A simulation model for nitrogen cycling in natural rooted papyrus wetlands in East Africa

Papyrus (Cyperus papyrus) wetlands around East African lakes provide important ecosystem services, including retention of nutrients, to millions of people. To understand the processes contributing to nitrogen retention in the wetland and to evaluate the effects of papyrus harvesting, a dynamic model for carbon and nitrogen cycling in rooted papyrus wetlands was constructed. The model consisted of sub-models for the permanently (P) and seasonally (S) flooded zones and was based on data from a papyrus wetland in Naivasha, Kenya. In each zone, water, nitrogen and carbon flows were calculated based on descriptions of hydrological (river flow, lake level, precipitation, evaporation) and ecological (e.g. photosynthesis, nitrogen uptake, mineralisation, nitrification) processes. Literature data were used for parameterization and calibration. The model simulated realistic concentrations of dissolved nitrogen and papyrus biomass density of papyrus. Daily harvesting up to about 84 (S-zone) and 60 (P-zone) g/m2 days dry weight reduced the aboveground biomass and increased nitrogen retention (expressed as $$ \left( {{\text{N}}_{{{\text{inflow}}}} {\text{ }} - {\text{ N}}_{{{\text{outflow}}}} } \right)/{\text{N}}_{{{\text{inflow}}}} *100\% $$ ) to 38 % (S-zone) and 50 % (P-zone). A further increase in daily harvesting resulted in collapse of the aboveground biomass. Papyrus biomass, however, recovered fully from annual harvesting of up to 100 % of the biomass. The model showed that papyrus re-growth after harvesting is nitrogen-limited in the P-zone.