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Studies of nitrogen (N) transformations under pastures are necessary to improve N agronomic efficiency (AE). A 2-year field experiment was conducted at Balcarce (37°45′S, 58°18′W), Argentina, on a pasture of tall wheatgrass (Elytrigia elongata) on a Typic Natracuol. The objective was to determine the effect of urea applied at varying rates (0, 90 and 180 kg N/ha) in autumn (late March) on soil mineral N, NH3 volatilisation losses (VL), herbage dry matter (DM) and plant accumulated N (Nplant) 46 days after fertilisation. In 1999, the accumulated VL were 3, 14 and 63 kg N/ha for 0, 90 and 180 kg applied N/ha, respectively, whereas in 2001, the accumulated VL were 1, 12 and 19 kg N/ha, respectively. Plant accumulated N was 32, 75 and 108 kg/ha in 1999, whereas in 2001, Nplant was 12, 34 and 58 kg/ha for 0, 90 and 180 kg N/ha, respectively. In both years, N AE ranged from 10 to 15 kg DM/kg applied N and the N recovered fraction (RF) ranged from 46 to 25% of applied N in 1999 and 2001, respectively. In 1999, unaccounted N (Nunac.) ranged from 41 to 23%, whereas in 2001, Nunac. ranged from 67 to 69% of applied N. Results of this study indicate that VL and denitrification affected the RF, and, for weather conditions of this area, VL would be the main N loss mechanism. These results indicate that management strategies to improve N use efficiency could be the utilisation of split N rates and N sources without urea. |
