Floodplain rehabilitation and the importance of nitrogen dynamics for plant communities

Abstract

This thesis addressed the question whether changes in river water quality and sediment characteristics due to human impact, in combination with strongly altered flooding regimes, would form a constraint for the ecological development of floodplain habitats with species-rich vegetation types. Besides floodplain restoration activities in the lower reaches of rivers, the project also aimed at investigating opportunities for restoration of estuaries. Several extensive field surveys in highly impacted, rehabilitated and also more natural floodplains of non-tidal rivers (the Netherlands and Poland) and in estuarine floodplain locations (the Netherlands and Ireland) were carried out to investigate correlations between vegetation parameters and nutrient-related sediment and water characteristics. In large-scale mesocosm experiments, highly frequent, detailed measurements of environmental parameters, plant variables as well as process rates in soil-vegetation monoliths were carried out. The focus was on the effects of flooding during the growing season and renewed influence of tide and brackish water. The field studies in the non-tidal river floodplains revealed that there were strong positive correlations of peak standing crop to the soil nitrogen content for the pristine river floodplains, indicating that plant growth is limited by nitrogen, which was confirmed by our analysis of tissue nutrient ratios in aboveground living biomass. These correlations were absent for regulated, impacted river systems, suggesting that primary productivity is no longer controlled by nitrogen there. Under experimental conditions it turned out that in soil monoliths of these heavily regulated systems vegetation was much less adapted to a flooding event during the growing season compared to communities from more natural sites. A similar effect of eutrophication on plant growth limitation and productivity was expected for estuarine floodplains. Instead, tidal amplitude in relation to an elevation gradient, rather than nutrient richness, turned out to be determining the distribution and productivity of the dominant helophyte plant species in the brackish zone of these estuaries. In experimental conditions the tidal movement increased nitrogen transformation rates in former estuarine sediments with and without agricultural use. Mineralization was enhanced by tidal water level fluctuations, resulting in higher inorganic nitrogen availability. In contrast to expectations, brackish treatments had no negative effects on plant biomass production in the agricultural grassland soils, while it did have a negative effect on productivity of the vegetation from the former brackish marsh. The different responses of the floodplain types investigated are directly relevant in discussions on rehabilitation strategies for modified non-tidal river floodplain systems. Measures promoting summer flooding should be considered with care, as it might lead to enhanced nutrient availability in the soil and is potentially detrimental for the plant communities present in the area. In estuarine systems, the emphasis of restoration practices should be on re-establishing a tidal gradient with sufficient amplitude as this is more important for species distribution compared to a salinity gradient, additionally reducing available nutrient concentrations is of much less importance as plant growth is not limited by nutrient availability in the soil