Changes In Soil Ph N <sub> 2 </ Sub> O Emission Mechanism

Nitrous oxide (N₂O) is the important constituent of the global nitrogen cycle. Increase in the atmospheric concentration of N₂O contributes not only to global warming but also directly to destruction of stratospheric ozone layer. Agricultural soils are the main resources of N₂O emission, and soil pH is one of the key factors affecting on N₂O emission from Agricultural soils. So it is important and valuable to understand the relationship between the N₂O emission and soil pH change and reduce the amount of N₂O emission from soils by regulating the soil pH.In this laboratory experiment, the effects of soil pH change on the N₂O emissions from nitrification and denitrification were studied.The main results are as follows:1 By cultivating after adding Ca(OH)₂ to acid soil, three kinds of soil with pH4.8, pH 6.0 and pH 8.8 were obtained. In 65% water holding capacity (WHC), three kinds of soil received 100 mg N kg^-1 as ammonium sulphate and were incubated for 16 days. There were almost no N₂O emission from both the pH4.8 soil and pH6.0 soil, while the amount of N₂O emission from the pH8.8 soil was 0.16% of total N added. The emission from soil results mainly from biological process and less from pure chemical process.2 By cultivating after adding Ca(OH)₂ or H2SO4 to neutral soil, three kinds of soil with pH4.50, pH 7.23 and pH 7.90 were obtained. The nitrification and denitrification processes can occur simultaneously during incubation in 65% WHC condition. When the soils were fertilized by 100 mg N kg-1 as ammonium sulphate, the pH 4.5 soil has larger N₂O emission than others. The emission from soil results mainly from biological process and less from pure chemical process. Heterotrophic nitrification was the major resource of N₂O emission from the pH 4.5 soil. Autotrophic nitrification became gradually dominant along with soil pH increasing. The pure chemical process contributed 25% of total N₂O emission from the pH 4.5 soil, while it contributed little to N₂O emission from the soils of both pH 7.2 and pH 7.9. The ratio of N₂O from the pure chemical process to total N₂O emission was decreased quickly with soil pH increase. 3 Three kinds of soil with pH5.82, pH 6.95 and pH 7.55 were obtained from the neutral soil. During the aerated incubation, the soil trend more N₂O emission with higher pH. The experiment results showed that more N₂O emitted from soil with the pH increasing. The ratios of amount of nitrogen loss by N₂O emission to amount of nitrogen applied were 0.11%, 0.11% and 0.72% for pH5.82, pH6.95 and pH7.55 soil, respectively.4 In 120% water holding capacity (WHC), three kinds of soil received 100 mg NO₃^--N kg^-1 and were incubated. The results showed that the activities of some enzymes related the denitrification are different in different pH soil. Original activities of nitrate reductase are increasing with soil pH increasing. For the original activities of nitrite reductase, pH6.95 > pH7.55 > pH5.82; for the original activities of N₂O reductase: pH7.55>pH6.95>pH5.82. Nevertheless, all of the activities of nitrate reductase, nitrite reductase, and N₂O reductase increase with soil pH increasing.