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

To understand the global impact of anthropogenic activities on litter decomposition, it is necessary to obtain large decomposition datasets using a standardized method. The Tea Bag Index (TBI) approach determines the decomposition constant (k) of an asymptote model of litter decomposition and a stabilized ratio of the hydrolyzable fraction (stabilization factor S) using a single measurement of the mass loss ratios of green and rooibos teas. This approach requires the assumption that S is equal for rooibos and green tea. Here, we performed a field experiment in four temperate forest stands that demonstrated that this assumption is not always true. In a forest on limestone soils, more than half of the mass loss ratios of rooibos tea exceeded the decomposable fraction predicted by the assumption and the green tea data, which implies that the S was higher for green tea than rooibos tea. This indicated that the main assumption is not always true. We also performed a random simulation study that demonstrated that the derivation of k based on this assumption leads to a biased positive relationship between k and S, regardless of whether the assumption is correct. Thus, we suggest that the use of k and S in an identical analysis causes biased results because these variables are not independent of each other under the assumption. These risks should be considered depending on the purpose of studies that use the TBI approach.

Automated summary

To understand the global impact of human activities on litter decomposition, standardized methods for obtaining large decomposition datasets are necessary. The Tea Bag Index (TBI) approach uses mass loss ratios of green and rooibos teas to determine the decomposition constant and stabilization factor. However, the assumption that the stabilization factor is equal for both teas is not always true. This study demonstrates the potential bias caused by this assumption and suggests considering the risks when using the TBI approach.