Nitrogen Removal and Use on an Australian Long-Term Fertilizer Experiment.

The Dahlen nitrogen and phosphorus experiment was established in 1996 and has been in a canola, wheat, barley and pulse rotation each year since then. The experiment has four rates of P (0, 9, 18, 36 kg/ha) applied at sowing, and five rates of N (0, 20, 40, 80, 160 kg/ha) either all at sowing or split. Crop and soil data has been collected for each crop year, which included two years where crops were sown but not harvested due to drought. A progressive N balance over the duration of the experiment has shown that N removal-to-fertilizer use ratios for the 9 kg P/ha/y were 2.96 (20 kg N/ha/y), 1.57 (40 kg N/ha/y), 0.92 (80 kg N/ha/y) and 0.47 (160 kg N/ha/y). Fixed N estimates were made based on pulse growth, checked with natural abundance measurements on site which ranged from 40 to 120 kg N/ha/crop depending on season. When fertilizer and legume N inputs are considered, the removal-to-supply ratios were 1.36 (20 kg N/ha/y), 1.04 (40 kg N/ha/y), 0.74 (80 kg N/ha/y) and 0.42 (160 kg N/ha/y). We conclude that this continuous cropping system was maintained as N neutral with the use of one pulse crop in 4 years, and the addition of 40 kg N/ha/y in the non-pulse crops. Soil organic carbon levels were unaffected by N application.