Influence of a nitrification inhibitor and of placed N-fertilization on N₂O fluxes from a vegetable cropped loamy soil.

Arable soils are a major source of the climate relevant trace gas nitrous oxide (N2O). Although N2O emissions from soils increase with the amount of N-fertilizer, there is still a lack of data for intensively fertilized systems, such as vegetable production. We investigated the effect of an ammonium sulfate nitrate (ASN) fertilization either placed or broadcast applied combined with a nitrification inhibitor (3,4-dimethylepyrazole phosphate (DMPP)) on soil surface N2O fluxes as compared to conventional broadcast ASN fertilization in a lettuce–cauliflower rotation over two years of measurement. Except for a lower cauliflower yield in the second experimental year with placed fertilization, no differences in yields between the fertilized treatments were observed. Annual cumulative N2O emissions of the conventionally fertilized treatment were 8.8 and 4.7 kg N2O-N ha−1 yr−1 for the first and second experimental year, respectively, indicating a high inter-annual variability. The addition of the nitrification inhibitor significantly reduced N2O emissions during the cropping season and also during the winter period, resulting in an annual reduction of 45 and 40% as compared to the conventionally fertilized (CONV) treatment. The reason for the lower N2O release in the DMPP treatment as compared to the conventionally fertilized treatment remained unclear. Since we did not find any significant differences in the mineral N pools during periods with distinctive inhibition, this can be ruled out as reason for the lower N2O release in the DMPP treatment. We found lower soil respiration in the DMPP treatment during several months starting about six weeks after fertilizer application. In contrast to the treatment with nitrification inhibitor, the placed fertilization as an N-depot (fertilizer bands inserted into the soil) did not reduce annual N2O emissions, although the ratio of ammonium (NH4+) to nitrate (NO3−) in the first weeks after N-application indicate inhibition of nitrification in the fertilizer depot. We assume that, even though NH4+ concentrations in the depots were high, toxicity was not sufficient for a complete inhibition of microbial activity in the surrounding of the depots, resulting in considerable N2O production. The emission factors calculated for CONV treatment were 1.6 and 0.8% for the first and second experimental year, respectively. For the treatment with nitrification inhibitor (NI), they were only 0.9 and 0.5%; for the treatment with placed fertilization as an N-depot (DEPOT) 2.0 and 0.8%. They were thus within the range proposed by the guidelines of the IPCC (2006). However, although the N-input related N2O emission factors were within the range proposed by the guidelines of the IPCC, the absolute N2O emissions from the intensively fertilized vegetable field were high. For effective, but environmentally sound vegetable production, a deeper understanding of nitrification inhibitory strategies is necessary