Two oxidative catalytic systems, titanium dioxide (TiO2) nanoparticles and a biomimetic Mn-porphyrin linked to montmorillonite through a spacer (MnP-M), were used to photo-polymerize soil organic matter (SOM) directly in situ on soil, with the aim to sequester organic carbon (OC) and reduce CO2 emissions from soil. We first validated, by infrared spectroscopy, thermogravimetric analysis (TGA), and proton spin–lattice relaxation time (T1ρH) in 13C cross-polarization magic angle spinning (13C CPMAS) nuclear magnetic resonance (NMR) spectra, the capacity of the nano-TiO2 catalyst to photo-oxidatively couple humic molecules together by covalent bonds, as already proved under MnP-M. Both catalysts turn the loosely associated humic superstructures into more stable and larger molecular masses through a photo-oxidative free radical coupling mechanism. Then, the addition of either nano-TiO2 or MnP-M to soil induced an in situ photo-polymerization of SOM, since exposure of catalyst-treated soils to solar light determined a significant reduction of respired CO2 and an increase in thermal stability of humic components and, consequently, in SOC content. This suggests that both nano-TiO2 and immobilized metal-porphyrin photo-catalysts effectively promote OC sequestration in soil and reduction of CO2 emissions from soil to atmosphere. Soil treatments with heterogeneous photo-catalysts may become a promising technology for the development of future agricultural practices aimed to increase SOM stabilization and contribute to mitigate global climate change.