Experimental manipulation of water levels in two French riverine grassland soils

In this experimental study, we simulated the effects of different river flooding regimes on soil nutrient availability, decomposition and plant production in floodplain grasslands. This was done to investigate the influences of soil water contents on nutrient cycling. Water levels were manipulated in mesocosms with intact soil turfs from two French floodplain grasslands. Three water levels were established: a 'wet' (water level at the soil surface), an 'intermediate' (water level at -20 cm) and a 'dry' treatment (water level at -120 cm). With increasing soil moisture, soil pH became more neutral, while redox-potential and oxygen concentration decreased. The 'dry' treatment showed much lower values for process rates in soil and vegetation than the 'intermediate' and 'wet' treatments. Regressions showed that soil C-evolution and N-mineralization were positively related to soil moisture content. Not all mineralized N was available for plant uptake in the wet treatment, as a considerable part was denitrified here. Denitrification was especially high as soil water contents increased to levels above field capacity, where redox-potentials sharply dropped. Further, soil P availability was higher under wet conditions. In the 'dry' treatment, soil water content was close to the wilting point and plant production was low. In the 'intermediate' treatment, plant production was most likely limited by nitrogen. The 'wet' treatment did not result in a further increase in plant production. Dam construction and river bed degradation can result in lower river levels and summer drought on floodplains. This experimental study suggests that summer drought on floodplain soils reduces decomposition of soil organic matter, nutrient availability, denitrification, plant production and nutrient uptake. This can affect the capacity of floodplains to remove or retain nutrients from river water in a negative way. (C) 2000 Editions scientifiques et medicales Elsevier SAS.