The experimental earthwork at Wareham, Dorset after 33 years, 3, Interaction of soil organisms with buried materials

The Wareham Experimental Earthwork was constructed in 1963 in an area of he athland in the south of England with acidic sandy soil to investigate the p rocesses that occurred early in the establishment of the archaeological rec ord. Amongst its objectives was monitoring the changes to various archaeolo gical materials that were buried in the earthwork. In this paper we present data on the interaction of soil micro-organisms with linen, flax, goatskin and hemp buried in 1963 and recovered during the 1996 excavation of the ea rthwork. These materials were originally buried on the old land surface ben eath a stack of freshly cut turves (the turf environment), where the total C and N contents were relatively high, and higher up in the earthwork (the sand environment). In the 33 years since the earthwork was constructed all the visible remains of the linen, flax and hemp had been lost except where preservation had been aided by the presence of copper alloy or steel discs. The size of the soil microbial community (microbial biomass) and microbial activity (respiration rate) were greater in the turf environment than in t he sand environment, but all the values were very small compared with typic al ranges for soils. There was some evidence, in the form of increased resp iratory responses to added glucose and greater respiration rates, that a le gacy from the flax and the hemp persisted after they ceased to be visible. However, the results from adenosine triphosphate (ATP) analysis, which prov ides a measure of physiologically active micro-organisms, and from substrat e utilization profiles (which provide an indication of the metabolic capabi lity of the microbial community) did not support this. A short-term(7 month s) laboratory experiment using samples of the original materials from the e xperimental archive and soil samples from the earthwork was conducted to at tempt to simulate the conditions at the outset of the experiment. The rate of decomposition of the materials during this experiment was initially rapi d and after 7 months between about 10 and 20% of the carbon in the buried m aterials could be related to their chemical composition, with the communiti es associated with plant-derived (carbohydrate-rich) materials giving large responses to carbohydrates, and the goatskin (mostly proteinaceous tissues ) giving large responses to amino acids (components of proteins). Solid-sta te nuclear magnetic resonance (NMR) spectroscopy was used to investigate ch anges in the distribution of carbon between different functional groups ind icative of some of the main types of biochemicals in the materials during t he short term decomposition experiment. Only small changes were observed in the NMR spectra of the plant-derived materials. Consistent with the substr ate utilization profiles, there was evidence of protein depolymerization to amino acids in the NMR spectra for the decomposing goatskin. When the mate rials were incubated in a reference soil that contained more N and a larger community of active micro-organisms than either of the soil types from the earthwork, the short term decomposition rate of all the materials was more rapid than in the earthwork soils, with the increase in decomposition rate being more marked for the plant-derived materials. It is hypothesized that this observation is due to a lack of available N restricting decomposition of the plant-derived materials in the earthwork soils.