Effect of potassium phosphate fertilization on production and emission of methane and its C-stable isotope composition in rice microcosms

Rice fields are an important source for atmospheric CH₄, but the effects of fertilization are not well known. We studied the turnover of CH₄ in rice soil microcosms without and with addition of potassium phosphate. Height and tiller number of rice plants were higher in the fertilized than in the unfertilized microcosms. Emission rates of CH₄ were also higher, but porewater concentrations of CH₄ were lower. The δ¹³C values of the emitted CH₄ and of the CH₄ in the porewater were both 2-6% higher in the fertilized microcosms than in the control. Potassium phosphate did not affect rate and isotopic signature of CH₄ production in anoxic soil slurries. On the other hand, roots retrieved from fertilized microcosms at the end of incubation exhibited slightly higher CH₄ production rates and slightly higher CH₄-δ¹³C values compared to roots from unfertilized plants. Addition of potassium phosphate to excised rice roots generally inhibited CH₄ production and resulted in increasingly lower δ¹³C values of the produced CH₄. Fractionation of ¹³C during plant ventilation (i.e. δ¹³C in pore water CH₄ versus CH₄ emitted) was larger in the fertilized microcosms than in the control. Besides plant ventilation, this difference may also have been caused by CH₄ oxidation in the rhizosphere. However, calculation from the isotopic data showed that less than 27% of the produced CH₄ was oxidized. Collectively, our results indicate that potassium phosphate fertilization stimulated CH₄ emission by enhancing root methanogenesis, plant ventilation and/or CH₄ oxidation, resulting in residence times of CH₄ in the porewater in the order of hours.