Effect of a late season urea fertilization on methane emission from a rice field in Italy

The application of NH₄⁺-based fertilizers is a common practice in rice production. The immediate effect of a late season urea application on the processes involved in CH₄ emission from a rice field was investigated on sandy loam located in the valley of River Po at Vercelli, northern Italy, and planted with rice (Oryza sativa, type japonica, variety Koral). Urea, applied at a rate of 50 kg N ha⁻¹ on the 75th day after flooding, significantly stimulated both CH₄ production and CH₄ oxidation. During the following 9 days, the rates of CH₄ production and CH₄ oxidation in the 0–3 cm soil layer increased by 24–52 and 18–41%, respectively, of the fertilized plot compared to the unfertilized control plot. Methane oxidation on roots was also stimulated by urea, indicated by shorter lag times. Porewater concentrations of CH₄ in the 0–6 cm soil layer increased with time and soil depth, but were not affected by fertilization with urea. Urea application also exhibited little impact on CH₄ emission. During the 7 days following fertilization, urea only slightly reduced the CH₄ flux. Ammonium originating from urea hydrolysis was completely depleted from the porewater within 3 days, mainly due to plant uptake. Fertilizer application did also not affect the contribution of CH₄ oxidation to the net flux of CH₄, determined by measuring δ¹³CH₄ and by inhibition of CH₄ oxidation. The absence of any effect of urea on net CH₄ emission in this study was presumably caused by the rapid depletion of urea, the counterbalance between the increase of CH₄ production and the increase of CH₄ oxidation after fertilization, and methanogenesis in deeper soil layers.