Morphology and pedogenesis of placic horizons in podzolic Ultisols with high clay content in humid subtropical forests

Abstract

Placic (Bsm) horizons are commonly found in different types of soils worldwide, but are very rare in Ultisols. The formation of Bsm horizon and podzolization are different processes in pedogenesis. This study collected five podzolic Ultisols containing Bsm horizons from the subalpine forests in Taiwan to illustrate the morphological characteristics and genesis of the Bsm horizons in these soils with both clay illuviation and podzolization. The distinctive Bsm horizons were 3 to 10 mm thick and had abrupt and wavy boundaries with overlying albic (E) and underlying argillic (Bt) horizons. All Bsm horizons contained over 35% of clay fraction which significantly increased from E horizon to B horizons in the pedons. The Bsm horizons showed much higher bulk density than adjacent horizons above or below them, elucidating that such iron pans were relatively impermeable to water and plant roots. The maximum contents of pedogenic Fe occurred in the Bsm horizons mainly cemented by poorly crystalline Fe oxides associated with high amounts of organic matter and epiaquic conditions. Pedogenic Al appeared to be accumulated in the Bt horizons where spodic materials were identified according to the oxalate-extractable amount of Fe and Al. The polygenetic processes of these podzolic Ultisols with Bsm horizons were proposed as three stages. Firstly, clay illuviation predominantly occurred to form Bt horizon in the interglacial periods during the mid to late Pleistocene. Then, podzolization gradually became a major process to accumulate spodic materials in the Bt horizon because of the coniferous forest formation under the cool and humid climates close to the late Pleistocene; however, water was perched at the soil surface due to poor drainage and abrupt textural change between E and Bt horizons after the Holocene. Eventually, the reduced iron from E horizon was gradually re-oxidized and precipitated on the top of Bt horizon where exhibited oxidizing condition to form Bsm horizon until today. Because of the occurrence of Bsm horizon in these Ultisols, a new subgroup Placic Epiaquults, was therefore proposed to the keys to Soil Taxonomy.