Root and nitrate-N distribution and optimization of N input in winter wheat

Scientific management of nitrogen (N) fertilizer has a significant effect on yield while also reducing the environmental risks. In this study, we conducted field experiments over three years at two different sites (Zhengzhou and Shangshui) in Henan Province, China, using different N application rates (0, 90,180, 270, and 360 kg ha^-1) to determine the relationships between soil N supply and N demand in winter wheat (Triticum aestivum L.). Optimal N input was then determined. Both sites showed the same trend. Namely, aboveground N uptake and soil nitrate N (NO₃^--N) increased with increasing N, while NO₃^--N decreased with increasing soil depth, gradually moving downwards with growth. A significant correlation (p < 0.001) between increasing aboveground N uptake and increasing NO₃^--N was also observed under N application, with the best relationships occurring in the 20-60 cm layer during jointing-anthesis (R² = 0.402-0.431) and the 20-80 cm layer at maturity (R² = 0.474). Root weight density showed the same spatial-temporal characteristics as NO₃^--N, following a unimodal trend with increasing N, and peaking at 90 kg ha^-1. The root weight density was mainly distributed in the 0-60 cm layer (above 80%), with the 20-60 cm layer accounting for 30% of the total root system. In this layer, the root weight density was also significantly positively correlated with aboveground N uptake. Wheat yield reached saturation under high N (>270 kg ha^-1), with a sharp decrease in N use efficiency (NUE) and linear increase in residual NO₃^--N. To balance yield and the risk of environmental pollution in the experimental area, an N application rate of 180-270 kg ha^-1 is recommended under sufficient irrigation, thereby supporting a well-developed root system while ensuring balance between N supply and demand.