The Ecologically Optimum Application of Nitrogen in Wheat Season of Rice–Wheat Cropping System

Because excessive application of N fertilizer for crop production leads to environmental pollution and low N utility efficiency, a better understanding of the effects of N application rates on crop yields and NO 3 –N leaching is required for developing optimum ecological N management that reduces NO 3 –N leaching while keeping crop yield. Field experiments at two sites in the Taihu region of China were conducted to study the ecologically optimum application of N in wheat ( Triticum aestivum L.) season of rice ( Oryza sativa L.)–wheat cropping system. The experiment at either site had five N rates on wheat (0–360 kg N ha −1 in 90‐kg increments) and NO 3 –N in leachate were collected by wedge‐shaped fiberglass wick lysimeters. At either site, the N‐wheat yield quadratic response curve was fitted quite well and a significantly linear relationship between N rates and seasonal NO 3 –N masses in leachate was also found. The calculated economically optimum N rate for wheat was more site related than depending on changing growing conditions from year to year, while the ecologically optimum N rate was significantly different both at sites and growing conditions ( P = 0.01). The results suggest that applying the ecologically optimum N rates of 120–180 kg N ha −1 to wheat is beneficial for maximally reducing NO 3 –N leaching loss but minimally decreasing yield.