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 and 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 strength of the sink, i.e., the ability of the host plant to accumulate N, (3) the amount of available soil N and (4) environmental constraints to N-2 fixation. Much of the N fixed by grain legumes is removed at harvest, the remainder 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 overall 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 included, and some possible explanations for the observed stagnation in efforts to increase N-2 fixation under field conditions are presented. (C) 2000 Elsevier Science B.V. All rights reserved.