Peatlands represent a vast store of global carbon. Observations of rapidly rising dissolved organic carbon concentrations in rivers draining peatlands have created concerns that those stores are beginning to destabilize. Three main factors have been put forward as potential causal mechanisms, but it appears that two alternatives--warming and increased river discharge--cannot offer satisfactory explanations. Here we show that the third proposed mechanism, namely shifting trends in the proportion of annual rainfall arriving in summer, is similarly unable to account for the trend. Instead we infer that a previously unrecognized mechanism--carbon dioxide mediated stimulation of primary productivity--is responsible. Under elevated carbon dioxide levels, the proportion of dissolved organic carbon derived from recently assimilated carbon dioxide was ten times higher than that of the control cases. Concentrations of dissolved organic carbon appear far more sensitive to environmental drivers that affect net primary productivity than those affecting decomposition alone.

Microbial activity and enzymic decomposition processes were followed during a field-based experimental lowering of the water table in a Welsh peatland. Respiration was not significantly affected by the treatment. However, the enzymes sulphatase,/3-glucosidase and phosphatase were stimulated by between 31 and 67% upon water table drawdown. A further enzyme, phenol oxidase, was not significantly affected. The observation of elevated enzyme activities without an associated increase in microbial respiratory activity suggests that drought conditions influence peatland mineralisation rates through a direct stimulation of existing enzymes, rather than through a generalised stimulation of microbial metabolism (with associated de-novo enzyme synthesis). Hydrochemical data suggest that the stimulation may have been caused by a reduction in the inhibitory action of iron and phenolics in the peat pore waters. Overall, the findings support the recent hypothesis that drier conditions associated with climate change could stimulate mineralisation within wetlands.

Abstract:Groundwater flooding occurred in the upper parts of many chalk rivers in the UK during the exceptionally wet winter of 2000-01. This provided a rare opportunity to investigate the spatial distribution of groundwater discharge and flooding along the normally dry intermittent headwaters of a chalk catchment. The extent of flooding along the River Pang, upstream of the seasonal head, was mapped using aerial photography, and point measurements of flow and water temperature were used to identify the contributing reaches of the river. The results are discussed in the context of the geological and groundwater conditions. The occurrence of flooding can largely be explained by the regional groundwater flow directions, but increased flow in some locations may be as a result of preferential groundwater flow along lines of geological structure.  Crown

Abstract. The Plynlimon experiment in mid-Wales, designed to determine the extent to which coniferous plantation increases evaporation losses and reduces streamflow relative to upland grassland, has now been yielding data since 1969 from the grassland Wye and the 67% forested Severn catchments. Water balance analyses of the early data indicated significantly higher evaporation rates from the forested catchment and studies of the hydrological processes involved attributed this to the high loss rates of precipitation intercepted by the forest canopy. Models based on these process studies predicted losses from the forested catchment that were similar but marginally higher than those determined by the catchment water balance. As the data sets from the catchments increased in length and a detailed reassessment of the ratings of the streamflow gauging structures was completed the updated water balances continued to show a significantly greater evaporation loss from the forested catchment, but the gap between the forest water balance and the model predictions widened. Furthermore Hudson and Gilman (1993), using the best data sets then available, identified downward trends in the evaporation from both catchments which the models did not reproduce and for which no obvious physical or physiological explanation was forthcoming. This dictated a major reassessment of the longer data sets, using the more powerful data processing techniques now available, to identify and eliminate any errors and inconsistencies. This paper describes the reassessment of the precipitation data and the estimates of potential evaporation and presents the water balance results emerging from the revised data sets. The revised results indicate that the evaporation losses from the grassland Wye catchment remained broadly similar to the potential evaporation estimates throughout the 1969-1995 period. The losses from the forested area of the Severn catchment declined from a level some 61% above that of the grassland in 1972 to a level only 18% higher before the start of felling in 1985. This downward trend continued as the felling and re-planting progressed. Over the period  since 1990 the forest catchment losses appear to have stabilised at some 5-10% below those of  the grassland catchment. Using the revised precipitation and potential evaporation data, process based models over-predict the forest catchment evaporation throughout the period and do not mirror the re-felling decline. Possible reasons for this apparent decline in evaporation rates are discussed.

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