Crop rotations including legumes have increased in importance because of their potential to reduce large inorganic N fertilizer needs for corn [Zea mays L.) and other crops. This study was conducted to determine N fertilizer recovery by corn in monoculture and rotational systems. Corn was grown under rainfed conditions on a Sharpsburg silty clay loam (fine, montmorillinitic, mesic, Typic Argiudoll) in four cropping systems: (i) continuous corn monoculture, ii) a 2-yr soybean [Glycine max (L.) Merr.]-corn rotation, (iii) a 4-yr rotation of oat [Avena sativa L.)] + clover [80% Melilotus officinalis (L.) and 20% Trifolium pratense]-grain sorghum [Sorghum bicolor (L.)]-soybean-corn, and (iv) a 4-yr rotation of soybean-grain sorghum-oat + clover-corn 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 by corn in each cropping system using isotopic methods. Nitrogen recovery determined by isotopic methods was significantly higher for corn in rotation vs. corn in monoculture, averaging 58.6 vs. 52.3% and 49.8 vs. 43.4% at the 90 and 180 kg N ha-1 rates, respectively. In contrast, fertilizer N recovery estimated by the difference method was much greater in continuous corn vs. N recovery in corn following oat + clover in the 4-yr rotation. These differences indicated that N fertilizer applied to corn in each cropping system appeared to be entering different sizes and types of organic soil N pools, resulting in apparent differences in N immobilization. Our results demonstrate problems exist in estimating fertilizer N recovery with both methods (isotope or difference) and before accurate N recovery estimates by corn or any other crop can be made in complex soil and crop management systems, procedures must be developed to explicitly follow N fertilizer pathways (immobilization, denitrification, volatilization, leaching, etc.). Until that time, correct interpretations with either method can be obtained within a system if all available information is used regarding both soil and crop factors.
