δ15N values of sorghum grains harvested on a Vertisol in the semi-arid tropics were positively related to doses of fertilizer N but negatively with the frequency of legume cultivation

The relation between δ15N values of sorghum grains grown on Alfisol or Vertisol fields and the fields' cropping and N-fertilizer histories was investigated. Fields were cultivated with legumes or cereals at different frequencies for 10 y, prior to sowing of sorghum. During that period, the fields for study were amended with different N-fertilizers at rates ranging between 236 and 1,472 kg N ha-1. The δ15N values were markedly different between the two soil types. In the Vertisol fields, the grain δ15N values were positively correlated with the cereal-cropping frequency and amounts of applied fertilizers, but negatively correlated with the legume-cropping frequency. Such correlations were not observed in the Alfisol fields. It was concluded that only the amount of applied fertilizer influenced the δ15N values of sorghum grains, and that this effect was caused by the slight alkalinity of the Vertiso