Effects of ammonium-based fertilisation on microbialprocesses involved in methane emission from soilsplanted with rice

The emission of the greenhouse gas CH4 from ricepaddies is strongly influenced by management practicessuch as the input of ammonium-based fertilisers. Weassessed the impact of different levels (200 and 400kgN.ha−1) of urea and (NH4)2HPO4on the microbial processes involved in production andconsumption of CH4 in rice field soil. We usedcompartmented microcosms which received fertilisertwice weekly. Potential CH4 production rates weresubstantially higher in the rice rhizosphere than inunrooted soil, but were not affected by fertilisation.However, CH4 emission was reduced by the additionof fertiliser and was negatively correlated with porewater NH4plus concentration, probably as theconsequence of elevated CH4 oxidation due tofertilisation. CH4 oxidation as well as numbersof methanotrophs was distinctly stimulated by theaddition of fertiliser and by the presence of the riceplant. Without fertiliser addition,nitrogen-limitation of the methanotrophs will restrictthe consumption of CH4. This may have a majorimpact on the global CH4 budget, asnitrogen-limiting conditions will be the normalsituation in the rice rhizosphere. Elevated potentialnitrifying activities and numbers were only detectedin microcosms fertilised with urea. However, asubstantial part of the nitrification potential in therhizosphere of rice was attributed to the activity ofmethanotrophs, as was demonstrated using theinhibitors CH3F and C2H2.