Nitrogen Uptake by Spring Wheat, Soil Distribution, and Recovery of N Fertilizer from Alternate Crop-fallow and Recrop Field Management Systems

Knowledge of N fertilizer utilization, movement, and recovery is required to attain maximum efficiency of applied N when shifting from fallow to recrop management. Field experiments involving spring wheat (Triticum aestivum) were established at two locations with different soil and climatic regimes in southwest Montana on fields which had been fallowed (fallow) or cropped (recrop) the previous year to evaluate the uptake, leaching, and recovery of ¹⁵NH₄¹⁵N0₃ fertilizer. Ammonium nitrate fertilizer was applied to these fields at rates of 30, 60, and 90 kg N/ha under dryland conditions. The fallow and recrop fields had high (greater than 100 kg/ha/l20 cm depth) and low (less than 50 kg/ha/l20 cm depth) levels of nitrate-N at seeding, respecitvely. The N content of grain was 48% greater on recrop than on fallow with higher N content associated with higher rates of N fertilizer. The N content of straw showed trends similar to that for grain. In addition, increased translocation of N from straw to grain was associated with recropping and increased rates of N fertilizer. The proportion of N in the plant derived from fertilizer averaged twice as great on recrop as on fallow and increased 2- to 20-fold with the addition of 90 kg N/ha. These responses reflected the variation in initial NO₃-N level of each field. Residual NO₃-N and total ¹⁵N levels in the soil at harvest were low and decreased with depth to 120 cm. Soil NO₃-N levels ranged from 5 to 15 kg/ha. Maximum recovery of N fertilizer was at the soil surface with little difference between fallow and recrop. No leaching below 120 cm was indicated. Total recovery of N fertilizer ranged from 45 to 90%. Overall there was 18% more N fertilizer recovered on recrop than on fallow. For each component, grain, straw, and soil, there was 22, 8, and 20% respectively more N fertilizer recovered on recrop than on fallow. Total recovery of N fertilizer varied from 67 to 77% for the rate of N applied. Grain and straw N recovery increased significantly with increasing N rate while soil recovery was highly variable.