Cropping systems, nitrogen (N) fertilizer levels, and climate largely dictate patterns of N use and influence problems arising from N fertilization. Nitrogen use was assessed in cropping systems with a nitrogen removal-index (NRI), defined as the ratio of N removed in the grain to total N supply including that from N fixation by legumes grown in rotation. Results are reported from analyses of NRIs of cropping systems that comprised a 12-yr continuous and sequential growing of maize [Zea mays L.], soybean [Glycine max. (L.) Merr.], sorghum [Sorghum bicolor (L.) Moench], and oat/clover [Avena sativa (L.)/80% Melilotus officinalis (L.) Lam., 20% Trifolium pratense] in eastern Nebraska. Rotations involving maize or sorghum had higher NRIs than continuous cereals at 0 N application levels. Increasing N rates reduced NRI and resulted in an increase of residual nitrate in all but the continuous soybean system. Also, NRI was highest in continuous soybean, lower in continuous maize, and lowest in continuous sorghum. Rotations and lower N rates both contributed to higher NRI and lower soil residual nitrate. Biological windows that comprised the cumulative number of days in the entire year when soil is moist and temperature above a specific threshold correlated positively and significantly with NRI, whereas NRI and August temperature were negatively related. Between 43 and 87% of variability of NRI in maize and soybean systems was attributed to August temperature plus August precipitation index. Biological window (moist soil, temperature above 5°C ) plus May temperature explained up to 76% of variability of NRI of maize and soybean. Nitrogen removal index for sorghum was unrelated to weather variables. Estimated additions to the soil organic N reserve from the return of crop residues averaged between 16 and 80 kg ha-1 yr-1 with higher levels from sorghum and from all treatments with high levels of N fertilizer. Crop rotations generally increased the N-removal index, reduced the year-to-year variability in N-removal-index, and at 0 N-application rate, increased the return of N in residue to the soil N pool, compared to continuous cropping of single species.
