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