Objectives Replacing part of chemical fertilizer with manure can reduce chemical fertilizers input, improve soil fertility; however, it also has risk of decreasing crop yield. So we studied the effect of different manure substitution ratios on soil fertility, nutrient supply, maize growth and yield, to propose a suitable substitution ratio of manure for crop production in black soil of Heilongjiang Province.

Methods A 7-year field experiment was carried out in Harbin, Heilongjiang Province during 2017−2024, the cropping system was maize-maize-soybean rotation, with one crop a year. The five treatments included merely chemical fertilizer (NPK), and the organic manure replacing rate of 25% (1/4M), 50% (2/4M), 75% (3/4M), and 100% (4/4M) at the same total N, P, K input with the NKP treatment. At soybean harvesting in 2023, soil samples were collected to analysis soil organic matter and nutrient contents. At the main growing stages of maze in 2024, soil samples were collected for the measurement of available nutrient contents, and the maize plant height was measured at the same stage. And maize yields were investigated at harvest in 2024.

Results Compared to the NPK treatment, the 1/4M treatment did not alter soil nutrients, whereas the 2/4M treatment significantly increased soil NO₃⁻-N and total P (TP) by 21.1% and 10.7%, respectively, and the 3/4M and 4/4M treatments significantly enhanced total N (TN), NO₃⁻-N, TP, total K (TK), available P (AP), and available K (AK) by 15.4% and 30.8%, 40.5% and 47.0%, 14.2% and 26.8%, 9.4% and 18.1%, 23.6% and 106.5%, and 8.6% and 25.5%, respectively, at harvest time of 2023. In the 0-20 cm soil layer, soil NO₃⁻-N gradually decreased with increasing organic manure substitution rates at the jointing and tasseling stages, and soil NO₃⁻-N of each treatment showed a change with initial increase followed by a decrease during maize growth period; however, the 3/4M and 4/4M treatments enhanced NO₃⁻-N contents relative to other treatments at the late filling stage. Soil NH₄⁺-N presented a similar change to NO₃⁻-N among different treatments at the early and middle maize growth stages, but NH₄⁺-N did not differ significantly among treatments at the late filling stage. Across maize growth season, soil AP was the lowest in 1/4M treatment, followed by the NPK treatment, and AP increased with increasing organic manure substitution ratios in other treatments. Soil AK showed an opposite change to NO₃⁻-N, which was the lowest in the NPK treatment, and then increased with increasing organic manure substitution ratios. In the 20−40 cm soil layer, the dynamics of NO₃⁻-N, NH₄⁺-N, and AK among different treatments were similar to those in the 0−20 cm layer during maize season; however, all nutrients were lower than those in the topsoil at the same stage, and the difference in AP among treatments was less pronounced compared to that in the 0−20 cm layer. At the jointing and tasseling stages, maize plant height did not differ significantly between the 1/4M and NPK treatments, whereas the 2/4M, 3/4M, and 4/4M treatments reduced plant height by 10.8%−15.3%, 16.4%−16.9%, and 18.4%−19.8%, respectively, relative to the NPK treatment. Maize yield did not differ between the NPK and 1/4M treatments across different years, but the 2/4M, 3/4M, and 4/4M treatments reduced maize yield by 6.5%−21.7%, 15.6%−50.4%, and 23.4%−60.0% than the NPK treatment, respectively. Correlation analysis revealed that soil inorganic N supply level at the early and middle growth stages of maize was the main factor affecting maize growth and yield.

Conclusions Although replacing over 25% of chemical fertilizer N with organic manure could maintain soil fertility, it reduced maize yield because of insufficient N supply to maize at vegetative stage. So the organic substitution rate should be controlled within 25% in black soil of Heilongjiang Province.