1. Microplastics (MPs) in soil affect plant–soil systems depending on their shape and polymer type. However, previous research has not yet considered the effects of degraded plastics, which are the plastic materials actually present in the environment. 2. We selected eight MPs representing different shapes (fibres, films and foams) and polymer types, and exposed them to UV-C degradation. Each MP was mixed with soil at a concentration of 0.4% (w/w). The phytometer Daucus carota grew in each pot. At harvest, soil properties and plant biomass were measured. 3. Photodegradation altered MP physical and chemical properties, impacting plant–soil systems. MP degradation effects on plant and soil were observed with fibres and foams, but there were negligible effects with films. The latter could be explained by the polymer structure of films and manufacturer's additives, potentially delaying their degradation. 4. Degraded fibres increased soil respiration more than their non-degraded counterparts, as photodegradation increased the positive effects of fibres on soil water retention. The emergence of oxygenated groups during degradation may have increased the hydrophilicity of fibres, enhancing their ability to retain water. Degraded foams increased soil respiration, which could be related to the possible leaching of organic substances with lower partition coefficients, which may promote soil microbial activity. 5. In contrast, degraded foams decreased soil aggregation, likely as degradation produced larger holes increasing their permeability. Also, the increase in hydrophilic molecules could have decreased soil particle cohesiveness. Degraded fibres and foams increased shoot and root mass as a result of MP effects on soil properties. Photodegraded MPs affected root traits, which could be linked to MP effects on soil water status and plant coping strategies. 6. Synthesis and applications. Photodegradation can intensify the effects that microplastics (MPs) have on plant–soil systems, which would have frequently been underestimated had we only worked with pristine MPs. Plastic companies, agricultural practitioners and researchers should consider that plastics are being degraded as they enter the soil. Policies should promote practices to minimize MP accumulation in soils and ensure their proper disposal.
