Testing the Tea Bag Index as a potential indicator for assessing litter decomposition in aquatic ecosystems

Abstract

The Tea Bag Index (TBI) approach is a standardized method for assessing litter decomposition in terrestrial ecosystems. This method allows determination of the stabilized portion of the hydrolysable fraction during the decomposition process, and derivation of a decomposition constant (k) using single measurements of the mass-loss ratios of green and rooibos teas. Although this method is being applied to aquatic systems, it has not been validated in these environments, where initial leaching tends to be higher than in terrestrial ecosystems. Here, we first validated a critical assumption of the TBI method that green tea decomposition plateaus during the standard incubation period of 90 days, and then tested the accuracy of a TBI-based asymptote model using a second model obtained from fitting actual decomposition data. Validation data were obtained by incubating tea bags in water samples taken from a stream, a pond, and the ocean in Kumamoto, Japan. We found that green tea decomposition did not plateau during the 90-day period, contradicting a key assumption of the TBI method. Moreover, the TBI-based asymptote models disagreed with actual decomposition data. Subtracting the leachable fraction from the initial tea mass improved the TBI-based model, but discrepancies with the actual decomposition data remained. Thus, we conclude that the TBI approach, which was developed for a terrestrial environment, is not appropriate for aquatic ecosystems. However, the use of tea bags as a standard material in assessments of aquatic litter decomposition remains beneficial.

Risk of misinterpreting the Tea Bag Index: Field observations and a random simulation

“…As discussed above, our results for the limestone site combined with the report by Mori, Ono, and Sakai (2021) suggest that the S of rooibos tea does not equal that of green tea. Because the two types of tea have different chemical compositions (Duddigan et al, 2020), we suggest that the S of rooibos tea is not always equal to that of green tea.…”

“…As the overturned assumption (i.e., S of rooibos tea equals that of green tea) may cause as much as a 20-times differences in the decomposition constant k (Mori, Ono, & Sakai, 2021), we suggest that the risk arising from the assumption should be considered, depending on the study purpose. Although the TBI was originally developed to obtain large-scale data with a simplified method (e.g., citizen science), the index was increasingly used in scientific research to clarify decomposition processes.…”

“…Again, determining k using temporal changes in rooibos tea could be an option, because this approach makes k and S independent of each other. If a study contains an analysis that simultaneously uses k and S (i.e., early decomposition rates and long‐term carbon stability), we recommend determining k using temporal changes in rooibos teas as in Mori (2021) or Mori, Ono, and Sakai (2021).…”

Validation of the Tea Bag Index as a standard approach for assessing organic matter decomposition

“…5b). This result agrees with previous studies performed in aquatic ecosystems, which demonstrated that rooibos tea had a larger S than green tea (Mori, 2021(Mori, , 2021c. Thus, the present study provides strong evidence for the assumptions in several previous studies: green tea and rooibos tea should have different S values because their chemical compositions are completely different (Mori et al, 2022(Mori et al, , 2021b.…”

“…1b), suggesting that the second premise is not always true. In aquatic ecosystems, the decomposition constant k calculated by the TBI was much larger than those determined by fitting time-series data of rooibos tea decomposition to an asymptote model (Mori, 2021;Mori et al, 2021c), possibly due to quick loss of the soluble fraction in the aquatic ecosystem. It is essential to test the validity of the TBI in terrestrial ecosystems.…”