Nitrogen application rate for keeping nitrogen balance in wheat–soil system in Weibei rainfed areas under different rainfall years

Objectives  The apparent nitrogen balance of wheat–soil system in different rainfall years in Weibei dryland were investigated, and the suitable nitrogen rates were proposed for different rainfall years, aiming to keep stable wheat yield, balance the soil nitrogen and avoid excessive nitrogen residue.

Methods  The rainfall from July 2011 to June 2012 was 710.1 mm, which was 23.1% higher than the average, belonging to humid year; That from July 2012 to June 2013 was 391.4 mm, 32.2% lower than the average, belonging to drought year; From July 2013 to June 2014 was 603.8 mm, 4.6% higher than average and belonging to normal year. A field experiment with five nitrogen application rates of N 0, 75, 150, 225, 300 kg/hm² (expressed as N0, N75, N159, N225, and N300) had been conducted during the three years. The wheat yield, wheat N uptake, apparent nitrogen balance and the nitrate residue were compared among the three years, and the suitable nitrogen application rate was calculated by the input and output of soil nitrogen, and tested by crop uptake, yield and nitrate accumulation in 100 cm soil layer.

Results Under the same rainfall, the increase of N application rate led to increment at first and then decrease of wheat yield. Although the highest yields in the three years were all recorded in the N225 treatment, the differences between the N225 and N150 were not significant. The pattern of wheat N uptake was similar with grain yield. The residual NO₃^–-N in 0–100 cm soil layer after wheat harvest was also increased with the increase of N application rate, of which 50.8%–75.5% concentrated in 0–40 cm and gradually decreased with the extension of the years. The residual NO₃^–-N in 40–100 cm soil layer was increased year by year and leached downward deeper. In case of no NO₃^–-N surplus, the nitrogen application rates were N 127, 54 and 103 kg/hm² respectively in wet, dry and normal years; When the allowable NO₃^–-N surplus was 40 kg/hm² for stable wheat yield and soil fertility, the nitrogen application rates were N 170, 99 and 150 kg/hm² respectively in turn, with which the wheat N uptake and wheat yield were close to the theoretical maximum values, and the nitrogen amounts were 4.7%, 142% and 21.3% less than those for maximum N uptakes, and 5.3%, 120% and 30.7% less than those for the maximum yields. The NO₃^–-N surplus in 0–100 cm layer after wheat harvest were 101.4, 104.2 and 113.7 kg/hm², respectively, which were within the safe threshold of residual nitrate level (N 90–100 kg/hm²).

Conclusions Considering the differences of nitrogen balance in winter wheat–soil system with different N application rates in different rainfall years, it is suggested that the nitrogen application rates in wet, dry and normal years in Weibei dryland should be N 170, 99 and 150 kg/hm² respectively to ensure stable crop yield and utilization efficiencies of nitrogen.