Winter wheat grain yield and summer nitrate leaching: Long-term effects of nitrogen and phosphorus rates on the Loess Plateau of China

- Effects of N and P rates on winter wheat grain yield and N uptake varied by year.
- Summer nitrate leaching was increased by N application but decreased by P.
- Nitrate leaching occurred mainly in top 40 cm and accumulation in 40-300 cm deep soil.
- During summer, 1 mm rain caused a 1.6-3.6 mm downward movement of nitrate in soil.
- High grain yield and low nitrate leaching with 160 kg N ha−1 and 100 kg P2O5 ha−1.

Excessive N application in agriculture may lead to nitrogen losses in the environment, particularly nitrate leaching, which is a growing concern worldwide. However, only limited information is available about nitrate leaching during summer fallow in dryland areas, especially after the long-term application of N or P fertilizer. In 2004, we initiated a nine-year field experiment with five N and five P rates in the Loess plateau of China to investigate the nitrate leaching from top soil and its accumulation in deep soil. The objective was to determine suitable N and P rates for maintaining high grain yields and reducing nitrate leaching. The results showed that the winter wheat grain yield and N uptake were affected by the N and P rates, but they showed no response to N fertilizer in dry years. The nitrate leaching occurred mainly from the top 40 cm soil, and it was affected by the N and P rates, as well as the summer precipitation intensity. In the wet summer of 2011, the nitrate leaching increased from 14.6 to 250 kg N ha−1 as the N rate increased from 0 to 320 kg N ha−1, and only the P rate of 100 kg P2O5 ha−1 significantly decreased the nitrate leaching compared with the other P rates. In the normal summer of 2012, the nitrate leaching occurred only at 240 and 320 kg N ha−1, and no leahcing occurred in the dry summer of 2013. The nitrate leached from top soil was found to be accumulated in deep 40-300 cm soil, and the amount of deep soil accumulated nitrate was clearly higher than that leached from the top soil. The accumulation increased as the N rate increase, i.e., 37.7-387 kg N ha−1 in 2011, and 53.9-193 kg N ha−1 in 2012 when the N rate increased from 0 to 320 kg N ha−1, whereas it decreased from 196 to 134 kg N ha−1 in 2011, and from 134 to 55.9 kg N ha−1 in 2012 as the P rate increased from 100 to 200 kg P2O5 ha−1, but no accumulation was observed in deep soil in 2013. The downward movement of nitrate lagged behind that of the soil water, and 1 mm rainfall could cause a 1.6-3.6 mm downward movement of nitrate in the soil profile in summer. In conclusion, the application of less than 160 kg N ha−1 and around 100 kg P2O5 ha−1 could reduce nitrate leaching from top soil and decrease its accumulation in deep soil, while still maintaining a relatively high grain yield and N uptake in the dryland area of the Loess Plateau.