Fate of 15N-labelled ammonium nitrate with or without the new nitrification inhibitor DMPSA in an irrigated maize crop

Nitrification inhibitors, originally proposed for nitrate leaching mitigation, are recommended as effective nitrous oxide (N2O) mitigation strategies. Several compounds have been trialled and used in the past including dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP). Yet, little is known about the new nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA). A field experiment using N-15 single-labelled ammonium nitrate ((NH4NO3)-N-15 or (NH4NO3)-N-15) was conducted to understand the effectiveness of DMPSA on a biochemical basis in an irrigated maize (Zea mays L.) crop. Gaseous fluxes, i.e. N2O, (N2O)-N-15, N-15(2), methane (CH4), and carbon dioxide (CO2) were measured, as well as soil mineral N, yield components and N-15 recovery in plant and soil. During the maize cropping period, the use of DMPSA significantly reduced cumulative N2O emissions (118 g N ha(-1)) compared to ammonium nitrate without inhibitor (231 g N ha(-1)). The N-15 analyses revealed that most N2O losses (particularly during the emission peak) came from (NH4NO3)-N-15 (i.e. nitrification, nitrifier denitrification and/or coupled nitrification denitrification) rather than (NH4NO3)-N-15 in this calcareous low C-content soil. As expected, DMPSA decreased N2O losses from (NH4+)-N-15 oxidation, but an effect on non-target microorganisms was noticed, as shown by the significant reduction of respiration rates and N2O losses coming from (NO3-)-N-15. No significant effect of DMPSA on CH4 oxidation or N-15(2) fluxes was observed. The DMPSA did not lead to a significant improvement of the dry weights of grain or biomass, although an increment of root biomass by 64% was found. This compound also tended to increase plant N recovery (average 67.8%) and to decrease soil N recovery (average 18.3%) but differences were not statistically significant. Conversely, the nitrification inhibitor significantly reduced the residual fertilizer-N in the 15-30 cm and 30-45 cm soil layers. The use of DMPSA was confirmed as a highly effective tool to reduce N2O emissions from irrigated crops in semi-arid areas.