Improved nitrogen use efficiency would be beneficial to agroccosystem sustainability in the northern Great Plains of the USA. The most common rotation in the northern Great Plains is fallow-spring wheat. Tillage during fallow periods controls weeds, which other-wise would use substantial amounts of water and available nitrogen, decreasing the efficiency of fallow. Chemical fallow and zero tillage systems improve soil water conservation, and may improve nitrogen availability to subsequent crops. We conducted a field trial from 1998 through 2003 comparing nitrogen uptake and nitrogen use efficiency of crops in nine rotations under two tillage systems, conventional and no-till. All rotations included spring wheat, two rotations included field pea, while lentil, chickpea, yellow mustard, sunflower, and safflower were present in single rotations with wheat. Growing season precipitation was below average in 3 of 4 years, resulting in substantial drought stress to crops not following fallow. In general, rotation had a greater influence on spring wheat nitrogen accumulation and use efficiency than did tillage system. Spring wheat following fallow had substantially higher N accumulation in seed and biomass, N harvest index, and superior nitrogen use efficiency than wheat following pea, lentil, chickpea, yellow mustard, or wheat. Preplant nitrate-N varied widely among years and rotations, but overall, conventional tillage resulted in 9 kg ha(-1) more nitrate-N (060 cm) for spring wheat than did zero tillage. However, zero tillage spring wheat averaged 11 kg ha(-1) more N in biomass than wheat in conventional tillage. Nitrogen accumulation in pea seed, 45 kg ha(-1) was superior to that of all alternate crops and spring wheat, 17 and 23 kg ha(-1), respectively. Chickpea, lentil, yellow mustard, safflower, and sunflower did not perform well and were not adapted to this region during periods of below average precipitation. During periods of drought, field pea and wheat following fallow had greater nitrogen use efficiency than recropped wheat or other pulse and oilseed crops.
