Effects of levees on soil microbial activity in a large river floodplain

In recent years, large river floodplains have received attention because of their importance in water quality amelioration and biodiversity support. However, few studies have addressed the effects of the extensive anthropogenic modification of floodplains caused by levee construction. To assess the effects of levees on soil properties, we measured microbial activities (electron transport system and extracellular enzyme activities), soil properties (organic matter, pH, ions, soil texture, etc.), coarse woody debris and litter accumulations in a section of the Wisconsin River floodplain in 1999 and 2000, where levees were constructed more than 100 years ago. No significant differences in the amount of organic matter or coarse woody debris were observed outside the levee compared to areas inside the levee. However, significantly higher microbial activities (dehydrogenase, ß-glucosidase, phosphatase) per gram organic matter were found for soils inside the levee. These changes were attributed to (1) modified hydrology, which caused differences in water availability and soil chemistry, and (2) tree species composition (e.g. higher number of oaks outside the levees), which caused differences in substrate quality. These results suggest that modification of the hydrologic regime by levees may alter not only the structure of plant communities but also the organic matter dynamics of floodplain.