Effects of tillage methods and nitrogen application rate on maize yield and soil aggregates in northeastern China under straw returning

Northeastern China is a major maize-producing region, but decades of unsustainable tillage practices and excessive nitrogen fertilization have degraded black soils and threatened the sustainable production of maize. Conservation tillage practices based on straw return and reduced or no tillage are key approaches to improving soil quality and maintaining crop productivity, and optimizing nitrogen inputs is essential for enhancing resource-use efficiency. To determine suitable tillage practices and optimal nitrogen rates under straw incorporation, and to promote coordinated improvements in maize yield and soil quality in northeastern China, a fixed-site field experiment was conducted from 2022 to 2024 at Yanjiagang farm, Harbin, Heilongjiang province. The experiment used a split-plot design with tillage as the main-plot factor—conventional tillage (CT), rotary tillage (RT), strip tillage (ST), and no tillage (NT)—and nitrogen rate as the subplot factor: 0 (N0), 135 (N1), 180 (N2), and 225 kg hm⁻² (N3) as pure N. Across the three years, mean grain yield ranked ST > CT > RT > NT, although differences among tillage treatments were not significant. Relative to NT, ST increased yield by 14.5% in 2023 and 6.6% in 2024, with no significant differences between ST and CT or RT. Nitrogen rate significantly increased yield (P < 0.001); yield under N2 and N3 were 183.1%-217.2% higher than under N0. No significant tillage×nitrogen interaction was detected for yield. Mean soil bulk density in the 0-20 cm layer followed the order NT > ST > RT > CT, with significant differences among tillage treatments; NT was 20.2%-31.4% higher than CT. Both tillage and nitrogen rate significantly affected aggregate-size distribution, mean weight diameter (MWD), and geometric mean diameter (GMD) (P < 0.05). NT increased the proportion of 0.250-2.000 mm aggregates in the 0-20 cm layer by 23.7%-56.7%, and MWD and GMD ranked NT > RT > ST > CT. The N2 and N3 treatments increased the proportion of 0.250-2.000 mm aggregates by 9.2%-29.7% and decreased the proportion of < 0.053 mm aggregates by 30.3%-51.5%. Correlation analysis showed significant positive associations between tillage practice and nitrogen rate under straw return and maize yield (P < 0.01), and the ST treatment achieved the maximum yield at an estimated N rate of 200.2 kg hm⁻². Overall, under straw incorporation, strip tillage combined with 180-225 kg hm⁻² N can achieve high maize yield while improving soil aggregation, and is therefore recommended for the cold regions of northeastern China.