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Nitrous oxide (N 2 O) contributes 8% to global greenhouse gas emissions. Agricultural sources represent about 60% of anthropogenic N 2 O emissions. Most agricultural N 2 O emissions are due to increased fertilizer application. A considerable fraction of nitrogen fertilizers are converted to N 2 O by microbiological processes (that is, nitrification and denitrification). Soil amended with biochar (charcoal created by pyrolysis of biomass) has been demonstrated to increase crop yield, improve soil quality and affect greenhouse gas emissions, for example, reduce N 2 O emissions. Despite several studies on variations in the general microbial community structure due to soil biochar amendment, hitherto the specific role of the nitrogen cycling microbial community in mitigating soil N 2 O emissions has not been subject of systematic investigation. We performed a microcosm study with a water-saturated soil amended with different amounts (0%, 2% and 10% (w/w)) of high-temperature biochar. By quantifying the abundance and activity of functional marker genes of microbial nitrogen fixation (nifH), nitrification (amoA) and denitrification (nirK, nirS and nosZ) using quantitative PCR we found that biochar addition enhanced microbial nitrous oxide reduction and increased the abundance of microorganisms capable of N 2 -fixation. Soil biochar amendment increased the relative gene and transcript copy numbers of the nosZ-encoded bacterial N 2 O reductase, suggesting a mechanistic link to the observed reduction in N 2 O emissions. Our findings contribute to a better understanding of the impact of biochar on the nitrogen cycling microbial community and the consequences of soil biochar amendment for microbial nitrogen transformation processes and N 2 O emissions from soil.

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 N 2 O 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 NH þ 4 to NO À 3 in soils. The inhibition of the AMO by NIs directly decreases the nitrification rate and it reduces the NO À 3 concentration which serves as substrate for denitrification. Hence, the two main pathways of N 2 O 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 NH 3 , results from literature suggest that the enzymatic activity of NH 3 oxidizing bacteria is the most important target for NIs because it was much stronger affected. The application of NIs to reduce N 2 O 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 N 2 O 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 N 2 O 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 NO À 3 -N leaching losses are reduced. It has clearly been demonstrated that NIs added to cattle slurry are very effective in reducing N 2 O as well as NO emissions after surface application and injection of slurry into grassland soils. In flooded rice systems NIs can reduce CH 4 emission significantly, whereas the effect on CO 2 emission is varying. On the other hand, as an effect of the delay of nitrification by NIs, NH 3 emission might increase when N fertilizers are not incorporated into the soil. As compared to other measures NIs have a high potential to reduce N 2 O emissions from agricultural soils. Further, no other measure has so consistently been proofed according its efficiency to reduce N 2 O 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.

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This study was conducted to determine the effect of soil compaction and N fertilization on the fluxes of N2O and CH4 in a soil (fine‐silty Dystric Eutrochrept) planted with potato (Solanum tuberosum L.). Fluxes of N2O and CH4 were measured weekly for 1 yr on two differently fertilized (50 and 150 kg N ha‐1) fields. For the potato cropping period (May–September) these fluxes were quantified separately for the ridges (soil bulk density ρb = 1.05 Mg m‐3) covering two‐thirds of the total field area, and for the uncompacted (ρb = 1.26 Mg m‐3) and the tractor‐traffic‐compacted (ρb = 1.56 Mg m‐3) interrow soils, each of which made up one‐sixth of the field area. The annual N2O‐N emissions for the low and the high rates of N fertilization were 8 and 16 kg ha‐1, respectively. The major part (68%) of the total N2O release from the fields during the cropping period was emitted from the compacted tractor tramlines; emissions from the ridges made up only 23%. The annual CH4‐C uptake was 140 and 118 g ha‐1 for the low and high levels of fertilization, respectively. The ridge soil and the uncompacted interrow had mean CH4‐C oxidation rates of 3.8 and 0.8 µg m‐2 h‐1, respectively; however, the tractor‐compacted soil released CH4 at 2.1 µg CH4‐C m‐2 h‐1. The results indicate thas soil compaction was probably the main reason for increased N2O emission and reduced CH4 uptake of potato‐cropped fields.

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