The molecular properties of humic substances isolated from a UK upland peat system: A temporal investigation

Abstract

The study concerns the possible changes in the molecular characteristics of humic materials isolated from the same source as a function of time. A great deal of data has been reported concerning the contrast in molecular characteristics of humic substances isolated from different environments. This has primarily been an attempt to identify source-specific molecular characteristics. However, data presented in this paper suggests that humic substances isolated from a single catchment have significant changes in molecular characteristics over time. Two naturally occurring peat pools (X and Y) situated upon a small organic catchment on Great Dun Fell, Cumbria, UK were sampled monthly between November 1994 and November 1996. Dissolved organic matter (DOM) from the pool water samples was fractionated using macroporous nonionic resins (XAD8 and 4), and the humic, fulvic and hydrophilic acids were collected. These fractions were analysed for elemental composition (C, H and N), weight average molecular weight, functional group content and adsorption (340 nm) of a 1 g l −1 solution measured in a 1-cm spectrophotometer cell. The molecular characteristics were compared to those of natural DOM described by Scott et al. (1998). Scott et al. reported that drought conditions and seasonal climatic changes could have appreciable effects upon molecular characteristics of natural DOM. Results showed that the atomic H/C ratio of the humic substances increased immediately after strong drought conditions experienced in the summer of 1995. This change was temporary with atomic H/C ratio decreasing gradually over the following months. A similar decrease was observed in the carboxyl group content of the isolated compounds. The data set suggested that atomic H/C ratio in the fulvic and hydrophilic fractions exhibited seasonal characteristics of higher ratios during the late summer/early autumn months. This was not observed in the humic fraction. Humic acids exhibited a seasonal pattern of higher weight average molecular weight during the summer months. These trends were explained in terms of summer production of DOM in the catchment soils, their sequestering in the soil due to limited soil water movement during the summer months and their relative ease of dissolution when rainfall and soil water movement increased during the late summer/early autumn period. The results were found to support seasonal and long-term patterns observed in natural DOM as reported by Scott et al. (1998).