Replacement ratio of chemical fertilizer nitrogen with manure under the winter wheat–summer maize rotation system in Lou soil

Objectives Replacing certain amount of chemical nutrients with manure is an important way to achieve the goal of zero-increase of total fertilizer consumption in the near future in China. This paper studied the responses of crops to the replacement, in order to realize the rational use of organic manure resource.

Methods A field experiment was carried out in two-year winter wheat-summer maize rotation system in Guanzhong Plain of Shaanxi Province. There were 6 treatments, i.e. no fertilization input as control (CK), 100% chemical fertilizers (NPK), 25%, 50%, 75% and 100% of the N supplied by manures, recorded as 25%M, 50%M, 75%M, and 100%M. The yields and nitrogen use efficiencies of crops were analyzed.

Results Crop yields were increased with the increase of organic N substitution ratios, and the treatment of 75%M had the highest crop yields. During two rotation years, the N recovery efficiencies were 35%–43% and 38%–61%; the N agronomic efficiencies were 12.3–18.3 kg/kg and 17–24 kg/kg; the N partial factor productivities were 41.4–46.5 kg/kg and 44–51 kg/kg; the N physiology efficiencies were 31–47 kg/kg and 34–46 kg/kg, respectively. All the N efficiencies were the highest under the 75%M treatment. The residual NO₃^–-N contents at 0–100 cm depth ranged from 11 to 133 kg/hm², and the highest residual was in NPK treatment after the two-years’ experiment. The N balance showed that the treatments of 50%M, 75%M and 100%M had N surplus of 33–120 kg/hm², with the highest surplus in the 100%M treatment, while the 25%M and NPK treatments had N deficit of 19.6 kg/hm² and 40.3 kg/hm², respectively. The average annual economic benefit was in a decreased order as 75%M > 50%M > 25%M > NPK > 100%M. The highest annual income was about 19906 yuan/hm² and the lowest was about 16634 yuan/hm².

Conclusions Substituting 75% of chemical N input with manure could effectively increase crop yields, N efficiency and economic benefits, and reduce soil nitrate nitrogen residues. However, it still needs to be further studied how combining organic manure with chemical fertilizer affects crop productivity and nutrient use efficiency in a long-term.