Nitrogen fixation in fababeans as affected by plant population density in sole or intercropped systems with barley

Nitrogen-fixing legumes are commonly intercropped with various non-N-fixing crops in the tropics. Little information is available on the quantitative estimation of the effect of competition by a non-legume on the N fixing ability of the associated legume in an intercrop. Our primary objectives were therefore to assess the effects of different densities of intercropped barley ( Hordeum vulgare. L.) on the amount or proportion of N fixed by fababeans ( Vicia faba, var. minor L.) and also to examine whether fixed N was transferred from the legume to barley. The proportion of N fixed by fababeans, and N uptake from soil and fertilizer by fababeans and barley were distinguished by the use of 15N-labelled urea. Compared to the sole crop, the dry matter yield and total N in the intercropped fababeans were reduced and consequently there was a slight reduction in the quantity of N fixed in the presence of barley. However. the proportion of the N in fababeans that was derived from fixation was significantly increased in the intercropped system. Although the sole crop fababeans absorbed more soil N than sole barley, this was reversed in the intercrop. Increasing the competition for soil N through increased plant population densities of either fababeans in the sole crop or barley in the intercrop resulted in substantial increases in the proportion of N in fababeans derived from fixation. This density effect was much greater with intercropped barley than that induced through increased fababeans density. There was no evidence that fixed N was transferred from fababeans to barley in this study.