The isotopic fingerprint of Fe cycling in an equatorial soil–plant–water system: The Nsimi watershed, South Cameroon

• Iron isotope measurements on the soil–plant–water continuum
• Investigation of a small watershed from equatorial Cameroon
• Contrarily to lateritic soils, the swamp soils show notable Fe isotope fractionation.
• Dissolved Fe in the waters is heavy isotopically and the vegetation is light.
• These Fe isotope variations show the impact of organic matter in the Fe cycling.

Following an initial study of a tropical lateritic hillside system showing little iron isotope fractionation despite a strong accumulation within the soil profile, the present work investigates iron isotope signatures within the organic matter rich swamp system that represents 20% of the studied watershed surface (from Nsimi, South Cameroon). This study considers the soil–plant–water continuum in order to better understand the Fe elemental and isotopic transfer out of the ecosystem.Within the swamp system, the iron isotope compositions of gleysol samples (δ57FeIRMM-14 ~+ 0.6‰) are significantly heavier than both the continental crust baseline and the reference lateritic soils from the hillslope (δ57FeIRMM-14 = + 0.1‰). This enrichment towards heavy isotopes is attributed to a preferential removal of light iron isotopes during soil forming processes. Pedogenic transformations (i.e., gleyzation, organic complexation of metal and leaching) are responsible for the reductimorphic features observed in ferralitic horizons (i.e., incomplete degradation of organic matter in surface and soil whitening favored by good draining conditions). The organic carbon-rich waters of the swamp system are prone to redox processes and strong metal chelation.The dissolved iron (i.e., fraction < 0.22μm) of the Mengong stream shows positive δ57Fe signatures, with a downstream enrichment in heavy isotopes, from + 0.511 ± 0.266‰ to + 1.076 ± 0.240‰. The binding of iron (FeIII) with organic matter can explain the observed enrichment in heavy isotopes in the dissolved fraction. On the contrary, plant leaves are significantly enriched in light Fe (δ57Fe of − 0.665 ± 0.035 and − 1.119 ± 0.080‰) relative to (i) the litter compartment (− 0.166 ± 0.078 to − 0.262 ± 0.013‰ for δ57Fe) and (ii) the most superficial soils. Iron isotopic compositions in plants and litter vary as a function of both plant species and season. Hence, the differences in Fe isotopic compositions between the various studied compartments suggest that Fe isotopes can be used (i) to study elemental transfers during soil pedogenesis in tropical environment and (ii) to better appraise and constrain iron biogeochemical cycle between surface horizon of soils, surface waters and the vegetation.