Simulation of the effects of nutrient enrichment on nutrient and carbon dynamics in a river marginal wetland

Abstract

A computer simulation model was built to investigate the interactions of carbon, nitrogen and phosphorus in a riverine wetland, and to simulate the effects of management and other human influences in or outside the wetland on nutrient-related functions, such as nutrient retention or transformation. The purpose of this study was to examine the usefulness of the model in predicting the effects of nutrient enrichment on the system. This was done by performing a computer simulation of a fertilisation experiment and comparing the results with a field fertilisation experiment that was performed at the same site that the model simulated. The model simulations increased the understanding of the functioning of the system, but did not accurately predict the results of the field experiment. Both the field experiment and the model simulations indicated co-limitation of plant growth by N and P, but the model simulations predicted a far greater role for N than was found in the field experiment. This is likely due to the fact that environmental conditions in the field at the time of the experiment were not exactly the same to those in the computer simulation, because of (1) probable losses of applied N due to high denitrification during peak weather events in the field, and (2) the fact that the computer simulation is based on measurements of a larger area than the area in the field experiment. Furthermore, the field experiment showed a greater role for P limitation than could be explained by the model; according to the model (which is based on experimental phosphate adsorption isotherms of the local soil, and field measurements), there is ample P in the soil. This could mean that P isotherms determined in a laboratory do not give a true picture of the situation in the field. In the field experiment it was difficult to find effects of the added N and P on the size of labile or more refractory soil nutrient pools due to the high heterogeneity of the soil. Long term computer simulations showed a strong accumulation of added N and P. Added N accumulated mainly in soil organic matter. Added P accumulated in the soil pool of adsorbed phosphorus. The nutrient accumulations predicted are in accordance with the nutrient storage function that have often been found in wetland studies, though sometimes a more rapid P saturation has been found or expected.