Management of moist grassland in a fresh-water marsh of the Weser river: effects on soil, vegetation, and fauna

In the Niedervieland, a fresh-water marsh of the Weser river, up to 150 ha of grassland have been artificially inundated for ten years as a part of a mitigation plan. The area has become one of the most important resting and breeding places for birds around Bremen. The duration of inundation varies within small distances due to microtopography. The soils have a high content of clay, organic matter, and nutrients. Their water conductivity is low. The effects of management on soil, vegetation, and fauna were studied with special regard to interactions between them. Before the area was inundated, the vegetation was mainly a Lolio-Cynosuretum typicum. Species composition of flora and soil fauna changed completely at sites inundated longer than 90 days. Muddy-ground vegetation, seasonally flooded grassland vegetation (Agrostietalia), or reeds developed, while moist grassland species did not return and no species-rich moist meadows (Molinietalia) developed. The number of isopods, snails, earthworms and carabid beetles reproducing in autumn decreased considerably or disappeared. In addition, the earthworm fauna was reduced by wading birds. The population of hygrophilous and small, flying invertebrates increased during the last ten years. Ridges function as refugia for animals which are able to migrate into inundated sites. At very long-lasting inundation the mineralisation rate is reduced due to lower soil temperatures, anaerobic conditions, and low abundance of decomposer organisms. The limited nutrient release appears to be compensated by subsurface runoff from the ridges into the shallow ditches. This may explain the higher plant-available nutrient supply in the shallow ditch soil compared to the ridge soil. The prohibition of fertilisation and the nutrient loss by runoff result in decrease of nutrient availability at the ridge sites. The abundance of plant species with low nutrient requirements increased in the mown grassland and the species diversity of the ridge sites increased. Nutrient impoverishment was only achieved at sites with short or no inundation.