Abstract/Summary

There has been an observed increase in dissolved organic carbon (DOC) concentrations in soil solutions and surface water bodies in acid sensitive areas of Europe and North America over the past four decades. This has been linked to changes in atmospheric chemistry and associated acid deposition, due to increased solubility of DOC in response to recovery from acidification. However, as DOC production (and consumption) is under biological control through the decomposition (and mineralisation) of organic material, there is uncertainty as to whether this increasing DOC trend is solely a chemical (solubility) response or whether there is a biological element also . In addition, there have been inconsistencies in DOC release from catchments receiving similar acid deposition loads, which suggests that differences in catchment characteristics may result in variations in the magnitude of response to acidification and recovery. Despite this and that fact that many catchments consist of peat and organo-mineral soil, much research has focused solely on peat. In order to investigate both the chemical and biological responses to changing acidity behind these DOC trends, an acidity manipulation field experiment was run over two National Parks with contrasting historical pollution levels, which included both peat and peaty podzol soil. The chemistry of pore water, as well as soil and surface litter extracts, were monitored alongside a decomposition experiment to separate out the changing solubility of DOC from the biological production through decomposition. Bacterial and fungal communities were also sequenced to assess how microbial communities were affected by changes in acidity. There was a clear chemically mediated DOC response to acidity in pore water, supporting previous findings and building on evidence of the pH-DOC hypothesis that recovery from acidification is increasing DOC solubility in organic soils. The DOC in the upper organic layer of peat and organo-mineral soil was found to be acid sensitive, but the surface litter DOC was not. However, overall there were limited responses of litter decomposition, Tea Bag Index (TBI) parameters and microbial diversity to acidity manipulations, and so there is little evidence that short-term changes in acidity effect microbial communities and biologically mediated processes (decomposition and associated DOC production). Regardless of experimental insignificance, bacterial community diversity was found to be positively and significantly related to both soil pH and extract DOC, which suggests that there may be a functional response to changing acidity as well as changes in community structure. Further work is needed to assess the mechanistic functional response of bacteria in terms of DOC production and consumption in response to changing acidity.