Nitrogen fixation and transfer in intercrop systems

Nitrogen, in intercropping systems that consist of legumes growing in association with another crop, is considered in terms of fixation, release, transformation, and gains and losses from the system. Methods of determining nitrogen fixation and transfer are briefly reviewed, with cautionary comments on each of these. Knowledge concerning nitrogen in the soil organic matter in intercrop systems is limited, yet it may play a more important role with respect to the nitrogen efficiency of intercrop systems, than is presently attributed to it. The success of intercrop farming systems depends initially on effective nitrogen fixation and more importantly, on subsequent transfer of nitrogen to the non-legume. While there is a good understanding of the factors concerning biological nitrogen fixation, this does not apply to transfer. A more thorough knowledge is needed of the processes involved in direct, indirect or residual nitrogen transfer from the legume to a companion or succeeding crop. Such knowledge could provide avenues for agronomic research, leading to new and more nitrogen-efficient practices. It is proposed that research concerning nitrogen in intercrop systems should be conceived within a framework of the system as a whole, and should be undertaken in the context of models that are constantly being reviewed, an iterative process. One type of model should be process-oriented. Another, more empirical model should be concerned with nitrogen balances, within segments of the whole system. Experimental work ought to be undertaken within the framework of such models, to establish sizes of pools, rates of transformations and of flows and movements of nitrogen within different parts of the system. It is suggested that the international agricultural research institutes currently involved in intercropping research are well placed to develop such models and to provide a focus for empirical research within their regions. Groups of research workers, associated in networks in different regions, should be able to contribute to a global research effort on intercropping in a coordinated way.