Dissolved Trivalent Manganese Under Defined Redox Conditions in Redoximorphic Soils: Laboratory and Field Evidence

The reduction–oxidation (redox) cycling of manganese (Mn) regulates biogeochemical element cycles, including the carbon cycle, in soils. Dissolved trivalent Mn (Mn 3+ ), stabilised by complexation with natural organic ligands (NOLs), is a key component mediating these processes. Accurate quantification of this species is therefore needed. Our objectives were to (i) speciate the total dissolved Mn (Mn T ) pool under defined redox conditions and (ii) explore the occurrence of Mn 3+ –NOL complexes under field conditions. Bulk soil from two horizons (AhBg and Bg) of a Calcaric Gleysol was incubated in a microcosm, running two redox cycles (I: oxidising → weakly reducing → moderately reducing → reoxidising; II: oxidising → strongly reducing → reoxidising) as expressed by the redox potential (E H ) to simulate typical redox fluctuations driven by waterlogging–aeration cycles. In addition, soil solutions were collected in situ from the Calcaric Gleysol and a nearby Salic Tidalic Fluvisol. Mn speciation analysis was conducted following a spectrophotometric protocol based on kinetic modelling. During microcosm experiments, Mn T was absent under the initial oxidation conditions, while Mn 2+ dominated subsequent reducing regimes. During redox cycle I, mean Mn 3+ –NOL complex proportions increased from weakly reducing conditions (AhBg: 8.5% ± 7.8%, Bg: 4.0% ± 4.8%) to moderately reducing conditions (AhBg: 10.8% ± 12.3%, Bg: 25.3% ± 24.2%). Reoxidising conditions showed contrasting trends for each suspension (AhBg: 12.2% ± 12.6%, Bg: 3.6% ± 3.5%). In redox cycle II, mean Mn 3+ –NOL complex proportions were lower at the threshold between moderately reducing and strongly reducing conditions (AhBg: 4.2% ± 4.3%, Bg: 15.5% ± 5.5%) than under moderately reducing conditions in redox cycle I. Under reoxidising conditions, Mn 3+ –NOL complexes increased (AhBg: 36.4% ± 29.8%, Bg: 30.0% ± 6.4%), indicating stabilisation against oxidation. The results demonstrate that Mn 3+ –NOL complexes form under both oxidising and reducing conditions. Field data confirmed the occurrence of these complexes in the Calcaric Gleysol (≤ 52.5% ± 2.7%) and Salic Tidalic Fluvisol (≤ 90.0% ± 0.9%). These findings highlight that redox fluctuations induced by waterlogging–aeration cycles are a key driver of Mn 3+ –NOL complex formation and stabilisation.