Nitrous oxide emissions from black soils with different pH.

N2O fluxes as a function of incubation time from soil with different available N contents and pH were determined. Cumulative carbon dioxide (CO2) emissions were measured to indicate soil respiration. A 144-hr incubation experiment was conducted in a slightly acidic agricultural soil (pH(H2O) 5.33) after the pH was adjusted to four different values (3.65, 5.00, 6.90 and 8.55). The experiments consisted of a control without added N, and with NH(4+)-N and NO(3-)-N fertilization. The results showed that soil pH contributed significantly to N2O flux from the soils. There were higher N2O emissions in the period 0-12 hr in the four pH treatments, especially those enhanced with N-fertilization. The cumulative N2O-N emission reached a maximum at pH 8.55 and was stimulated by NO(3-)-N fertilization (70.4 microg/kg). The minimum emissions appeared at pH 3.65 and were not stimulated by NO(3-)-N or NH(4+)-N fertilization. Soil respiration increased significantly due to N-fertilization. Soil respiration increased positively with soil pH (R2 = 0.98, P < 0.01). The lowest CO2-C emission (30.2 mg/kg) was presented in pH 3.65 soils without N-fertilization. The highest CO2-C emissions appeared in the pH 8.55 soils for NH(4+)-N fertilization (199 mg/kg). These findings suggested that N20 emissions and soil respiration were significantly influenced by low pH, which strongly inhibits soil microbial nitrification and denitrification activities. The content of NO(3-)-N in soil significantly and positively affected the N20 emissions through denitrification.