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

Abstract

The Tea Bag Index (TBI), a novel approach to assessing organic matter decomposition using commercial tea bags, has been increasingly utilized as a standard method in academic studies worldwide. This approach was designed to obtain an early-stage decomposition constant (k) indicative of early-stage decomposition rates and a litter stabilization factor (S) indicative of long-term carbon stability by using two types of teas—green and rooibos. However, despite the worldwide usage of the method, the accuracy of this approach has never been validated in terrestrial ecosystems. Here, the validity of this approach was tested by examining the two essential premises of the TBI using a laboratory incubation experiment. The first premise of the TBI—namely, that the unstabilized hydrolyzable fraction of green tea is mostly decomposed within 90 days—did not hold in the present study, which caused overestimations of the S of rooibos tea, as well as k. The second premise—namely, that the ratio of stabilized to total hydrolyzable fractions (i.e., S) of rooibos tea is equal to that of green tea—was also rejected, which resulted in substantial underestimations of the S of rooibos tea and k. Overall, the TBI largely underestimated the S of rooibos tea and k (more than 1.5 and 5 times smaller than those determined by time-series data, respectively). The present study suggests that time-series mass loss data of rooibos tea should be obtained to accurately determine k, rather than assuming that the S of rooibos tea is equal to that of green tea.