How the anthropogenic addition of nutrients, especially nitrogen (N), impacts litter decomposition has attracted extensive attention, but how environmental factors other than nutrients affect the impacts of N addition on litter decomposition is less understood. Since different local litters could respond differently to N addition, standard materials are necessary for comparing the impacts among various environments. The present study tested if tea bags used for the Tea Bag Index (TBI) approach, i.e., constructing an asymptote model by using a green tea decomposition datum and a rooibos tea decomposition datum (single measurement in time), can be standard materials for testing the impacts of N addition on litter decomposition in aquatic ecosystems. A laboratory incubation experiment was performed using a water sample taken from a stream in Kumamoto, Japan. Since a recent study suggested that the TBI approach may be inapplicable to aquatic ecosystems, a time-series data approach, i.e., fitting models to time-series mass loss data of tea bags, was also used for testing if tea bag decomposition can pick up the impacts of N addition on aquatic litter decomposition. The time-series data approach demonstrated that N addition significantly suppressed rooibos tea decomposition, whereas green tea decomposition was not affected by N addition. The TBI approach was unsuitable for testing the sensitivity of the response of tea bag decomposition to N addition because the TBI-based asymptote model failed to predict the observed data, confirming the suggestion by a previous study. Overall, the present study suggested that the tea bags can be used as standard materials for testing the impacts of N addition on litter decomposition in aquatic ecosystems, but only when using a time-series measurement and not the TBI.
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