Nitrogen cycling in a calcareous fen peatland: stresses, controls, and variability

This thesis describes nitrogen (N) cycling in Greywell Fen (38-ha) - a calcareous, nitrateenriched groundwater-fed peatland in the Loddon catchment in the south of England. Peatlands are important environments for N cycling, and Greywell Fen is of particular interest because of plans to abate further ecological degradation by ceasing groundwater abstraction; essentially, this would involve rewetting the site with the nitrate-enriched groundwater. This is the first study to quantify the N dynamics in Greywell Fen. A year-long intensive monitoring programme was carried out, focussing on fortnightly measurement of the hydrochemistry and hydrology of fen porewaters, groundwater, and surface waters. Analysis of N concentrations and water table heights established that groundwater abstraction had a relatively limited effect on the fen water table and that nitrate concentrations in the fen varied spatially more than temporally, where discrete nitrate-rich porewater sites existed. Multivariate statistical analysis of N and dissolved solutes demonstrated that the hydrological transfer and cycling of nitrate in the fen were controlled mainly by discrete inflows of nitrate-enriched groundwater, which was regulated by the thickness of a clay layer dividing the fen and the Chalk aquifer. The effects of soil rewetting were then investigated through a laboratory experiment. Intact soil columns, extracted from the fen, were rewetted with nitrate-enriched water, and the porewater hydrochemistry and nitrous oxide gas emissions were measured. Higher nitrate concentrations at the peat surface and nitrous oxide release were both observed in soil columns with a thinner clay layer, particularly during partially and fully water saturated conditions. In terms of site management, the research demonstrated that groundwater abstraction shutdown may not be necessary to prevent further ecological degradation of the fen. In a broader context, the research demonstrates the importance of considering hydrogeology when restoring peatlands for the specific use of nitrate buffering, as well as the risk of nitrate saturation and increased nitrous oxide release resulting from direct nitrateenriched groundwater inflow.