Film Mulching Enhances Wheat Productivity in Tilled Systems but Not in No-Till Systems by Differentially Regulating Root-Zone Temperature During the Spring Season in the North China Plain

Enhancing winter wheat yield in early spring relies on optimal soil temperature (ST) conditions and robust root systems, particularly in cold and dry areas. However, the long-term combined effects of conservation tillage and plastic film mulching (PFM) on the crop root system during early spring (the period of rejuvenation and jointing) remain unstudied. This study is based on a 22-year field experiment involving two long-term conservation tillage methods: mouldboard plowing with crop residue incorporation (MC) and no-tillage with crop residue cover (NC). The main treatments were further divided by applying black (B) and white (W) plastic films to each, resulting in MC with black (MCB) and white (MCW), and NC with black (NCB) and white (NCW) films. ST was recorded at depths of 0–40 cm during the afternoon, evening, and morning, while root characteristics (RCs) were measured at the peak flowering stage at depths of 0–60 cm, and crop yield and attributes were recorded at harvest during the 2023–2024 cropping season. Compared with MC and NC, MCB and MCW increased afternoon ST by 2.5 °C and 0.94 °C, and evening ST by 1.94 °C and 1.87 °C, while NCB and NCW decreased ST. MCB and MCW also increased accumulated ST during overwintering (131–161 °C) under the tilled system. PFM on MC increased the root length and weight densities by 10–17% and 25–32%, respectively; NCB and NCW decreased RCs by 8–15.2% across the soil depth. Additionally, afternoon and evening STs at 5–20 cm positively correlated with RCs and yield attributes (r > 0.84), whereas morning ST and a 40 cm depth were negatively correlated (r < −0.77). Under tilled conditions, both MCB and MCW substantially increased grain yield (10–12%) and biomass (31–38%) compared with MC. In contrast, NCB and NCW showed no yield and biomass advantage and even reductions (16–12% and 14–3%, respectively) compared with NC. FPM improved STs, RCs, and yield under tilled conditions but not in no-till systems, highlighting the need for supplementary practices to optimize ST in no-till systems.