Objectives This paper studied the optimum rate of organic manure N in the total N input of winter wheat–summer maize rotation system from an aspect of economy under long-term fertilization, in order to provide a reliable technical support for efficient organic N application.

Methods The data was collected from a long-term field experiment in Dezhou, Shandong Province. The duration was from 2007 to 2017, the tested cultivars from October 2007 to October 2009 were Triticum aestivum L. cv. Yannong19 for wheat and Zea mays L. cv. Jundan20 for maize, and after October 2009 were Triticum aestivum L. cv. Jimai22 for wheat and Zea mays L. cv. Zhengdan958 for maize. A complete random block design with triplicate were setup in the experiment, including 4 organic fertilizer N rates (cattle manure) of 0, 45, 120, 240 kg/hm² and 6 urea N rates of 0, 45, 90, 120, 180, 240 kg/hm². The chemical N was half basal applied and half top dressed at jointing stage of wheat or at big bell stage of maize. The total manure was either basal applied at once on wheat or separately basal applied on wheat and maize at twice a year. All the phosphate and potash fertilizers were only basal applied on wheat and maize in rate of 150 kg/hm² each year in the treatments containing NPK. At harvest, the wheat and maize samples of above ground parts were collected and grain yields were investigated. The annual productive benefits for wheat and maize system were calculated, and the N nutrient price dependent benefits were analyzed.

Results For the 10-year average annual yield of wheat plus maize, no significant difference was observed between single chemical fertilizer and single manure treatments, while both were significantly lower than those of the combined application treatments of organic and chemical fertilizer. The annual yield of wheat and maize decreased with time in the treatments of inorganic N input < 180 kg/hm², while those in treatments of N > 180 kg/hm² maintained relatively stable and increased with time in the treatments of combined organic and chemical fertilizer application. For the maximum production value, the proportion of organic N decreased from 60% to 50% during the 10 years, while for the optimal economic benefit (marginal production ratio was 5∶1), the optimum proportion of organic N increased from 0% in the first two years to 49.1% in 2017 when the price of organic N was epual to chemical fertilizer nitrogen. In fact, the price of organic N was twice as much as urea N. The best ratio of organic N increased from 0% in the first three years to 34% in 2017 based on the actual fertilizer price.

Conclusions The long-term experiment proves the high yield and efficiencies are obtained by combined application of organic and chemical fertilizers than those of single organic or chemical fertilizer application. As the price of organic N is generally twice of that of chemical N, and keeping the optimum economic benefit, higher proportion of organic N could be applied with the elongation of organic and chemical fertilizer combination study. Under the tested conditions, the proportion of organic N could be increased to 34% after 10 years of organic and chemical fertilizer combination.