Water level drawdown and perennial vegetation impact litter decomposition in the sediment of a eutrophic wetland in the Netherlands

Managed wetlands often lack natural dynamics, such as water level fluctuations that normally cause dry–wet cycles. Several dry years result in a (partial) water level drawdown, after which an excess of rainfall creates the wet phase. To mimic these natural cycles, a water level drawdown can be manually induced. However, this will have major consequences for the cycling of carbon and nutrients. Yet, it is hereto unknown how water level drawdown, and the associated changes in vegetation composition and environmental conditions, affect plant litter decomposition. In the sediment of a eutrophic wetland, we buried green and rooibos tea bags, and local reed litter within a full-factorial design of water level drawdown (yes or no) and perennial vegetation (yes or no) (n = 9). Inundated conditions had lower temperature and higher conductivity and moisture content in the sediment. We found lower tea decay in the water level drawdown area, indicating more tea mass remaining after 90 days. The decomposition rate of tea was higher in the areas with perennial vegetation, pointing towards faster breakdown of organic matter. Results on remaining reed litter mass were in line with those from the tea-bags; increasing with a water level drawdown. Our results suggest that a multi-year water level drawdown reduces the decomposition rate in the sediment, while increasing the potential for more carbon sequestration. After a drawdown, establishment of perennial vegetation and re-inundation could speed up decomposition rates in the sediment, showing the importance of allowing water level dynamics to maintain high productivity.