Grain sorghum [Sorghum bicolor (L.) Moench] has become a major dryland crop for the Great Plains, but information on production in rotations is limited. This study was conducted to determine N fertilizer recovery and use by grain sorghum in monoculture and rotational systems. Grain sorghum was grown under rainfed conditions on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic, Typic Argiudoll) in: (i) continuous grain sorghum monoculture, (ii) a 2-yr soybean [Glycine max (L.) Merr.]-grain sorghum rotation, (iii) a 4-yr rotation of soybean-corn [Zea mays L.]-oat [Avena sativa (L.)] + clover [80% Melilotus officinalis (L.) and 20% Trifolium pratense (L.)]-grain sorghum, and (iv) a 4-yr rotation of oat + clover-corn-soybean-grain sorghum at Mead, NE. Broadcast applications of 15N-depleted NH4NO3 were made at 90 and 180 kg N ha(-1) in 1985 and 1986 to evaluate N fertilizer recovery. Fertilizer N recovery determined by isotopic methods was significantly higher for grain sorghum in monoculture (64.9%) vs. grain sorghum in rotation (54.9%). Fertilizer N recovery estimated by the difference method ranged from 118.5% in continuous grain sorghum to 9.1% in sorghum following oat + clover. Differences in N-recovery results by the isotope and difference methods indicated N fertilizer applied to grain sorghum in various cropping systems appeared to be entering different organic soil-N pools. These results substantiate the importance of the mineralization-immobilization turnover (MIT) effect in N-isotope experiments and the necessity for careful interpretation of N-recovery results, especially between complex cropping systems.
