Carbon, nitrogen and phosphorus cycling in river marginal wetlands; a model examination of landscape geochemical flows

The importance of landscape geochemical flows was investigated using a dyna mic model simulating carbon, nitrogen and phosphorus cycling in riverine we tlands, which has been described in a previous paper. The hydro-geomorphic unit (HGMU) concept was incorporated in the model by defining a separate, c omplete unit-model for each unit (HGMU) within the wetland. These unit-mode ls were connected by defining the flows of nitrogen and phosphorus between them. These flows, also called landscape geochemical flows, usually consist of flows of water containing N and P. The model was applied to a site at Kismeldon Meadows, in south-western Engl and. This site consists of two units, a slope and a floodplain, separated b y a ditch, which catches most of the run off and shallow groundwater flows from the slope. Only an estimated 1% of the N and P that leaves the slope u nit in the water outflow reaches the floodplain unit; the rest is caught in the system of ditches, which prevent the geochemical flows taking their na tural course. To examine the influence of this system of ditches, the model was run for the same site, but without the ditches. This is comparable to a situation of a restored site, where run off and shallow groundwater flows containing nutrients, can freely get from the slope to the floodplain. The computer simulation experiment reconnecting the slope and floodplain sh owed that this (1) increased the nutrient input into the floodplain, causin g a higher biomass production, and (2) increased the wetness of the floodpl ain, causing slower decomposition, which together (3) led to a faster soil organic matter accumulation in the floodplain. Nutrient inflows became rela tively more important compared to atmospheric deposition, especially for ph osphorus. By connecting the slope and the floodplain more nitrogen and less phosphorus flowed into the river.