<p>Nanoparticles (NPs) pose a potential risk to the environment, including to soil. Previous studies have confirmed their negative effects on soil biota, but research on their impact on soil properties and water dynamics is limited. This study investigates the effect of two forms of 20-nm-sized titanium dioxide (TiO₂) nanoparticles on soil organic matter (SOM) properties and moisture in organic soil at different relative humidities (RH) over short-term (one-month ageing) and long-term (one-year ageing) periods, using differential scanning calorimetry (DSC).</p><p>At 43, 70 and 95% RH, different types of water predominate: water adsorbed on organic matter, water in molecular bridges (WaMB) and phase water, respectively. Nanoparticles were found to both decrease (at 70% RH in all cases and 95% RH after one-year ageing for rutile) and increase the stability of WaMB (after one-year ageing for both rutile and anatase at 43% RH and 95% RH for anatase). Generally, nanoparticles reduced the desorption/evaporation enthalpy of water, indicating a faster moisture elimination.</p><p>Effects on total soil water were observed, with varying impacts of rutile and anatase at low RH in the long-term experiment, likely due to their different hydrophilicity, which diminished at higher RH. Measurements of ice melting enthalpy and water evaporation enthalpies showed an increase in ice melting enthalpy and a decrease in evaporation enthalpy, confirming nanoparticle’s impact on water properties. The proportion of aliphatic crystallites in SOM was reduced by both rutile and anatase nanoparticles in most cases. The findings corroborate concerns about nanoparticle’s negative effects on soil processes, highlighting the need for attention should be paid to their impact on both living and non-living SOM.</p>