The Experimental Earthwork at Wareham, Dorset After 33 Years: Changes to the Buried LFH and Ah Horizons

Abstract

This paper reports upon the effects of 33 years of burial upon the micromorphology, chemistry and magnetic susceptibility of the topsoil of a lowland podzol, buried beneath the bank of the Experimental Earthwork built at Wareham, Dorset, U.K., in 1963. The turf-cored sand bank and associated ditch were constructed to replicate features of archaeological monuments found on acid heathland podzols. As local soils were not fully recorded in 1963, unburied control profiles as well as turf- and sand-buried soils of the 1996 excavated section were studied. The most marked changes affected the bLFH horizon, which was reduced from a likely maximum thickness of 70 mm to 1–4 mm, a transformation already strongly established after only 17 years (1980). Compression, meso-faunal mixing and incipient “ferruginization” accompanied decomposition and the transformation of an open, plant tissue-rich excremental fabric to a dominant amorphous form. The bAh was less obviously affected. While the extent of organic decomposition in the buried soil cannot be established with certainty from organic C data, continuing decomposition is indicated by a lowering of the C/N ratio, an increase in the pyrophosphate ext. C:organic C ratio, an increase in alkali soluble humus, and reductions in organic matter recorded in image analysis of thin sections, especially in the sand-buried soil. Major differences between the two burial environments (sand and turf) may result from the buried turf having a greater water holding capacity compared to the sands, the moister environment more strongly inhibiting decomposition and activity by mesofauna compared to the sand-buried soil. This has led to greater transformations of void space and microstructure in the sand-buried soil where the proportion of organ residues (e.g. roots) is the lowest, roots being the easiest and most rapidly decomposed organic matter in the buried soil. pH, χ max and χ conv in the sand-buried Ah horizon rose, probably due to the influence of overlying unweathered sands, while χ and χ conv had become slightly diminished under the turf stack, possibly as a result of weak gleying affecting the buried soil. The higher moisture content here was also recorded by the formation of a coated and bridged (“welded”) microfabric in the turf stack and uppermost turf-buried Ah horizon. These phenomena and processes such as continued leaching and podzolization are discussed in the light of some archaeological podzols. At Wareham continued leaching and podzolization led to a lossof K but an increased proportion of alkali soluble humus and pyrophosphate ext. C in the bAh horizon, with a suggestion of incipient sesquioxide impregnation of buried plant materials (e.g. bLFH) also being recorded.