Urea-based fertilization strategies to reduce yield-scaled N oxides and enhance bread-making quality in a rainfed Mediterranean wheat crop

Urea fertilizer applications to calcareous soils can result in significant nitrous oxide (N2O) and nitric oxide (NO) emissions, predominantly via nitrification rather than denitrification. To address this, we explored several mitigation strategies based on improved urea management in a rainfed winter wheat (Triticum aestivum L.) crop during two consecutive cropping seasons with contrasting rainfall quantities and distribution. The strategies we investigated included the split application of urea at top dressing, the use of nitrification inhibitors (e.g. 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture, DMPSA, and nitrapyrin), the urease inhibitor N-butyl thiophosphorictriamide (NBPT), or the double inhibitor DMPSA + NBPT. Emissions of N2O, NO, methane (CH4), as well as measurements of grain and straw yield and bread-making quality (protein content, reserve protein composition: glutenins and gliadins) were measured. Nitrogen (N) use efficiency (NUE) and N surplus were also calculated. Results were affected by rainfall, since the first cropping season experienced typical rainfall quantity and distribution, whilst the second cropping season was very dry, thus increasing significantly the yield-scaled emissions and N surplus, and markedly decreasing the NUE. In comparison to the single application of urea without inhibitors, all treatments generally decreased surface-scaled and yield-scaled emissions, with urea+DMPSA being the most effective and consistent mitigation option. Split urea and NBPT did not mitigate surface-scaled emissions in the dry cropping season, because of the marked peaks in N oxides after flowering, caused by inefficient crop N uptake. In the typical rainfall cropping season, the use of the double NBPT+DMPSA inhibitor led to the best balance between mitigation of yield-scaled N oxides emissions, N efficiency, crop yield and bread-making quality (i.e. increments in total protein, gliadins and glutenins). We did not observe any effect of nitrification inhibitors on grain yield (except in the dry cropping season) or the composition of gluten proteins. Our results suggest that the use of DMPSA with or without NBPT should be recommended to mitigate yield-scaled emissions of N oxides in rainfed semi-arid crops.