Multi-element features of active acid sulfate soils across the Swedish coastal plains

Storage and Distribution of Organic Carbon and Nutrients in Acidic Soils Developed on Sulfidic Sediments: The Roles of Reactive Iron and Macropores

“…77 A previous study showed that (i) 1 M HCl extracted substantial proportions of the aqua regia-extractable P pools (26−88%, median = 64%) in the oxidized zones of Swedish coastal AS soils and (ii) the extractabilities were closely related to those of Fe(III). 66 The close association between 1 M HCl-extractable P and Fe(III) is also evident for the oxidized zones of the studied AS soil field (Figures 1b and 2a). Since the surfaces of schwertmannite and/or ferrihydrite enriched on the macropore surfaces are positively charged under acidic conditions prevailing in the oxidized zones, they have the potential to trap large amounts of PO 4 3− via the formation of strong inner-sphere surface complexes.…”

“…Given the widespread occurrence and massive surface areas of the macropores in the oxidized zones of the AS soils on the experimental field and elsewhere in boreal AS soils, the Fe hydroxides and oxyhydroxysulfates on the macropores should trap large quantities of mobile Fe pools released by pyrite oxidation and pH-promoted weathering of Fe-bearing phyllosilicates during the formation and ripening of AS soils. It is well-documented that, in contrast to a large number of other metals, Fe is lost to only a limited extent, and can even accumulate in the oxidized zones of typical boreal AS soil profiles. ,,, As elaborated previously, the accumulation of Fe hydroxides and oxyhydroxysulfates on macropore surfaces was likely further contributed by continuous oxidation of dissolved Fe­(II) species, diffused locally from the interiors of columnar blocks and supplied via evaporation-driven capillary movement of soil porewater or moisture from underlying transition zones. The relevance of the latter process is further supported by abundant aqueous and organically complexed Fe­(II) in the transition zone samples (Table ).…”

“…It is well-documented that, in contrast to a large number of other metals, Fe is lost to only a limited extent, and can even accumulate in the oxidized zones of typical boreal AS soil profiles. 36 , 66 , 68 , 76 As elaborated previously, 40 the accumulation of Fe hydroxides and oxyhydroxysulfates on macropore surfaces was likely further contributed by continuous oxidation of dissolved Fe(II) species, diffused locally from the interiors of columnar blocks and supplied via evaporation-driven capillary movement of soil porewater or moisture from underlying transition zones. The relevance of the latter process is further supported by abundant aqueous and organically complexed Fe(II) in the transition zone samples ( Table 2 ).…”

“…The macropore surfaces had consistently low pH values (mostly between 3.4 and 4.0; Table S1 ), consistent with typical active AS soils in the boreal zone. 35 , 66 − 68 The results of previous studies with samples from one site in the same field revealed strong concomitant enrichments of both near-total as well as 1 and 4 M HCl-extractable fractions of Fe and S on macropore surfaces relative to the interiors of solid columnar blocks. 40 , 41 This should be a widespread feature for the whole experimental field given that the macropore surfaces sampled from all 11 sites across the field were significantly and concomitantly enriched with 1 M HCl-extractable Fe and S fractions relative to the interior counterparts ( Tables 1 and S1 ).…”

“…As compared to the reduced zones, the oxidized zones of the AS soils in those studies had aqua regia- extractable P concentrations that ranged from “not significantly depleted” (down to 0.03%) to “strongly enriched” (up to 2.37%). , The strong P enrichments cannot be counterbalanced by P loss from other soil zones and thus were assumed to reflect immobilization/retention of external P, that is, excess P derived from phosphate-bearing fertilizers in the plow layer . A previous study showed that (i) 1 M HCl extracted substantial proportions of the aqua regia -extractable P pools (26–88%, median = 64%) in the oxidized zones of Swedish coastal AS soils and (ii) the extractabilities were closely related to those of Fe­(III) . The close association between 1 M HCl-extractable P and Fe­(III) is also evident for the oxidized zones of the studied AS soil field (Figures b and a).…”