A synthetic, water-soluble iron-porphyrin [meso-tetra(2,6-dichloro-3-sulfonatophenyl) porphyrinate of Fe(III) chloride] has recently been proposed as a biomimetic catalyst in the process of oxidative polymerization of terrestrial humic acids, to increase their conformational stability and thus contribute to a reduction of soil CO 2 release into the atmosphere. This study was aimed at investigating changes in selected soil chemical properties, CO 2 efflux, and maize root morphotopology after the addition of iron-porphyrin as a microcosm-style experiment, located in a greenhouse. The addition of mature compost was also included as an experimental factor in order to reveal synergistic effects in regard to freshly added organic materials. Iron-porphyrin determined a negligible effect on soil organic budget in both unplanted and planted microcosms. Conversely, the biomimetic catalyst was found to have significant and contrasting effects on soil respiration, apparently reflecting different iron porphyrin-plant-compost interactions. Consequently, iron-porphyrin significantly reduced CO 2 efflux from the bare (unplanted) soil, which was, conversely, stimulated in maize-planted microcosms. Additionally, combined iron-porphyrin and compost addition synergistically acted in increasing soil respiration in planted microcosms. Moreover, root biomass was increased with the addition of iron-porphyrin, and a further effect on maize root morphology was noted when used in combination with compost; notably the length of coarse and fine roots increased. We hypothesized that the efficacy of iron-porphyrin in reducing CO 2 efflux from soil may be mediated by morphological changes in the plant-root system.