Iron mobilization under oscillating redox conditions in South Patagonian volcanic ash soils

• Redox oscillations control Fe-cycling via microbial organic matter turnover. • The formed Fe-compounds contain high proportions of bioavailable Fe. • The strong regional hydroclimate variability controls seasonal Fe mobilization. • Large eruptions can entail a long-term Fe supply for more than 6000 years. Iron is a critical micronutrient for organisms in ecosystems globally and plays a crucial role in (co)limiting the biogeochemical cycles of other essential elements. Large uncertainties exist regarding the processes that govern the terrestrial production of bioavailable Fe in the fjord zone of the southernmost Patagonian Andes. We therefore investigated the Fe-cycling in alternately saturated peaty Andosols by analyzing the organic and inorganic geochemical composition of rhyolitic tephra, soil horizons as well as leachates and soil-tephra substrates. In batch experiments, changing redox conditions simulated the influence of rainfall-induced water-level fluctuations on weekly to seasonal time scales. Our data demonstrate that successive biotic-abiotic dissolution-(re)crystallization processes predominantly form Fe-compounds, which are typically considered bio-accessible. These comprise dissolved Fe and metastable, organic matter (OM)-associated Fe-phases that are leached or (trans)formed from tephra and Fe-(hydr)oxides. Under oscillating redox, this biogeochemical cycling is primarily driven by intense OM turnover and microbial activity, constantly producing solutes that contain essential elements, which are either dissolved or exist as colloidal OM complexes. Under the variable hyper-humid climate, the composition of Andosol-sourced solutes varies seasonally, providing ∼ 80 % of the annually released Fe during austral winter. At the land-fjord interface in this region, the Andosol-sourced Fe flux should considerably surpass that from glacial meltwaters and last several thousand years. Therefore, we consider volcanic ash deposits and the soils derived from it as the prevailing contributor of bioavailable Fe for the primary producers in the fjords of one of the most pristine ecosystems worldwide.