Relations of iron, aluminum, and carbon along transitions from Udults to Aquods

Abstract

Spodosols of the S.E. USA occur mainly on poorly-drained “flatwoods” landscapes. Their Bh horizons constitute a globally significant pool of subsurface carbon (C) but mechanisms of the hydrologic linkage are uncertain. ObjectivesThis study was to determine relations between iron (Fe), aluminum (Al) and C along transitions between poorly-drained Aquods and better drained Udults and to evaluate mechanistic implications. Materials and methodsTwenty three soil profiles were described and sampled along four 80–120m transects corresponding to drainage gradients documented by wells over a 2.5-year period. Soils showed a typical transitional trend of upward fading Bh horizon as the drier end of the gradient was approached. Iron and Al concentrations in extracting solutions of pyrophosphate (Fep and Alp), ammonium oxalate (Feaao and Alaao), and citrate–dithionite–bicarbonate (Fecdb and Alcdb) were determined by ICP. Pyrophosphate-extractable (Cp) and total C (Ct) were determined by flash combustion. Results and discussionDeclining strength of Bh expression with increasing mean water table depth suggests that regional podzolization is largely affected by the frequency at which near-surface saturation occurs at a sufficient duration to reach certain moisture-driven biogeochemical thresholds. Results confirmed declining Fe from weak Bh horizons of drier soil to well expressed Bh horizons of Aquods. Pyrophosphate-extractable Al, but not Fep, related positively with Cp and Ct. Thus depletion of Fe and retention of C and Al in Bh horizons accompany increasing wetness and degree of podzolic expression, implicating Fe as a possible inhibitor of the process. Alternatively, preemptive redox depletion of Fe could occur prior to onset of podzolic thresholds associated with wetness (e.g., DOC activity). Persistence of Fe and its correlation with Al in weak Bh horizons indicate spatial overlap in its redox and podzolic redistribution. The pH decreased with increasing C for all horizons suggesting that acidity is controlled more by organic acids than by metal hydrolysis. Ratios of Cp/Ct and Alp/Ct as well as slope between Cp and Ct could be used to distinguish Bh- from A-horizon materials. Eluvial–illuvial relations between E and Bh were confirmed for clay as well as metals. ConclusionsThe morphological transition from Udults to Aquods parallels hydrological, chemical, and compositional changes. Implications of these changes are that Al and C distributions are mainly controlled by podzolization whereas Fe concentrations diminish with progressive wetness and podzolization. These results implicate Fe as a possible inhibitor of the process for southern Spodosols. However, its higher concentration and correlation with Al in weak Bh horizons indicate limited podzolic redistribution of Fe in marginally wet zones. The results are pertinent to the effects of hydrologic change on C and metal distributions in soils of S.E. USA coastal plain landscapes.