Denitrification in extreme alkaline saline soils of the former lake Texcoco

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Abstract

Extreme alkalinity and salinity affects different processes involved in the N cycle, such as mineralization and nitrification, so it might affect the denitrification process and the N2O-to-N2 ratio. An alkaline saline soil of the former lake Texcoco with electrolytic conductivity (EC) 12.6 dS m−1 and pH 10.0, and an ‘extreme’ alkaline saline soil with EC 56 dS m−1 and pH 10.4 were amended with 100 mg NO3− -N kg−1, purged of all O2, shaken and incubated anaerobically for 72 h. A ‘normal’ agricultural soil of Acolman with EC 0.8 dS m−1 and pH 6.3 was used as control. Treatments were with or without acetylene (C2H2) (found to inhibit reduction of N2O to N2) and with or without chloramphenicol (found to inhibit de novo synthesis of reduction enzymes), while emission of CO2 and N2O and concentrations of NO3− and NO2− were monitored. The NO3− reduction rate was not affected by C2H2 or chloramphenicol in the Acolman soil, but decreased >3.5 times when chloramphenicol was added to the Texcoco soils. The concentration of NO2− increased 3 times when chloramphenicol was added to the Acolman soil after 72 h, was similar in the Texcoco soil with EC 12 dS m−1, but chloramphenicol decreased it 14 times in the Texcoco soil with EC 56 dS m−1. The emission of N2O in the unamended Acolman soil was 0.09 mg N kg−1 and N2 6.5 mg N kg−1 soil after 72 h resulting in a N2O-to-N2 ratio of 0.014, while N2O production was 0.05 mg N kg−1 in the Texcoco soil with EC 12.6 dS m−1 and N2 1.6 mg N kg−1 resulting in a N2O-to-N2 ratio of 0.031. The emission of N2O in the Texcoco soil with EC 56 dS m−1 was 1.3 mg N kg−1 soil after 72 h and that of N2 was 2.0 mg N kg−1 resulting in a N2O-to-N2 ratio of 0.65. It was found that the activity of NO2− and N2O reductase decreased with increased pH and EC, so that the concentrations of NO2−, N2O and the N2O-to-N2 ratio increased with increased EC and pH, although that the effect of other soil characteristics can not be excluded.

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