Agricultural management of grain legumes: has it led to an increase in nitrogen fixation?

Biological nitrogen (N-2) fixation is an important aspect of sustainable an d environmentally friendly food production and long-term crop productivity. The amount of N-2 fixed is primarily controlled by four principal factors: (1) the effectiveness of the rhizobia-host plant symbiosis, (2) the streng th of the sink, i.e., the ability of the host plant to accumulate N, (3) th e amount of available soil N and (4) environmental constraints to N-2 fixat ion. Much of the N fixed by grain legumes is removed at harvest, the remain der becomes available to subsequent crops following mineralization, may be incorporated into the soil organic matter, or as with fertilizer N, may be lost from the cropping system. This paper reviews some of the agronomic management practices that affect N -2 fixation by grain legumes, asking whether grain legumes can provide an o verall net N benefit to the soil when grown in rotation with other crops. A survey of long-term trends in N-2 fixation by selected grain legumes is in cluded, and some possible explanations for the observed stagnation in effor ts to increase N-2 fixation under field conditions are presented. (C) 2000 Elsevier Science B.V. All rights reserved.