Ammonia (NH₃) emissions should be mitigated to improve environmental quality.  Croplands are one of the largest NH₃ sources, they must be managed properly to reduce their emissions while achieving the target yields.  Herein, we report the NH₃ emissions, crop yield and changes in soil fertility in a long-term trial with various fertilization regimes, to explore whether NH₃ emissions can be significantly reduced using the 4R nutrient stewardship (4Rs), and its interaction with the organic amendments (i.e., manure and straw) in a wheat–maize rotation.  Implementing the 4Rs significantly reduced NH₃ emissions to 6 kg N ha^–1 yr^–1 and the emission factor to 1.72%, without compromising grain yield (12.37 Mg ha^–1 yr^–1) and soil fertility (soil organic carbon of 7.58 g kg^–1) compared to the conventional chemical N management.  When using the 4R plus manure, NH₃ emissions (7 kg N ha^–1 yr^–1) and the emission factor (1.74%) were as low as 4Rs, and grain yield and soil organic carbon increased to 14.79 Mg ha^–1 yr^–1 and 10.09 g kg^–1, respectively.  Partial manure substitution not only significantly reduced NH₃ emissions but also increased crop yields and improved soil fertility, compared to conventional chemical N management.  Straw return exerted a minor effect on NH₃ emissions.  These results highlight that 4R plus manure, which couples nitrogen and carbon management can help achieve both high yields and low environmental costs.

The inhibition of nitrification by mixing nitrification inhibitors (NI) with fertilizers is emerging as an effective method to reduce fertilizer-induced nitrous oxide (N₂O) emissions.  The additive 3,4-dimethylpyrazole phosphate (DMPP) apparently inhibits ammonia oxidizing bacteria (AOB) more than ammonia oxidizing archaea (AOA), which dominate the nitrification in alkaline and acid soil, respectively.  However, the efficacy of DMPP in terms of nitrogen sources interacting with soil properties remains unclear.  We therefore conducted a microcosm experiment using three typical Chinese agricultural soils with contrasting pH values (fluvo-aquic soil, black soil and red soil), which were fertilized with either digestate or urea in conjunction with a range of DMPP concentrations.  In the alkaline fluvo-aquic soil, fertilization with either urea or digestate induced a peak in N₂O emission (60 μg N kg^–1 d^–1) coinciding with the rapid nitrification within 3 d following fertilization.  DMPP almost eliminated this peak in N₂O emission, reducing it by nearly 90%, despite the fact that the nitrification rate was only reduced by 50%.  In the acid black soil, only the digestate induced an N₂O emission that increased gradually, reaching its maximum (20 μg N kg^–1 d^–1) after 5–7 d.  The nitrification rate and N₂O emission were both marginally reduced by DMPP in the black soil, and the N₂O yield (N₂O-N per NO₂^–+NO₃^–-N produced) was exceptionally high at 3.5%, suggesting that the digestate induced heterotrophic denitrification.  In the acid red soil, the N₂O emission spiked in the digestate and urea treatments at 50 and 10 μg N kg^–1 d^–1, respectively, and DMPP reduced the rates substantially by nearly 70%.  Compared with 0.5% DMPP, the higher concentrations of DMPP (1.0 to 1.5%) did not exert a significantly (P<0.05) better inhibition effect on the N₂O emissions in these soils (either with digestate or urea).  This study highlights the importance of matching the nitrogen sources, soil properties and NIs to achieve a high efficiency of N₂O emission reduction.

Denitrification-induced nitrogen (N) losses from croplands may be greatly increased by intensive fertilization.  However, the accurate quantification of these losses is still challenging due to insufficient available in situ measurements of soil dinitrogen (N₂) emissions.  We carried out two one-week experiments in a maize–wheat cropping system with calcareous soil using the ¹⁵N gas-flux (¹⁵NGF) method to measure in situ N₂ fluxes following urea application.  Applications of ¹⁵N-labeled urea (99 atom%, 130–150 kg N ha⁻¹) were followed by irrigation on the 1st, 3rd, and 5th days after fertilization (DAF 1, 3, and 5, respectively).  The detection limits of the soil N₂ fluxes were 163–1 565, 81–485, and 54–281 μg N m⁻² h⁻¹ for the two-, four-, and six-hour static chamber enclosures, respectively.  The N₂ fluxes measured in 120 cases varied between 159 and 2 943 (811 on average) μg N m⁻² h⁻¹, which were higher than the detection limits, with the exception of only two cases.  The N₂ fluxes at DAF 3 were significantly higher (by nearly 80% (P<0.01)) than those at DAF 1 and 5 in the maize experiment, while there were no significant differences among the irrigation times in the wheat experiment.  The N₂ fluxes and the ratios of nitrous oxide (N₂O) to the N₂O plus N₂ fluxes following urea application to maize were approximately 65% and 11 times larger, respectively (P<0.01), than those following urea application to wheat.  Such differences could be mainly attributed to the higher soil water contents, temperatures, and availability of soil N substrates in the maize experiment than in the wheat experiment.  This study suggests that the ¹⁵NGF method is sensitive enough to measure in situ N₂ fluxes from intensively fertilized croplands with calcareous soils.

