The nitrogen (N) fractions and release characteristics varied much among different types of organic manure. Study and estimation on N fertilizer replacement value of different livestock manures with chemical N as a reference could provide a theoretical basis for rational application of organic fertilizer and scientific combination application of organic and inorganic fertilizers.

In this study, composted pig manure, chicken manure, cattle manure and chemical fertilizer were selected as the research materials. There were six application rates of N set, i.e. 0, 40, 80, 120, 160 and 200 mg·kg^-1 dry soil, respectively. A soil column experiment was conducted to investigate the effects of N derived from manure and chemical fertilizers on wheat yield and N uptake. Furthermore, the relative replacement equivalence of N in three kinds of manure was calculated by using the response between crop N uptake or yield and N application rate.

(1) The grain yield and biomass of wheat were increased with the elevated N application rate. When 40-120 mg·kg^-1 dry soil input, the increase of grain and biological yield of chemical fertilizer and pig manure was higher than that of chicken manure and cow manure, while there was no significant difference among grain yield treated with chemical fertilizer, pig manure and chicken manure, except being significantly higher than the cow manure, when N input at 160-200 mg·kg^-1. (2) At the same N input rate, chemical fertilizer could significantly improve wheat grain N uptake and aboveground N uptake compared with manure treatment. For three kinds of manure treatment, the total aboveground N uptake associated with pig manure was the highest, that with chicken manure followed, and that with cow manure was the low. In this study, the N recovery rate under three kinds of manure treatments was firstly increased and then decreased with the elevated N application level, while the N recovery rate of chemical fertilizer showed a gradually decrease trend. (3) When N input at different application levels, the relative N replacement value of pig manure, chicken manure, and cow manure were 37.7%-84.2%, 23.1%-71.0% and 11.3%-34.2%, respectively, with wheat grain yield as a reference index; 49.2%-91.3%, 23.3%-78.3%, 7.4%-42.2% with aboveground biomass as a reference index; 31.1%-76.3%, 19.8%-67.1%, 6.0%-35.7% with grain N uptake as a reference index; 30.8%-97.1%, 19.8%-75.6%, 7.8%-43.8% with aboveground N uptake as a reference index. There was a positive correlation (P<0.01) between the N replacement value of organic manure and N application rate.

In this study, the relative N replacement value of organic manure was affected by both organic fertilizer types and N application rate. Within a certain range of N application level, the N replacement value of livestock manure increased with the increase of N application level. The average relative equivalent of N replacement by pig manure, chicken manure, and cow manure were 59.6%, 46.2%, and 23.6%, respectively.