Nitrogen dynamics and budgets in a winter wheat–maize cropping system in the North China Plain

Abstract

Nitrogen dynamics and budgets in a clay loam soil (Meadow Aqualf) in the North China Plain were investigated in a winter wheat ( Triticum aestivum L.) and maize ( Zea mays L.) cropping system comparing the effects of four N rates (0, 120, 240 and 360 kg N ha −1 as urea) applied twice to each crop over 2 years. Ammonium nitrogen (NH 4-N) in the soil profile remained at a low and constant level (except in the surface 20 cm layer) following application of fertilizer N. In contrast, nitrate nitrogen (NO 3-N) levels were significantly altered by the rate of applied N. A strong tendency of NO 3-N to move from the surface layer to the lower layers (20–100 cm) was observed during the wheat and maize growth seasons in treatments of 240 and 360 kg N ha −1 per crop (N240 and N360). The amounts of NO 3-N accumulated in the soil profile were significantly higher in N240 and N360 than those in N0 and N120 (treatments receiving 0 and 120 kg N ha −1 per crop). After 2 years, soil NO 3-N levels at 0–300 cm depth in N120, N240 and N360 amounted to 336, 815 and 1141 kg ha −1, respectively, with more than half of these amounts distributed in the 100–300 cm layer. The calculated total N balance indicates that most fertilizer N was available as NO 3-N in the top 300 cm of the soil profile using traditional fertilization and irrigation practices. Over the subsequent 2 years, N losses were calculated to be relatively low in N120 but significantly higher in N240 and N360. Measured gaseous N losses showed that NH 3 volatilization and denitrification comprised only a small fraction of total N losses during the 2-year rotation, while NO 3-N leaching from the top 100 cm of the soil profile accounted for most N losses across all N rates and experimental years. The N budget showed that accumulation and/or leaching of NO 3-N below 100 cm depth (beyond the reach of most roots) was the main pathway for N losses in the winter wheat–maize cropping system. The recommended N application rate of 120 kg N ha −1 minimized soil NO 3-N accumulation and leaching losses while maintaining high yields and N utilization by winter wheat and maize.