A comparative study of nitrogen and phosphorus cycling in tidal and non-tidal riverine wetlands

Abstract

This paper describes a study of nutrient dynamics in 12 tidal and non-tidal freshwater riverine wetlands in The Netherlands, Belgium, and Maryland (USA). The purpose of the study was to investigate the relationships between nutrient cycling processes in riverine wetlands that were geographically separated, that were dominated by different types of vegetation, and that had different hydrodynamics. We also compared restored and natural riverine wetlands. The results showed distinct differences in interstitial water chemistry between the sites in Maryland and Europe. No such regional differences were found in the soil variables, except for soil phosphorus, which was higher in The Netherlands. Soil organic matter, total nitrogen and phosphorus content, and bulk density were higher in tidal freshwater wetland soils. Forested wetland soils had higher organic matter and total nitrogen and lower bulk density and total phosphorus than soils from wetlands dominated by herbaceous species. Restored wetlands had lower soil organic matter and total soil nitrogen and phosphorus than similar types of natural riverine wetlands. There were no differences in nutrient-related process rates nor plant nutrient concentrations in tidal versus non-tidal riverine wetlands. Lower nitrogen and phosphorus concentrations in plants at the restored sites suggest that nutrient uptake by vegetation may be poorly coupled to rates of nutrient cycling during early stages of vegetation development. A principal components analysis of the data identified groupings of soil and water variables that were similar to those that had been previously identified when we applied the same methods to peatlands that were also geographically widely separated. Results of the study demonstrate that the techniques that we have been using are robust and repeatable. They are especially useful for making general comparisons of nitrogen and phosphorus cycling, when there are limitations on the number of wetland that can be sampled. The approach that we have developed may also be used to calibrate and refine nutrient cycling models that are incorporated into wetland assessment procedures.