The effect of nitrification inhibitors on the nitrous oxide (N2O) release from agricultural soils-a review

The use of nitrification inhibitors (NI) is a technique which is able to improve N fertilizer use efficiency, to reduce nitrate leaching and to decrease the emission of the climate-relevant gas N2O simultaneously, particularly in moderately fertilized agricultural systems adapted to plant N demand. The ammonia monooxygenase (AMO) is the first enzyme which is involved in the oxidation of NH4+ to NO3- in soils. The inhibition of the AMO by NIs directly decreases the nitrification rate and it reduces the NO3- concentration which serves as substrate for denitrification. Hence, the two main pathways of N2O production in soils are blocked or their source strength is at least decreased. Although it has been shown that archaea are also able to oxidize NH3, results from literature suggest that the enzymatic activity of NH3 oxidizing bacteria is the most important target for NIs because it was much stronger affected. The application of NIs to reduce N2O emissions is most effective under conditions in which the NI remains close to the N - fertilizer. This is the case when the NI was sprayed on mineral - N fertilizer granules or thoroughly mixed with liquid fertilizers. Most serious problems of spatial separation of NI and substrate emerge on pasture soils, where N2O hotspots occur under urine and to a lesser extent under manure patches. From the few studies on the effect of different NI quantities it seems that the amount of NI necessary to reduce N2O emissions is below the recommendations for NI amounts in practice. NIs can improve the fertilizer value of liquid manure. For instance, the addition of NIs to slurry can increase N uptake and yield of crops when NO3- - N leaching losses are reduced. It has clearly been demonstrated that NIs added to cattle slurry are very effective in reducing N2O as well as NO emissions after surface application and injection of slurry into grassland soils. In flooded rice systems NIs can reduce CH4 emission significantly, whereas the effect on CO2 emission is varying. On the other hand, as an effect of the delay of nitrification by NIs, NH3 emission might increase when N fertilizers are not incorporated into the soil. As compared to other measures NIs have a high potential to reduce N2O emissions from agricultural soils. Further, no other measure has so consistently been proofed according its efficiency to reduce N2O emissions. From the published data [Akiyama et al. (2010) and more recent data from the years 2010-2013; 140 data sets in total] a reduction potential of approx. 35% seems realistic; however, further measurements in different management systems, particularly in regions with intense frost/thaw cycles seem necessary to confirm this reduction potential. These measurements generally should cover a whole annual cycle.