Crop rotations including soybean [Glycine max (L.) Merr.] have increased in importance because of their potential to increase yields, reduce expenditures for pesticides and fertilizer, and increase net returns to the producer. Despite the importance of soybean in the USA, there is a lack of understanding of how N affects soybean in rotation systems. This study was conducted to determine the effects of rotation and N fertilization on N uptake and recovery by soybean. Soybean was grown under rainfed conditions on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic, Typic Argiudoll) in five cropping systems: (i) continuous soybean monoculture, (ii) a 2-yr corn (Zea mays L.)-soybean rotation, (iii) a 2-yr grain sorghum [sorghum bicolor (L.) Moench]-soybean rotation, (iv) a 4-yr rotation of corn-oat [Avena sativa L.)] + clover [80% Melilotus officinalis (L.) and 20% Trifolium pratense]-grain sorghum-soybean, and (v) a 4-yr rotation of grain sorghum-oat + clover-corn-soybean at Mead, NE. Broadcast applications of 15N-depleted NH4NO3 were made at 34 and 68 kg N ha-1 in 1985 and 1986 to evaluate N fertilizer recovery by soybean in each cropping system. Nitrogen recovery determined by isotope analyses was approximately 50% in all rotation systems and was not significantly affected by rotation or N rate. Large amounts of N were removed by the harvested soybean seed in all systems (150 to 200 kg N ha-1), regardless of N fertilizer applications. Soybean can act as a N sink and potentially aid in reducing the amount of soil N available for leaching. Since soybean grown in a rotation can utilize soil or fertilizer N along with the other benefits of rotations, soybean shows promise as a crop that may help reduce the leaching of nitrate-N in to ground water.
