The nitrogen advantage of legumes in crop rotation systems is well known, but the nitrogen advantage from legumes to associated crops in an intercropping system is unclear. The objective of this study was to determine whether plant-available nitrogen is released by cowpea and soybean during the growing season, since this could mean a saving in nitrogen fertilizer for an associated crop in an intercropping system. Nitrate nitrogen levels were monitored monthly throughout two growing seasons on cowpea, soybean and maize plots at two localities. There was no evidence of nitrogen enrichment of soil by the legumes in the early growing stages, but nitrogen was released into the soil when the legumes reached maturity, about four months after planting. The nitrogen enrichment of the soil occurred only within the legume rows, since no differences in nitrogen levels were found between the rows. This suggests that the mechanism of nitrogen enrichment of soil from these legumes is the rapid mineralization of nitrogen from roots and/or nodules when they reach maturity. It is, therefore, unlikely that legumes can provide a nitrogen advantage to associated crops within an intercropping system in the same season.