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Pigeonpea (Cajanus cajan L. Millsp) and sorghum (Sorghum bicolor L. Moench), are grown as intercrops on soils typified by low nitrogen (N), particularly Alfisols and Vertisols of the semi-arid tropical areas of India and East Africa. Proper management of fertilizer N in sorghum/pigeonpea intercropping systems is essential to enhancing biological nitrogen fixing ability of pigeonpea, and to increasing grain yields of sorghum. In a 3-year study, we examined the effect of time and rates of fertilizer N application on biological N fixation (BNF) and fertilizer N recovery by pigeonpea and sorghum grown as a sole crop, and as an intercrop on an Alfisol in India. In 1993, N (50 kg N ha(-1)) was all applied at planting (basal) or at 40 days after sowing (delayed). During 1994-1995, four N rates 0, 25, 50, and 100 kg ha(-1) were applied (i) 100% at planting (basal) or (ii) 67% at 40 days, and 33% at 60 days after sowing (delayed).

Delaying N fertilization until 40 days after sowing (DAS), rather-than applying at sowing significantly increased (p < 0.001) total dry matter, grain yield (GY) and total N content (TNC) of the sorghum, but not of pigeonpea. Highest GY and TNC was recorded at an N rate of 50 kg ha(-1) with sole crop sorghum, and at 25 kg ha(-1) for intercrop sorghum. Delaying N fertilization to sorghum significantly (p < 0.001) increased N-15 recovery in the shoot from 15 to 32% in sole crop, and 10 to 32% in intercrop. Similarly, there was a significant (p < 0.001) increase in N recovery (calculated without N-15, but by the difference between the nonfertilized N and fertilized plots) from 43-59% in sole crop and from 28-71% in intercrop sorghum. Pigeonpea fixed between 120-170 kg ha(-1) of atmospheric N throughout the cropping season. Nitrogen application to the sorghum row, but not to the pigeonpea, enhanced BNF by pigeonpea and maximized fertilizer recovery by sorghum in a sorghum/pigeonpea intercropping.