Soil development on moraines of Mendenhall Glacier, southeast Alaska .2. Chemical transformations and soil micromorphology

Soils on Holocene-age moraines of the Mendenhall Glacier in southeast Alaska range from morphologically undeveloped to soils having field-identified spodic horizons that are more strongly expressed with increasing soil age. A soil chronosequence was sampled to study development of spodic features and to compare the expression of these features with chemical and micromorphological properties, Organic C accumulates rapidly in these soils, with O horizons forming within 38 yr, Organic C is highly correlated with cation exchange capacity (R(2) = 0.92), variable charge (R(2) = 0.94), extractable acidity (R(2) = 0.83), and 1500-kPa water retention (R(2) = 0.97), The pH of the IO-yr old pedon is acidic with a soil pH (H2O) ranging from 5.0 to 6.4, The pH decreases with soil age, with the lowest pH in horizons at the organic/mineral contact. Increases in dithionite-citrate, sodium pyrophosphate, and acid-oxalate extractable Fe (Fe-o) are most evident in the > 240-yr old soil. Depth trends for these measured properties within pedons are most distinctive for Fe-o which increases from surface mineral horizons (A or E) to underlying horizons in all pedons. The Fe-o/Fe-d ratio increases with soil age in B horizons, and in pedons 170 yr, this ratio ranges from 0,3 to 0.5 in the E horizons and from 0.4 to 0.83 in B horizons immediately below, These data indicate a greater proportion of poorly crystalline Fe in illuvial horizons with increasing soil age. Allophane content is low (less than or equal to 6 g kg(-1)) in these pedons, and there is no consistent increase with soil age. The optical density of acid-oxalate extract (ODOE) and Al-o + 1/2 Fe-o are sensitive and consistent chemical indicators of spodic development. The ODOE increases with soil age in B horizons (0.06 to 0.56). The Al-o + 1/2 Fe-o also increases with soil age in B horizons(1.2 to 4.5 g kg(-1)) and within pedons from E to B horizons. Nonoccluded Ca-bound P (195 to 49 mg kg(-1)) and total P (700 to 300 mg kg(-1)) decrease in E horizons relative to underlying horizons with soil age. Surface horizons of the younger pedons are predominantly aggregates of uncoated sand-sized grains with few organic constituents. Early processes of eluviation are evidenced by the etching of mineral grains in the E horizon of the 70-yr old pedon. There is no optical evidence of Fe-oxide accumulation in these soils, with the exception of the BC horizon of the > 240-yr old pedon. This chronosequence study indicates that spodic development is achieved within a relatively short period of time in soils of this region.