<p>Soil water scarcity and the need to enhance carbon persistence pose major challenges for sustainable land management. The incorporation of superabsorbent polymers offers a potential strategy to improve water retention and influence soil carbon dynamics. This study evaluated the effects of a biodegradable superabsorbent nanocomposite (NSN) and a polyacrylate-based commercial superabsorbent (Com) on water retention, physical structure, microbial biomass, and carbon behaviour in two contrasting soils.&#xa0;A laboratory incubation was conducted using Cambisol and Andisol soils amended with the two polymers. Bulk density, total porosity, water retention at different matric potentials (pF), microbial biomass carbon (MBC), aggregate stability, and carbon mineralisation were assessed to characterise polymer–soil interactions.&#xa0;Both polymers reduced bulk density and increased porosity, with stronger effects in Cambisol (up to 41% reduction with Com) than in Andisol (22% with Com; 17% with NSN). Water retention increased at intermediate matric potentials (pF 2.5), particularly in Cambisol (49.9% with Com; 37.4% with NSN), whereas effects were limited at higher suctions (pF ≥ 3.3). Regarding carbon behaviour, Com increased the mineralisation of labile carbon, while NSN was associated with a comparatively less mineralised carbon fraction, indicating the potential for enhanced carbon stabilisation.&#xa0;The polymers improved soil physical properties and influenced carbon dynamics through distinct pathways. Com primarily affected short-term labile carbon turnover and intermediate-range water retention, whereas NSN contributed to structural enhancement and showed potential to favour carbon persistence. Their responses varied by soil type, underscoring the importance of selecting polymer–soil combinations for sustainable management.</p>