Microplastic (MP) presence in soil shows high variations in shape and dose. It is poorly understood how the variations affect soil quality and productivity, particularly in dryland agriculture where plastic film mulching is widely used. To address this issue, a two-year field experiment was conducted with four MP shapes (bead, fibril, splinter and particulate) across three concentrations in an arid irrigated maize field under the mulched (M) and non-mulched (NM) conditions. The data indicated that the effects of MPs on soil hydrothermal status and carbon-nitrogen cycling were significantly dependent on their shape and dose (p < 0.05), and plastic mulching amplified the effectiveness. High concentrations of bead and fibril MPs lowered maize leaf area index, biomass, and yield significantly (p < 0.05). Conversely, splinter MPs generally led to neutral effects (p > 0.05), while low-dose particulate MPs exhibited modest positive influences on maize yield. On the other hand, MPs significantly affected soil quality index (SQI) (p < 0.05), and the bead, splinter, and particulate MPs generally improved SQI with increasing concentration. However, the MP-induced improvements on SQI were not synchronously translated into yield gains (Pearson's r = 0.043, p > 0.05), showing a functional decoupling between soil quality and crop yielding. Structural equation modeling elucidated a cascading pathway, and MP shape and dose differentially affected soil physical, chemical, and biological traits. They tended to modify canopy structure and crop growth dynamics, ultimately driving yield changes. This multi-level disruption of the soil-crop system by MPs, particularly the observed SQI-yield decoupling, offers novel insights into agroecosystem vulnerability and underscores the profound threat that MPs pose to soil fertility and productivity.