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 dCH4 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. © 2001 Elsevier Science B.V. All rights reserved.