Soil translocation by water erosion from agricultural cropland into wet depressions (morainic kettle holes)

Abstract

Wet depressions are important habitats in the agricultural landscape. In NE Germany, many of these depressions are morainic kettle holes containing temporary or permanent water bodies. Such kettle holes were formed at the end of the last glaciation, about 12,000 yr ago, by slowly melting blocks of buried dead ice. Because of the considerable relief in the resulting small inland catchments, sedimentation of eroded soil from croplands represents a major threat to these important habitats. In one of these small lakes, with a catchment area of 3.8 ha and a maximum slope of 17.6%, we determined the amount of erosion by running water, using soil sequences, sediment cores from the lake, and cesium analysis. The kettle hole itself contained about 3600 m3 of sediment, with a further 7700 m3 of colluvium around the depression: a total of 11,300 m3. In terms of slope steepness, only about 50% of the catchment area is prone to erosion by runoff. This means that during some 600 yr of agricultural land use, the mean soil loss from those 1.9 ha of erodible land amounted to about 0.6 m, i.e., 9000 t ha−1 or 15 t ha−1 yr−1. From the 14th century to 1960, the mean accumulation rate around the kettle hole was about 0.1 to 0.2 cm yr−1, increasing to 0.5 cm yr−1 after 1960 owing to intensified cultivation. Analysis of 137Cs activity indicates low erosion rates at the hilltops, increasing erosion at midslope and sedimentation around and within the kettle hole. This caused eutrophication and the slow filling-up of the small morainic lake. Two conservation strategies were considered feasible: either to create 10–20 m wide fringes of grassland around the kettle hole, combined with measures to control erosion in the croplands, or to stop agricultural activity within the catchment. In the study area, the latter option was implemented, based on contracts between the nature conservation authorities and local farmers. Water quality parameters are being monitored continuously to determine the impact of the conservation measures. After 3 yr of measurements, 12 parameters showed positive results.