Microplastic biodegradability does not modify plant carbon input in soil but accelerate soil carbon loss in agroecosystems

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 13 CO 2 pulse‐labelling, we traced the flow of photosynthetically fixed C across the SPAC under low, medium and high MP concentrations. We quantified: (i) 13 C distribution in plant shoots, roots and bulk soil; (ii) 13 C 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 CO 2 emissions increased. On average, 13 C 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 13 C 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.