Field evaluation of nitrogen fixation and use of nitrogen fertilizer by sorghum/pigeonpea intercropping on an Alfisol in the Indian semi-arid tropics

A field experiment was conducted to obtain the N balance sheet for sole crops and intercrops of sorghum Sorghum bicolor (L.) Moench] and pigeonpeas Cajanus cajan (L.) Millsp.]. Intercropping gave a significant advantage over sole cropping in terms of dry matter production and grain yield, as calculated on the basis of the land equivalent ratio and area-time equivalent ratio. The N fertilizer use efficiency and atmospheric N₂ fixation by pigeonpea were estimated using ¹⁵N-labeling and natural abundance methods. The N fertilizer use efficiency of sorghum was unaltered by the cropping system, while that of the pigeonpea was greatly reduced by intercropping. Although intercropping increased the fractional contribution of fixed N to the pigeonpeas, no significant difference was observed between the cropping systems in total symbiotically fixed N. There was no evidence of a significant transfer of N from the pigeonpea to the sorghum. This study showed that use of soil N and fertilizer N by pigeonpeas was almost the same as that by sorghum in sole cropping, indicating the potential competence of pigeonpeas to exploit soil N. However, when N was exhausted by a companion crop in intercropping, the pigeonpea crop increased its dependency on atmospheric N₂ fixation. We conclude that knowledge of how N from different sources is shared by companion crops is a prerequisite to establishing strategies to increase N use, and consequently land productivity, in intercropping systems.