Microplastics (MPs) are emerging contaminants that disrupt terrestrial carbon (C) cycling, yet how their biodegradability modulates the turnover of plant-derived C remains unclear. Here, we investigated how two widely used MPs-non-biodegradable polyethylene (PE) and biodegradable polylactic acid (PLA)-affected the fate of photosynthetically fixed C in a dryland agroecosystem. The goal was to explore how MPs influenced C fluxes across the soil-plant-atmosphere continuum (SPAC) and assess their implications on climate change. We conducted a two-year field experiment to evaluate how PE and PLA-based MPs affected plant photosynthetic C fixation and its subsequent turnover in soil. Using 13CO2 pulse-labelling, we traced the flow of photosynthetically fixed C across the SPAC under low, medium and high MP concentrations. We quantified: (i) 13C distribution in plant shoots, roots and bulk soil; (ii) 13C allocation among soil aggregate size fractions; and (iii) microbial EEAs, CAZy gene abundance and soil respiration dynamics. Soil C sink capacity tended to decline for both MPs types, as cumulative soil CO2 emissions increased. On average, 13C retained in soil decreased from 50.8 to 41.1 mg m-2 in MPs treatments, relative to the control. Interestingly, the underlying mechanisms differed among MP types. Non-biodegradable PE-MPs weakened soil aggregation and reduced 13C retention in macroaggregates. However, biodegradable PLA-MPs generated marginal effects on aggregation and enhanced the activity of microbial hydrolase, which negatively affected C retention. Moreover, metagenomics confirmed that PLA-MPs enhanced microbial decomposition capacity by enriching C degradation and energy metabolism genes. Finally, photosynthetic C assimilation remained unchanged with increasing MP concentrations, regardless of MP types. Synthesis and applications. Both MP types can evidently impair soil C pools and differentially alter soil C cycling via the biodegradation-dependent mechanisms. These findings challenge the widely held assumption that biodegradable MPs are inherently environmentally benign, as their presence in soils undermines C storage capacity. The findings offer insights into future applications as follows: (1) to phase down the increment and stock of soil MPs, in favour of truly green alternatives of plastic mulching; (2) to update the estimation methods of soil C emissions in global terrestrial ecosystems considering the presence of soil MPs.