Anthropogenically-triggered iron pan formation in some Irish soils over various time spans

Podzolisation, with iron transport and deposition as coloured Bs or Bh horizons, is the fate of acid or decalcified, free-draining mineral soils in Irelandʹs cool temperate, humid climate. A second generation of iron accumulation, mainly in the form of thin seams (pans, placic horizons) has been studied at several locations in Ireland. These seams generally occur within a former cultivated horizon and within 20–25 cm of the current surface. The time span over which the iron pans developed ranges from a few decades to four or five centuries and invariably involved a change in land-use. The trigger for the new accumulation is a change in Eh/pH relations and/or acid/base conditions. Examples include (1) sudden and massive loading of a parkland pasture with peat debris in the 1950s, (2) acid litter from a single spruce crop (ca. 50 years) in a reclaimed podzol topsoil, (3) slow accumulation of peat debris over a number of decades in a peat fuel storage area, started about 100 years ago, (4) invasion of land abandoned ca. 1850 by heather and other acidophilic species and (5) run down of soil fertility and structure by grazing without amendments (pedogenic age, four to five centuries). The morphological and chemical characteristics of these soils show that where free iron is present in a horizon, any land-use change that establishes a pH or Eh/pH gradient is likely to initiate diffusion gradients and the local accumulation of iron oxides, irrespective of topographic position. This study investigates the effect of organic matter on iron accumulation and on the development of the second-generation eluvial/illuvial horizon sequence as affected by anthropogenic activities.