Nitrogen fertilizers amended with nitrification inhibitors (NIs) are used to increase nitrogen use efficiencies in agricultural systems. 3,4-Dimethylpyrazole phosphate (DMPP) is the most successful commercial NI to date but has a highly variable efficacy. To explore whether degradation could contribute to its inconsistent performance, incubation studies were performed with DMPP and 3,4-dimethylpyrazole glycolate (DMPG) in two alkaline clay soils that were treated with the fertilizer ammonium sulfate ((NH 4 ) 2 SO 4 ). Analysis of the soil extracts revealed a qualitative correlation between the amount of NI present in the soil and inhibition efficiency as well as several degradation products resulting from the oxidation of a methyl side chain and dimerization. A similar outcome was obtained for the degradation in sterilized soil and in accelerated weathering studies in the absence of soil. Our data suggest that chemical and not microbiological pathways are primarily responsible for the degradation of this inhibitor, which could potentially be initiated by reactive oxygen species (ROS) resulting from both biotic and abiotic processes in soils.

Gas-phase reaction of the aromatic distonic radical cations 4-Pyr + * and 3-Pyr + * with amines led to formation of the corresponding amino pyridinium ions 4-Pyr + NR 2 and 3-Pyr + NR 2 through amine addition at the site of the radical, followed by homolytic β-fragmentation. The reaction efficiencies range from 66-100 % for 4-Pyr + * and 57-86 % for 3-Pyr + * , respectively, indicating practically collision-controlled processes in some cases. Computational studies revealed that the combination of positive charge and spin makes nucleophilic attack by the amine at the site of the radical barrierless and strongly exothermic by about 175 � 15 kJ mol À 1 , thereby rendering 'conventional' radical pathways through hydrogen abstraction or addition onto π systems less important. Exemplary studies with 4-Pyr + * showed that this reaction can be reproduced in solution. A similar addition/radical fragmentation sequence occurs also in the gas-phase reaction of amines with the aliphatic distonic radical cation Oxo + C * , showing that the observed charge-spin synergism is not limited to aromatic systems.