but they may be much less effective than previously thought when both direct and indirect emissions are taken into account. Whilst nitrification inhibitors are effective at decreasing direct N2 O emission and nitrate (NO3- ) leaching, limited studies suggest that they may increase ammonia (NH3 ) volatilization and, subsequently, indirect N2 O emission. These dual effects are typically not considered when evaluating the inhibitors as a climate change mitigation tool. Here, we collate results from the literature that simultaneously examined the effects of nitrification inhibitors on N2 O and NH3 emissions. We found that nitrification inhibitors decreased direct N2 O emission by 0.2-4.5 kg N2 O-N ha-1 (8-57%), but generally increased NH3 emission by 0.2-18.7 kg NH3 -N ha-1 (3-65%). Taking into account the estimated indirect N2 O emission from deposited NH3 , the overall impact of nitrification inhibitors ranged from -4.5 (reduction) to +0.5 (increase) kg N2 O-N ha-1 . Our results suggest that the beneficial effect of nitrification inhibitors in decreasing direct N2 O emission can be undermined or even outweighed by an increase in NH3 volatilization.