Using inhibitors to trade greenhouse gas emission for ammonia losses in paddy soil: A zero-sum game

Nitrogen (N) amendment of soil ecosystems alters GHG fluxes to the atmosphere by increasing nitrous oxide (N₂O) emission and affecting methane (CH₄) uptake in well-aerated soils. Although nitrification inhibitors (NIs) can decrease N₂O emission, this may alter the rate of ammonia (NH₃) volatilization, crop yield, and soil CH₄ fluxes by directly or indirectly affecting methane-oxidizing bacteria, causing severe environmental and economic damages. Here, we measured the response of gaseous N loss (N₂O + NH₃) and GHG fluxes (CH₄ + direct N₂O + indirect N₂O) to the intensive application of three different rates of DCD (15 to 45 kg ha⁻¹) and DMPP (3 to 9 kg ha⁻¹) in N fertilized soils imposed upon six different rice cultivars. N addition increased NH₃ volatilization by several orders of magnitude, N₂O emission by 7- to 10-fold, and CH₄ emission by 2- to 4-fold. DCD and DMPP strongly inhibited the N₂O emission and, to a lesser extent, CH₄ emission but promoted greater NH₃ losses in a dose-dependent manner. The soil receiving urea with inhibitors exhibited up to 62.7% additional NH₃ volatilization than untreated (inhibitor-free) soils over the rice growth cycle. Considering the estimated indirect N₂O emission from deposited NH₃, the overall inhibitory effect of nitrification inhibitors (at different application rates) ranged from 17.4% to 36.6% (reduction) for global warming potential and -6.7% to -45.9% (increase) for nitrogen losses. Collectively, our results suggest moderate inhibition of global warming potential but inadequate reduction of gaseous nitrogen losses by NIs, which were outweighed by increased NH₃ volatilization. Consequently, NIs appear to be a potentially double-edged sword when targeting to diminish gaseous nitrogen losses from N fertilized soils.