Effects of inhibitors and slit incorporation on NH3 and N2O emission processes after urea application

The use of urea fertilizers in agriculture is associated with many negative environmental impacts and is a source of ammonia (NH₃) and nitrous oxide (N₂O) emissions. Such losses from urea fertilizer can be avoided by different mitigation techniques. Three different mitigation principles, urease inhibitor (N-(2-Nitrophenyl) phosphoric triamide, 2-NPT) (UI) alone and urease inhibitor in combination with nitrification inhibitors (N-[3(5)-methyl-1 H-pyrazol-1-yl) methyl] acetamide, MPA) (NI) and closed slit incorporation of urea fertilizer into the soil, were compared on a sandy loam soil at a soil water level of 70 % water-holding capacity. An in vitro microcosm approach with open dynamic incubation chambers was used to monitor NH₃ emissions over two weeks with NH₃ sampling by washing bottles. N₂O emissions were studied over ten weeks in slow throughflow mesocosms with continuous gas chromatographic (GC) measurements. To get insights into N₂O production and consumption processes, gas samples were taken after six weeks and N₂O isotopocules were analyzed by isotope ratio mass spectrometry (IRMS). Slit injection showed the greatest effect on NH₃ emission reduction by 79.6 % (40.6 % by UI, and 46.7 % by UINI) compared to surface applied urea. Minor pollution swapping to N₂O was observed at the beginning of the trial due to incorporation but not in the cumulative emissions over the entire incubation time. The reduction effect of UINI on N₂O emissions decreased over time with no cumulative emission reduction at the end of experimentation. N₂O isotopocules confirmed the high contribution of nitrification to N₂O production. In contrast and bacterial denitrification, nitrifier denitrification and fungal denitrification were involved on a much lower level and N₂O reduction to N₂ was not pronounced. All NH₃ mitigation measurements where effective to decrease NH₃ emissions while their effects on N₂O emission varied over time. Factors as crop N uptake and rainfall would further modify the overall effect on N₂O emissions and need to be considered for final pollution swapping assessment. Further research on the impact of NI on non-target microbial communities is warranted to elucidate potential environmental consequences and long-term efficacy of inhibitor compounds.