Suitable sowing rate of green manure for replanting oilseed rape under different tillage patterns after wheat in the Yellow River tank farm area of Inner Mongolia

To further clarify the suitable tillage patterns and sowing quantity parameters of rape green manure for multiple cropping after wheat, a field experiment was conducted in Dalad Banner, Inner Mongolia. Seven sowing quantity gradients (7.50, 11.25, 15.00, 18.75, 22.50, 26.25, and 30.00 kg·ha⁻¹) were set under three tillage patterns (straw removal tillage, straw returning tillage, and straw removal no-tillage). The suitable sowing rate was studied based on the biomass and nutrient accumulation of rape green manure. Results showed that the average dry matter accumulation of each sowing amount under the straw returning tillage planting mode reached 8.92 t·ha⁻¹, which significantly increased by 26.5% and 16.4% compared to the straw removal tillage and straw removal no tillage modes, respectively (P < 0.01). The sowing amount significantly affects the biomass and nutrient accumulation of rapeseed green manure (P < 0.01) and shows similar trends in the three modes. When the sowing amount ranges from 7.50 to 26.25 kg·ha⁻¹, the biomass of rapeseed green manure increases with the increase of sowing amount. At 26.25 kg·ha⁻¹, the biomass of rapeseed green manure in all modes can reach 7 t·ha⁻¹ or above. When the sowing rate exceeds 26.25 kg·ha⁻¹, there is no significant difference in the biomass of rapeseed. The trend of the accumulation of carbon, nitrogen, phosphorus, and potassium nutrients in rapeseed green manure with the change of sowing amount is consistent with the biomass. The specific suitable sowing amounts for the three planting modes are slightly different. According to the fitting equation between the maximum biomass, nutrient return, and sowing amount obtained from the theory of rapeseed green manure, the suitable sowing amount range for the tillage mode (wheat straw removal and return) is 24.39～28.36 kg·ha⁻¹. In contrast, the straw removal no tillage mode can increase the sowing amount by approximately 20%, with a suitable sowing amount range of 30.03 to 37.84 kg·ha⁻¹.