Hydrology and nitrogen balance of a seasonally inundated Danish floodplain wetland

This paper characterizes a seasonally inundated Danish floodplain wetland in a state close to naturalness and includes an analysis of the major controls on the wetland water and nitrogen balances. The main inputs of water are precipitation and percolation during ponding and unsaturated conditions. Lateral saturated subsurface flow is low. The studied floodplain owes its wetland status to the hydraulic properties of its sediments: the low hydraulic conductivity of a silt-clay deposit on top of the floodplain maintains ponded water during winter, and parts of autumn and spring. A capillary fringe extends to the soil surface, and capillary rise from groundwater during summer maintains near-saturated conditions in the root zone, and allows a permanently very high evapotranspiration rate. The average for the growing season of 1999 is 3(.)6 mm day(-1) and peak rate is 5(.)6 mm day(-1). In summer, the evapotranspiration is to a large degree supplied by subsurface storage in a confined peat layer underlying the silt-clay. The floodplain sediments are in a very reduced state as indicated by low sulphate concentrations. All nitrate transported into the wetland is thus denitrified. However, owing to modest water exchange with surrounding groundwater and surface water, denitrification is low; 71 kg NO3-N ha(-1) during the study period of 1999. Reduction of nitrate diffusing into the sediments during water ponding accounts for 75% of nitrate removal. Biomass production and nitrogen uptake in above-ground vegetation is high-8(.)56 t dry matter ha(-1) year(-1) and 103 kg N ha(-1) year(-1). Subsurface ammonium concentrations are high, and convective upward transport into the root zone driven by evapotranspiration amounted to 12(.)8 kg N ha(-1) year(-1). The floodplain wetland sediments have a high nitrogen content, and conditions are very favourable for mineralization. Mineralization thus constitutes 72% of above-ground plant uptake. The study demonstrates the necessity of identifying controlling factors, and to combine surface flow with vadose and groundwater flow processes in order to fully comprehend the flow and nitrogen dynamics of this type of wetland. Copyright (C) 2004 John Wiley Sons, Ltd.