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

The application of NH4+-based fertilizers is a common practice in rice prod uction. The immediate effect of a late season urea application on the proce sses involved in CH4 emission from a rice field was investigated on sandy l oam located in the valley of River Po at Vercelli, northern Italy, and plan ted with rice (Oryza sativa, type japonica, variety Koral). Urea, applied a t a rate of 50 kg N ha(-1) on the 75th day after flooding, significantly st imulated 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 in creased by 24-52 and 18-41%, respectively, of the fertilized plot compared to the unfertilized control plot. Methane oxidation on roots was also stimu lated 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 n ot affected by fertilization with urea. Urea application also exhibited lit tle impact on CH4 emission. During the 7 days following fertilization, urea only slightly reduced the CH4 flux. Ammonium originating from urea hydroly sis 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 delta (CH4 )-C-13 and by inhibition of CH4 oxidation. The absence of any effect of ure a on net CH4 emission in this study was presumably caused by the rapid depl etion of urea, the counterbalance between the increase of CH4 production an d the increase of CH4 oxidation after fertilization, and methanogenesis in deeper soil layers. (C) 2001 Elsevier Science B.V. All rights reserved.