Methane and nitrous oxide emissions from a rice field in relation to soil redox and microbiological processes

Paddy rice fields provide an environment for production of two important gr eenhouse gases, CH4 and N2O, because of variations in soil characteristics, moisture content, and microbial activity during the cropping season. Emiss ions of CH4 and N2O from a paddy rice field in northern China were measured in situ by static chamber technique during March to December in 1995 and 1 996. Factors affecting gas emission, including soil temperature, pH, and re dox potential (Eh), were measured as well. Emissions of CH4 and N2O were st rongly correlated with changes in soil redox potential. Significant CH4 emi ssion occurred only at soil redox potential lower than approximately -100 m V, while the emission of N2O was not significant below +200 mV. A significa nt inverse relationship between CH4 and N2O emissions was observed (r = -0. 49, n = 16, 5% confidence level). The results suggest the possibility of us ing management practices to maintain the redox potential in a range where b oth N2O and CH4 emissions are low. The activities of six related bacteria g roups (zymogenic bacteria, acetic acid and hydrogen-producers, methanogens, CH4 oxidizers, and nitrifiers and denitrifiers) in the soil were also meas ured in an effort to explain the relationship between gas emission and soil microbiological processes. Methane emission was significantly related to t he logarithm number of zymogenic bacteria (r = 0.76, n = 12, 1% confidence level), as well as to soil redox potential (r = -0.72, n = 12, 1% confidenc e level). Both zymogenic bacteria number and soil redox potential appear to be predicators of CH4 emission potential.