Response of the microbial community to water table variation and nutrient addition and its implications for in situ preservation of organic archaeological remains in wetland soils Isabel Douterelo a, * , Raymond Goulder b , Malcolm Lillie c a Department of Biological Sciences, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK b Department of Biological Sciences, University of Hull, Cottingham Road, Hull HU6 7RX, UK c Department of Geography, University of Hull, Cottingham Road, Hull HU6 7RX, UK article info Article history: Received 23 April 2009 Received in revised form 13 June 2009 Accepted 17 June 2009 Available online 15 July 2009 Keywords: Water table Redox potential Organic archaeological remains 14 C-Leucine assimilation Soil nutrients abstract Wetland environments can preserve organic archaeological remains because of their anaerobic nature. The ongoing discovery of archaeological sites in wetlands is associated with a lack of funds for excavation and preservation. This situation has led to the consideration of preservation in situ the preferred option for dealing with the majority of waterlogged archaeological remains in England. To expand our understanding of the burial environment, we studied changes in environmental variables along with counts of total bacteria and microbial 14 C-leucine assimilation down the soil profile at two wetlands in the North of England. Soil cores were sampled at five depth intervals between 10 and 100 cm. To test whether the addition of nutrients induces bacterial activity in the soil, inorganic phosphate and combined nitrogen were added to soil samples and the rate of 14 C-leucine assimilation was recorded. Redox potential readings were positive above the water table and negative below. The total number of bacteria and the 14 C-leucine assimilation rates differed among sites, but always decreased with increasing soil depth. Nutrient availability was limiting for the microbial communities in the upper soil horizons, but did not appear to limit those in the lower soil. These results allow a better understanding of the physico-chemical and microbiological conditions that potentially favour or inhibit the decomposition of organic archaeological remains at the studied wetlands. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Wetlands preserve organic archaeological remains because they tend to be highly reducing environments within which many of the biochemical reactions involved in decomposition processes are suppressed (Caple, 1996; Raiswell, 2001; Corfield, 2007). In general, under waterlogged conditions, oxygen exclusion protects organic archaeological remains from degradation because the activity of the aerobic microorganisms involved in biodegradation is limited and, as a consequence, the decay process is slowed down (Caple and Dungworth, 1995). Monitoring of physico-chemical variables, biological processes and the condition of the archaeological remains themselves is an efficient approach for archaeologists worldwide to better understand the factors influencing biodegradation of organic artefacts and it is considered to be essential for successful in situ conservation (Gregory, 1998; Jordan, 2001; Kenward and Hall, 2000; Caple and Hovmand, 2001; Hogan et al., 2001; Powell et al., 2001; Gregory et al., 2002; Gumbley et al., 2005). There are several major environmental factors that influence the microbial decomposition of organic materials in soil; these include organic matter content, moisture, oxygen, pH, ion species, temperature and clay content (Corfield, 1996). The process of deterioration also depends upon the depth at which organic artefacts are buried, as oxygen tends to decrease with depth. Nevertheless the decay of organic artefacts can still take place even in deep deposits where oxygen is severely limited (Bjo ¨rdal et al., 2000). The role of microorganisms in the decay of organic archaeological remains has been widely studied (Blanchette et al., 1991; Blanchette and Hoff- mann, 1994; Blanchette, 1995; Powell et al., 2001), and several studies of ancient waterlogged woods have shown that bacteria are respon- sible for wood deterioration under anaerobic conditions (Donaldson and Singh, 1990; Hedges, 1990; Blanchette et al., 1991; Blanchette and Hoffmann, 1994; Blanchette, 1995; Powell et al., 2001). Two of the key abiotic factors influencing organic decay are the height of the water table (i.e. level of saturation) and associated redox potential values (Eh) (Caple, 1996; Cheetham, 2004). As an indirect measure of the oxidation/reduction potential of the burial environ- ment, Eh values are perhaps the most important metric of the potential for the in situ preservation of archaeological remains in soil. * Corresponding author. Tel.: þ44 2476522697. E-mail address: I.Douterelo@warwick.ac.uk (I. Douterelo). Contents lists available at ScienceDirect International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod 0964-8305/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibiod.2009.06.010 International Biodeterioration & Biodegradation 63 (2009) 795–805