Experimental manipulation of water levels in two French riverine grasslandsoils

In this experimental study, we simulated the effects of different river flo oding regimes on soil nutrient availability, decomposition and plant produc tion in floodplain grasslands. This was done to investigate the influences of soil water contents on nutrient cycling. Water levels were manipulated i n mesocosms with intact soil turfs from two French floodplain grasslands. T hree water levels were established: a 'wet' (water level at the soil surfac e), an 'intermediate' (water level at -20 cm) and a 'dry' treatment (water level at -120 cm). With increasing soil moisture, soil pH became more neutr al, while redox-potential and oxygen concentration decreased. The 'dry' tre atment showed much lower values for process rates in soil and vegetation th an the 'intermediate' and 'wet' treatments. Regressions showed that soil C- evolution and N-mineralization were positively related to soil moisture con tent. Not all mineralized N was available for plant uptake in the wet treat ment, as a considerable part was denitrified here. Denitrification was espe cially 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 'intermedi ate' treatment, plant production was most likely limited by nitrogen. The ' wet' treatment did not result in a further increase in plant production. Da m construction and river bed degradation can result in lower river levels a nd summer drought on floodplains. This experimental study suggests that sum mer drought on floodplain soils reduces decomposition of soil organic matte r, nutrient availability, denitrification, plant production and nutrient up take. This can affect the capacity of floodplains to remove or retain nutri ents from river water in a negative way. (C) 2000 Editions scientifiques et medicales Elsevier SAS.