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 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.  

Lipoxygenases (LOXs) are a group of non-heme, iron-containing enzymes and extensively involved in plant growth and development, ripening and senescence, stress responses, biosynthesis of regulatory molecules and defense reaction. In our previous study, 18 LOXs in melon genome were screened and identified, and five 13-LOX genes (CmLOX08, CmLOX10, CmLOX12, CmLOX13 and CmLOX18) were predicted to locate in chloroplast. Phylogenetic analysis result showed that the five genes have high homology with jasmonic acid (JA) biosynthesis-related LOXs from other plants. In addition, promoter analysis revealed that motifs of the five genes participate in gene expression regulated by hormones and stresses. Therefore, we analyzed the expressions of the five genes and LOX activity in leaves of four-leaf stage seedlings of oriental melon cultivar Yumeiren under abiotic stress: wounding, cold, high temperature and hydrogen peroxide (H2O2), and signal molecule treatments: methyl jasmonate (MeJA), abscisic acid (ABA) and salicylic acid (SA). Real time qPCR revealed that wounding and H2O2 induced the expressions of all the five genes. Only CmLOX08 was induced by cold while only CmLOX13 was suppressed by high temperature. ABA induced the expressions of CmLOX10 and CmLOX12 while inhibited CmLOX13 and CmLOX18. MeJA increased the 3 genes expressions except CmLOX08 and CmLOX13, whereas SA decreased the effect, apart from CmLOX12. All the abiotic stresses and signal molecules treatments increased the LOX activity in leaves of oriental melon. In summary, the results suggest that the five genes have diverse functions in abiotic stress and hormone responses, and might participate in defense response. The data generated in this study will be helpful in subcellular localization and transgenic experiment to understand their precise roles in plant defense response.