Abstract: The use of single tillage methods and excessive application of chemical fertilizer on the black soil of northeast China have resulted in poor soil structure, hardening of the plow base layer, and decline of soil fertility. We investigated the effects of deep-till-rototill rotation on soil structure and enzyme activity in the black soil area of Songnen Plain. With full straw returning in black soil, we carried out an experiment with two tillage treatments (continuous rotary tillage and deeptillrototill rotation) and two fertilization treatments (chemical fertilizer and chemical fertilizer+organic fertilizer). Soil samples from the 0-20 cm layer were air-dried, sieved, and divided into 2-5, 1-2, 0.5-1, 0.25-0.5, and <0.25 mm particle size aggregates. The activities of peroxidase, sucrase, urease, and acid phosphatase were measured for each aggregate size. The results showed that soil aggregates in the two fertilization treatments under deep-till-rototill rotation were mainly in the 2-5 mm particle sizes, which accounted for 26.8%-31.3% of the total mass and significantly higher than other particle sizes. The proportion of aggregates of 2-5, 1-2, and 0.5-1 mm in the deep-till-rototill rotation treatment was significantly higher than that in the conti-nuous rotary tillage treatment. The activities of catalase, urease, and acid phosphatase were higher under the deep-till-rototill rotation treatment than the continuous rotary tillage treatment. The application of organic fertilizer had a positive effect on the enzyme activity of large-sized aggregates. The activities of catalase, urease, and sucrase increased with particle size, with the highest activity of urease and sucrase in 2-5 mm aggregates. The activity of acid phosphatase increased with the decreases of particle size, being highest in <0.25 mm aggregates. In conclusion, combining deep-till-rototill rotation with the application of organic fertilizer benefits the coalescence and formation of soil macroaggregates in the 0-20 cm soil layer and has a significant effect on the improvement of enzyme activities in all aggregate sizes.

Key words: black soil, rotation tillage, fertilization, soil aggregate, soil enzyme activity.