The upland agricultural soils in North China are distributed north of a line between the Kunlun Mountains, the Qinling Mountains and the Huaihe River.  They occur in arid, semi-arid and semi-humid regions and crop production often depends on rain-fed or irrigation to supplement rainfall.  This paper summarizes the characteristics of gross nitrogen (N) transformation, the fate of N fertilizer and soil N as well as the N loss pathway, and makes suggestions for proper N management in the region.  The soils of the region are characterized by strong N mineralization and nitrification, and weak immobilization and denitrification ability, which lead to the production and accumulation of nitrate in the soil profile.  Large amounts of accumulated nitrate have been observed in the vadose-zone in soils due to excess N fertilization in the past three decades, and this nitrate is subject to occasional leaching which leads to groundwater nitrate contamination.  Under farmer’s conventional high N fertilization practice in the winter wheat-summer maize rotation system (N application rate was approximately 600 kg ha^–1 yr^–1), crop N uptake, soil residual N, NH₃ volatilization, NO₃^– leaching, and denitrification loss accounted for around 27, 30, 23, 18 and 2% of the applied fertilizer N, respectively.  NH₃ volatilization and NO₃^– leaching were the most important N loss pathways while soil residual N was an important fate of N fertilizer for replenishing soil N depletion from crop production.  The upland agricultural soils in North China are a large source of N₂O and total emissions in this region make up a large proportion (approximately 54%) of Chinese cropland N₂O emissions.  The “non-coupled strong ammonia oxidation” process is an important mechanism of N₂O production.  Slowing down ammonia oxidation after ammonium-N fertilizer or urea application and avoiding transient high soil NH4+ concentrations are key measures for reducing N₂O emissions in this region.  Further N management should aim to minimize N losses from crop and livestock production, and increase the recycling of manure and straw back to cropland.  We also recommend adoption of the 4R (Right soure, Right rate, Right time, Right place) fertilization techniques to realize proper N fertilizer management, and improving application methods or modifying fertilizer types to reduce NH₃ volatilization, improving water management to reduce NO₃^– leaching, and controlling the strong ammonia oxidation process to abate N₂O emission.  Future research should focus on the study of the trade-off effects among different N loss pathways under different N application methods or fertilizer products.  

The objective of this study was to determine the efficiency of different plant systems in capturing deep soil nitrate (NO3 -) to reduce NO3 - leaching in a field plot experiment using 15N labelling. The study was conducted on a calcareous alluvial soil on the North China Plains and the plant systems evaluated included alfalfa (Medicago sativa), American black poplar (Populus nigra) and cocksfoot (Dactylis). 15N-labelled N fertilizer was injected to 90 cm depth to determine the recovery of 15N by the plants. With conventional water and nutrient management, the total recovery of 15N-labeled NO3 --N was 23.4% by alfalfa after two consecutive growth years. The recovery was significantly higher than those by American black poplar (12.3%) and cocksfoot (11.4%). The highest proportion of soil residual 15N from the labeled fertilizer N (%Ndff) was detected around 90 cm soil depth at the time of the 1st year harvest and at 110-130 cm soil depth at time of the 2nd year harvest. Soil %Ndff in 0-80 cm depth was significantly higher in the alfalfa treatment than those in all the other treatments. The soil %Ndff below 100 cm depth was much lower in the alfalfa than those in all the other treatments. These results indicated that 15N leaching losses in the alfalfa treatment were significantly lower than by those in the black poplar and cocksfoot treatments, due to the higher root density located in nitrate labeling zone of soil profile. In conclusion, alfalfa may be used as a plant to capture deep soil NO3 - left from previous crops to reduce NO3 - leaching in high intensity crop cultivation systems of North China Plain.