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Abstract

Intermittent drainage of rice fields is discussed as an option to
mitigate emission of CH4, an important greenhouse gas. However N2O, a
potentially more effective greenhouse gas, may be emitted during the
aeration phase. Therefore, the metabolism of NO, N2O, NH4+, NO2- and
NO3- and the kinetics of CH4 oxidation were measured after aeration of
methanogenic rice field soil. Before aeration, the soil contained NH4+
in relatively high concentrations (about 4 mM), while NO2- and NO3- were
almost undetectable. Immediately after aeration both NO and N2O were
produced with rates of about 15 pmol h(-1) gdw(-1) and 5 pmol h(-1)
gdw(-1), respectively. Simultaneously, NH4+ decreased while NO2-
accumulated. Later on, NO2- was depleted while NO3- concentrations
increased. Characteristic phases of nitrogen turnover were associated
with the activities of ammonium oxidizers, nitrite oxidizers and
denitrifiers. Oxidation of NH4+ and production of NO and N2O were
inhibited by 10 Pa acetylene demonstrating that nitrification was
obligatory for the initiation of nitrogen turnover and production of NO
and N2O. Ammonium oxidation was not limited by the available NH4+ and
thus, concomittant production of NO and N2O was not stimulated by
addition of NH4+. However, addition of NO3- stimulated production of NO
and N2O in both anoxic and aerated rice soil slurries. In this case, 10
Pa acetylene did not inhibit the production of NO and N2O demonstrating
that it was due to denitrification which was obviously limited by the
availability of NO3-. In the aerated soil slurries CH4 was only oxidized
if present at elevated concentrations (>50 ppmv CH4). At atmospheric CH4
concentrations (similar to 1.7 ppmv) CH4 was not consumed, but was even
slightly produced. CH4 oxidation activity increased after preincubation
at 20% CH4, and then CH4 was also oxidized at atmospheric
concentrations. CH4 oxidation kinetics exhibited sigmoid characteristics
at low CH4 concentrations presumably because of inhibition of CH4
oxidation by NH4+.