Characterization of microbial NO production, N2O production and CH4 oxidation initiated by aeration of anoxic rice field soil

Abstract

Intermittent drainage of rice fields isdiscussed as an option to mitigate emission ofCH4, an important greenhouse gas. HoweverN2O, a potentially more effective greenhouse gas,may be emitted during the aeration phase. Therefore,the metabolism of NO, N2O, NH\(_4^ + \),NO\(_2^ - \) and NO\(_3^ - \) and the kinetics ofCH4 oxidation were measured after aeration ofmethanogenic rice field soil. Before aeration, thesoil contained NH\(_4^ + \) in relatively highconcentrations (about 4 mM), while NO\(_2^ - \) andNO\(_3^ - \) were almost undetectable. Immediatelyafter aeration both NO and N2O were produced withrates of about 15 pmol h-1 gdw-1 and 5 pmolh-1 gdw-1, respectively. Simultaneously,NH\(_4^ + \) decreased while NO\(_2^ - \)accumulated. Later on, NO\(_2^ - \) was depletedwhile NO\(_3^ - \) concentrations increased.Characteristic phases of nitrogen turnover wereassociated with the activities of ammonium oxidizers,nitrite oxidizers and denitrifiers. Oxidation ofNH\(_4^ + \) and production of NO and N2O wereinhibited by 10 Pa acetylene demonstrating thatnitrification was obligatory for the initiation ofnitrogen turnover and production of NO and N2O.Ammonium oxidation was not limited by the availableNH\(_4^ + \)and thus, concomittant production of NOand N2O was not stimulated by addition ofNH\(_4^ + \). However, addition of NO\(_3^ - \)stimulated production of NO and N2O in bothanoxic and aerated rice soil slurries. In this case,10 Pa acetylene did not inhibit the production of NOand N2O demonstrating that it was due todenitrification which was obviously limited by theavailability of NO\(_3^ - \). In the aerated soilslurries CH4 was only oxidized if present atelevated concentrations >50 ppmv CH4). Atatmospheric CH4 concentrations (∼1.7 ppmv)CH4 was not consumed, but was even slightly produced.CH4 oxidation activity increased afterpreincubation at 20% CH4, and then CH4was also oxidized at atmospheric concentrations. CH4oxidation kinetics exhibited sigmoid characteristicsat low CH4 concentrations presumably because ofinhibition of CH4 oxidation by NH\(_4^ + \).