Factors controlling the retention and release of dissolved organic carbon in upland organo-mineral soils

Soils play a significant role in the movement of Dissolved Organic Carbon (DOC) within a catchment and are a major source of DOC to surface waters. Recent DOC increases in some surface waters in Europe and North America could be indicative of changes in soils leading to greater export of DOC. Organo-mineral soils have the ability to act as both sources and sinks for DOC due to the presence of a large organic layer as well as mineral horizon(s). However, the role of these soils in the movement of DOC within catchments requires further research. This study investigated the factors controlling the sorption and release of DOC in organo-mineral soils. Firstly, the sorptive properties of DOC leached from organic horizons beneath Calluna vulgaris (heather), Molinia caerulea (grass) and Picea sitchensis (forest) were investigated using a batch study and three isotherm models. The forest exhibited greatest adsorption to the mineral soil but released DOC with a high specific ultra-violet absorbance (SUVA). Both the heather and grass showed no net adsorption but released DOC with a lower SUVA. Secondly, the Langmuir isotherm was used to derive sorption parameters for 20 mineral soils collected from 11 UK upland sites. The sorption parameters showed no differences by soil type (gleysol or podzol) or by horizon (A, B, E). The maximum sorption capacity (Qmax) was greater for soils beneath graminoids than for soils beneath forest or shrubs. The amorphous forms of aluminium (Alo) were the most significant predictor of Qmax. However, the occurrence of adsorption was controlled by soil pH; adsorption increased with increasing soil pH. The most significant control on desorption within the soils was Alo. Iron content was not a major contributor to sorption within these soils. Thirdly, a column study was carried out to determine the effect of soil solution retention time on the adsorption of DOC to mineral soil. Sorption equilibrium was reached within 24 hours which would suggest that in soils prone to waterlogging most DOC sorption would occur in 24 hours. Additionally it was observed that there is a fixed sorption equilibrium concentration which differs only by DOC source. The work of this thesis suggests that vegetation cover could be used to manage DOC export in soils. The work here also suggests that the recovery of organo-mineral soils from previous acid deposition is likely to be having the effect of enhancing adsorption in the mineral horizons. However, as mineral horizons recover beyond their optimum pH for adsorption then it is likely that greater release of DOC from the mineral horizons will occur and increases in DOC will be observed in the catchments.