Dahlen Long-term Nitrogen and Phosphorus Recovery Experiment

The Dahlen nutrition experiment was established by Incitec Pivot Fertilizers in 1996 to investigate the interaction of different rates of N and P within a modern cropping system (i.e., retain stubble, no-till) on a Vertisol. Since establishment, the site has been in a canola, wheat, barley, pulse rotation (with oaten hay in 2011) on factorial combinations of annual applications of N (0, 20, 40, 80, and 160 kg N/ha as urea) and P (0, 9, 18, and 36 kg P/ha as triple superphosphate). Long term analyses of trends in soil properties have been undertaken and show that the best nutrient combination for productivity was also the best rate for improving soil carbon, which rose from by about 20% with balanced N and P supply compared to the unfertilized treatments.

A balance sheet of N and P was developed for each treatment set over the 18 years of the experiment, and approximately 88% of the P applied at the 9 kg P/ha application rate was recovered in crop products. This treatment showed a total of 40 kg P excess over removal which was seen as an increase in Colwell soil test P from 24 mg/kg to 40 mg/kg. This implies that each kg P applied in excess of removal resulted in a 0.4 mg/kg increase in soil test value. Similarly for N, approximately 92% of the applied N at the 40 kg N/ha rate could be recovered in crop products. Higher application rates led to lower recoveries.

Higher application rates of N or P led to lower recoveries in product, but much of the P could be accounted for by changes in soil test values, while the excess N seems to have been leached below the root zone, rather than lost via denitrification.