Fate of nitrogen-15-labelled fertilizer applied to maize-millet cropping systems in the mid-hills of Nepal

Maize grown in the mid-hills of Nepal traditionally received inputs of manure. However, N fertilizer is increasingly applied either alone or in combination with manure. This study investigated the effect of these different nutrient sources applied at three rates (0, 45, 90 kg N ha^-1) on crop yields in a maize-millet rotation at two locations (Pakhribas and Dordor Gaun) in the mid-hills of Nepal and measured the recovery of ¹⁵N-labelled urea applied as a top-dressing to maize at three rates (11.25, 22.5, 45 kg N ha^-1). Grain and straw yields of maize were greater following the application of fertilizer either alone or in combination with manure, rather than manure alone. Millet yields were unaffected by the rate or form of N inputs to maize. Little (<25%) of the applied fertilizer was recovered in the maize crop, with only a further 3% recovered by the subsequent millet crop. On average, 58% of the applied fertilizer was recovered in the 0- to 60-cm soil layer at maize harvest, mainly in non-mineral N forms. Transformations and movement of applied fertilizer N were shown to be rapid, occurring within 7 days of application. Approximately one-third of the applied fertilizer was unaccounted for in the crop-soil system at maize harvest. It was concluded that fertilizer was rapidly immobilized and that its subsequent rate of turnover was low so that an application of fertilizer to one crop made no substantial contribution to the nutrition of the next.