Effects of ammonium-based fertilisation on microbial processes involved in methane emission from soils planted with rice

The emission of the greenhouse gas CH4 from rice paddies is strongly influenced by management practices such as the input of ammonium-based fertilisers. We assessed the impact of different levels (200 and 400 kgN.ha(-1)) of urea and (NH4)(2)HPO4 on the microbial processes involved in production and consumption of CH4 in rice field soil. We used compartmented microcosms which received fertiliser twice weekly. Potential CH4 production rates were substantially higher in the rice rhizosphere than in unrooted soil, but were not affected by fertilisation. However, CH4 emission was reduced by the addition of fertiliser and was negatively correlated with pore water NH4+ concentration, probably as the consequence of elevated CH4 oxidation due to fertilisation. CH4 oxidation as well as numbers of methanotrophs was distinctly stimulated by the addition of fertiliser and by the presence of the rice plant. Without fertiliser addition, nitrogen-limitation of the methanotrophs will restrict the consumption of CH4. This may have a major impact on the global CH4 budget, as nitrogen-limiting conditions will be the normal situation in the rice rhizosphere. Elevated potential nitrifying activities and numbers were only detected in microcosms fertilised with urea. However, a substantial part of the nitrification potential in the rhizosphere of rice was attributed to the activity of methanotrophs, as was demonstrated using the inhibitors CH3F and C2H2.