Residual effect of nitrogen fixed by mungbean ( Vigna radiata ) and blackgram ( Vigna mungo ) on subsequent rice and wheat crops

Annual crop legumes, grown in rotation with cereal crops, contribute to the total pool of nitrogen in the soil and improve the yield of cereals. The present study aimed at the quantification of nitrogen fixation by mungbean and blackgram using 15N isotopic dilution methodology; and the quantification of grain and nitrogen yield differences of succeeding rice and wheat crops compared with a cereal–cereal rotation. There were 2 experiments in separate fields but with the same layout: in experiment 1, rice followed the mungbean and blackgram varieties and in experiment 2, wheat followed the mungbean and blackgram varieties. Nitrogen fixed ranged from 26 to 36 kg/ha in experiment 1 and from 30 to 36 kg/ha in experiment 2. Soil nitrogen spared by legume crops ranged from 2 to 26 kg/ha in experiment 1 and from 4 to 23 kg/ha in experiment 2. Rice paddy yields were 0.6–1.1 t/ha higher in the legume–cereal rotation than in the cereal–cereal sequence. Similarly, wheat grain yields were 0.5–1.1 t/ha higher in the legume–cereal rotation.