No abstract

Climatic change has great impacts on stream catchments and their ecology. Expectations are that more extreme climate events will result in undesired flooding in stream catchments. In the Netherlands, former floodplains with a history of agricultural use are put into use again as flooding areas for the purpose of water retention. This study focuses on the effects of winter flooding on various plant site conditions in the soil, such as redox, nutrient, pH, and base status. We compared the effects on groundwater-and rainwater-dominated floodplains. Water chemistry (pH, EC, HCO 3 , SO 4 , Cl, Ca, Mg, Na, K, p CO 2 , NO 3 , NH 4 , and PO 4 ) and soil nutrients (Total N and P, and bio-available P) were monitored for one year, including a 3-4 month period with winter flooding. In both floodplains no direct effect of the flood-water chemistry was detected in the pore water, because the soil pores had become saturated with groundwater or rainwater, respectively, just before flooding, flood-water did not penetrate the soil. We found that the increase in pH and ammonium concentration in the rainwater floodplain were due to changes in redox status, resulting from the completely water-filled state of the soil pores during the flooding event. Furthermore, we noticed an increase in soil nutrient contents and a shift in plant species composition in the rainwater floodplain: the vegetation included more plant species characteristic for N-richness. Finally, we conclude that winter flooding has more drastic effects on biogeochemical conditions and vegetation composition in the atmotrophic conditions characteristic for low-order subcatchments than in lithotrophic conditions in the larger, higher-order subcatchments of the stream basin.

Abstract. Reintroduction of winter flooding events will have strong effects on the plant growth conditions in the parts of stream valleys that have not been accustomed to flooding in recent years. The major goal of this research is, firstly, to investigate the plant growth conditions in floodplain soils in the period after a winter flood and, secondly, to assess whether a phytometer setup is suitable for the evaluation of winter flooding on plant growth conditions. Soil cores of three agricultural and three semi-natural grassland sites have been exposed to a simulated winter flooding event. Then, cores were subjected to spring conditions in a growth chamber and were planted with seedlings of Anthoxantum odoratum and Lythrum salicaria. The growth conditions changed in opposite directions for our two phytometer species, expressed as biomass and nutrient changes. We discuss possible causes of an increase or decrease in biomass, such as (1) soil nutrient effects (N, P and K), (2) toxic effects of NH4, Fe and Al, and (3) possible shortage of other macro- and micronutrients. The conclusions are that plant growth after winter flooding was affected by enhanced nutrient and toxicant availabilities in agricultural sites and mainly by soil nutrients in the semi-natural sites. The use of the two species selected had clear advantages: Lythrum salicaria is well-suited to assess the nutrient status in previously flooded soils, because it is a well-known invader of wetlands and not easily hampered by potentially toxic compounds, while A. odoratum is less frequently found at wetland soils and more sensitive to toxic compounds and, therefore, a better indicator of possible toxic effects as a result of winter flooding than L. salicaria.