The Regulatory Effect of Integrated Agronomic Management on the Root and Shoot Growth Relationship of Shallow-Buried Drip Irrigation Maize in the West Liaohe Plain

Water conservation and grain yield improvement are primary objectives for sustainable agricultural development in arid and semi-arid regions. Variety selection, planting density, and irrigation management represent crucial agronomic practices that regulate root–crown growth and grain yield in maize. A two-year field experiment was carried out from 2021 to 2022 in Tongliao, Inner Mongolia Autonomous Region, China. Two widely cultivated maize varieties, DK159 and ZD958, were used as test materials. Two planting densities were designed: 60,000 plants ha−1 (D1, local farmers’ conventional density) and 90,000 plants ha−1 (D2). Five irrigation levels were established: 450 mm (I5, local farmers’ practice, CK), 360 mm (I4), 270 mm (I3), 180 mm (I2), and 90 mm (I1). We investigated the interactive effects of variety, planting density, and irrigation amount on dry matter accumulation pre- and post-silking, root spatial distribution characteristics, and the coordination mechanism of root–shoot growth in maize under shallow-buried drip irrigation. The results indicated that grain yield under DK159 was 5.37–6.69% higher than that under ZD958, and the yield under D2 was 13.32–15.89% higher than that under D1. At the D1 density, no significant difference in grain yield was observed between I2 and I5, with yields ranging from 12.90 to 13.92 t ha−1. At the D2 density, grain yield under I3 was statistically similar to that under I5, ranging from 15.54 to 17.39 t ha−1. Compared with local farmers’ conventional planting density and full irrigation regime, increasing planting density and reducing irrigation amount altered the vertical root distribution of maize. The proportion of roots distributed in the 0–20 cm topsoil layer decreased, while appropriate water deficit markedly increased root proportion in the 40–60 cm subsoil layer. Increasing planting density and moderately reducing irrigation effectively promoted pre- and post-silking dry matter accumulation while maintaining a high harvest index (HI). At silking stage, the root–shoot ratio increased initially and then stabilized with increasing irrigation amount. At maturity, the root–shoot ratio gradually decreased and tended to be stable as irrigation increased. Therefore, the adoption of water-efficient maize varieties, combined with appropriately increased planting density and optimized irrigation regimes, can coordinate root–shoot relationships in the early growth period, facilitate early root establishment and late-stage nutrient accumulation, and thus improve maize yield. Under the conditions of shallow-buried drip irrigation in the supplementary irrigation area of the West Liaohe Plain, the adoption of water-saving maize varieties, appropriately increased planting density, and optimized irrigation regimes can coordinate the developmental relationship between root and above-ground growth, promote early root development and late-stage nutrient accumulation, and thereby simultaneously increase maize grain yield. These results provide practical theoretical and technical references for achieving high-yield and water-saving maize production under similar ecological conditions.