Tillage combined with straw return increases maize yield and water use by regulating root morphological distribution and nitrogen metabolism in Northeast China

The combination of tillage and straw return is effective for improving soil structure and increasing farmland productivity; however, few studies have explored the changes in the root morphological distribution and nitrogen (N) metabolism of crops, grain yield, and their potential interactions in Northeast China. A field study that commenced in 2017 was conducted with a two-factor split design and included nine treatments in total: three tillage depths of 10 (D10), 30 (D30), and 50 cm (D50), and three straw application techniques, i.e., mixing with soil (SM), burying in soil (SB), and straw removal (SR). Soil and plant samples were collected and analyzed in 2022 and 2023. The root growth and distribution of maize significantly increased under SM and SB, contributing to greater root length density, surface area density, and volume density in the 0 −90-cm soil layer. The responses of soil N availability and root N metabolism to straw return increased, as indicated by higher ammonium and nitrate N contents, and nitrate reductase, nitrite reductase, glutamine synthetase, glutamate dehydrogenase, and glutamate synthase activities. Combined with partial-least-squares path modeling and random forest, these morphological and physiological improvements were conducive to increasing the maize grain yield and water use efficiency. Tillage of the upper 30-cm soil layer combined with SM and tillage of the upper 50-cm soil layer combined with SB resulted in high productivity for maize. Our results emphasize the key roles and synergistic effects of tillage depth and straw return in enabling sustainably high maize yields in Northeast China.