Tillage Legacy Modulates Soil Mechanical Sensitivity, Root System, and Wheat Grain Yield Under Short‐Term Compaction

Long‐term conservation tillage improves soil condition, but episodic wheel traffic may impose mechanical constraints that alter root function and reduce yield. How tillage legacy regulates compaction sensitivity and root compensation remains unclear. From 2023 to 2025, on a 22‐year tillage platform, we compared moldboard ploughing with residue incorporation (MC) and no‐tillage with residue retention (NC), each subjected to short‐term compaction (STC) (CMC and CNC). We assessed the soil quality index (SQI; integrating 10 physical (soil bulk density, porosity, penetration resistance, soil moisture content), chemical (available N, P, K, soil organic carbon), and biological (microbial biomass carbon, dissolved organic carbon) indicators in the 0–20 cm layer), root traits (length density, RLD, root weight density, RWD, root surface area, root diameter, RD, and root volume) and trait‐specific root fraction at two soil depths (0–20 cm and 20–40 cm), and grain yield across two consecutive growing seasons (2023–2024 and 2024–2025) following a single compaction event. Compaction reduced SQI under both tillage systems. In the immediate season (2024), the decline was greater under MC (42%) than NC (27%); however, the residual‐season effect (2025) was more persistent under NC (45%) than MC (40%), reflecting the stronger legacy vulnerability of the no‐tillage system. SQI loss was driven mainly by increased soil bulk density and soil penetration resistance and reduced soil moisture, carbon fractions, and nutrient availability. Compaction suppressed shallow‐root proliferation (RLD decline: CNC −35%, CMC −18% vs. non‐compacted controls in 2024) and, particularly under CNC, increased root fraction allocation to deeper layers across most root traits; however, 60%–90% of roots remained in the 0–20 cm layer across all treatments, and under the non‐water‐limited conditions of this experiment, deeper rooting did not compensate for topsoil impairment. Grain yield was reduced by 18.4% under CNC and 9.7% under CMC in 2024, with persistent reductions of 15.1% and 7.3%, respectively, in 2025 (all p < 0.05). Topsoil SQI was the strongest predictor of shallow‐root development (standardized path coefficient β = 0.79–0.92, p < 0.001; where β represents the change in root traits per standard‐deviation change in SQI), and shallow‐root traits exerted dominant positive effects on yield (β = 0.67–0.74, p < 0.001), explaining 70%–94% of yield variation, whereas deep‐root pathways were non‐significant. Yield stability under STC was governed by preservation of topsoil quality and shallow‐root functioning. NC exhibited greater mechanical vulnerability to episodic traffic‐induced degradation.