We evaluated together the ability of large urea granules, low soil temperature, and a nitrification inhibitor to limit nitrification. This laboratory study was done to provide information on the conservation of N from fall-applied urea during the winter. Apparent nitrification of urea-N decreased as the granule size, or weight, increased from 0.02 to 3 g. For example, apparent nitrification at 15°C was 89% with 0.02 g, but only 16% with 3-g urea granules after a 35-d incubation period. Application at lower soil temperatures decreased nitrification of urea. The incorporation of dicyandiamide (DCD) into large urea granules further decreased nitrification. Apparent nitrification was reduced from 89 to 44% with DCD (100 g kg−1 urea) in 0.02-g granules at 15°C after a 35-d incubation period. Under similar conditions, nitrification with 3-g granules was decreased from 16 to 1.3% with DCD. There was no significant accumulation of NO2- during the transformations of urea-N in large granules. From this laboratory study we concluded that a combination of delayed application (at lower soil temperature), large granules, and a suitable nitrification inhibitor would conserve N in fall-applied urea for the next growing season
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