Dahlen Long-term Nitrogen and Phosphorus Recovery Experiment

The Dahlen long-term N and P experiment was established in 1996, and has been sown to a canola, wheat, barley, and pulse rotation each year since then. Treatments included four rates of P (0, 9, 18, 36) applied at sowing and 5 rates of N (0, 20, 40, 80, 160) applied either all at sowing or as a 50:50 split between sowing and the start of reproductive growth. Soil test values, including total P, Colwell P, total N, and mineral N contents were tracked over the course of the experiment to provide information on the long-term effects of fertilizer use in this region, including fertilizer nutrient use efficiency.

Colwell P at the start of the experiment was 24 mg/kg (PBI 115), and after 16 years its values were 17, 40, 72, and 125 mg/kg for the 0, 9, 18, and 36 kg P/ha/yr rates. Soil mineral N values were 24, 36 and 34 kg N/ha at 0 to 60 cm depth and 58, 226 and 529 kg N/ha at 0 to 150 cm depth for 0, 80 and 160 kg N/ha rates, respectively. Soil C values were not affected by N application and averaged 1.24 +/- 0.16, while soil C levels were 1.09, 1.25, 1.33, and 1.29 (LSD= 0.21 at p=0.05) for the 0, 9, 18, and 36 kg P/ha/yr rates, respectively. Based on these data, it can be inferred that P application rates that are similar to P removal will maintain both soil P and soil C contents. Over application of N resulted in a large accumulation of N in the subsoil but had no effect on soil C. ANZ-08