Introduction
Decomposition is a vital part of ecosystem functioning, contributing fundamentally to nutrient cycling (Bazzaz 1998). It is recognised as one of the most important ecological processes (Krishna & Mohan 2017), and so measuring it accurately is important; for this purpose it is necessary to find standardised methods, and the easier the method, the more widely it can be used.
The Tea Bag Index was proposed by (Keuskamp et al. 2013) as a standardised protocol to quantify microbial decomposition. It is a simple and attractive idea. Using just two different types of commercially available tea bags, bags are deployed for 90 days and, from their final weight, a measure of decomposition, known as the Tea Bag Index (TBI), is calculated. The method is intended for researchers and other interested people without a background in decomposition research, and is perfectly suited to citizen science projects (http://www.teatime4science.org/ , www.spotteron.com/teabagindex). The method has now been widely used across numerous countries and has formed the basis for a various scientific publications (Ogden 2017; Becker & Kuzyakov 2018).
The approach is ingenious. It uses two types of teabags: rooibos and green tea. The aim was to develop a method that will allow a single placement of teabags and enable the decay constant k to be calculated for the slower decaying tea. The theory behind this is well established – there is, in every decaying piece of plant material, a slow (“refractory”) and a fast (“labile”) decomposing fraction which, under ideal conditions, will all decay (i.e. those under conditions of highest decomposition). Not all of the labile fraction of the rooibos tea will decay, slowed due toa larger recalcitrant fraction in the rooibos tea than in the green tea. The labile fraction has been estimated chemically, as 0.842 in green tea and 0.552 in rooibos tea (see Keuskamp et al. 2013). Under most natural conditions, not even all of the labile fraction of the green tea will have decomposed; a measure of this is S, the stabilisation factor. S is calculated by assessing the proportion of the potential labile fraction that remains. Assuming that the rooibos tea shows a similar rate of loss of the labile fraction to the green tea, but that that fraction is a smaller proportion of the total, then proportional loss of the labile fraction per unit time of the rooibos tea can be calculated, and therefore k , the decomposition rate calculated. The measures k and S are then used as the “Tea Bag Indices”. Figure 3 in Keuskamp et al. (2013) shows these two estimates for sites across a range of habitats from rain forest to deserts with higher values in rain forest and lower values in desert.
Determing the Tea Bag Index relies on the important assumption that all agents of decomposition (i.e. microbe and invertebrates) are included in the assay. However, this may not be the case in all circumstances. In colder temperate and boreal regions fungal decay is by the far the most important factor in dead plant decay, but in hotter and wetter, more tropical, regions, a large portion of decomposition is carried out by macrofauna (especially termites; (Cornwell et al. 2009); (Handaet al. 2014; Griffiths et al. 2019). In temperate regions invertebrates such as earthworms, woodlice and millipedes also play an important role, albeit secondary to microbes (Wall et al. 2008; Smith et al. 2009).
Furthermore, termites need special consideration for two reasons. First, they are dominant decomposers in many habitats between 45° north and south (Tuma, Eggleton & Fayle In press).Second, termites can chew through many materials, including plastics (Lenz et al. 2012), and so may gain access to the tea leaves in the bags (i.e. chew the bags open – this is true for all types of litter bags). When this happens, the termites frequently consume all the contents of the tea bags; consequently the resulting data will have more zeros than those expected due to microbial decomposition alone. Under these conditions, it is impossible to calculate the real rate of ‘k ’, as noted by Keuskamp et al. 2013 for samples left for 12 months . Additionally, the assumption that the recalcitrant fraction is essentially untouched is violated by termites (or other invertbrates that gain access) because they may remove a large proportion of tea, of both fractions. Therefore, in the presence of termites, removal of the tea from the bag will appear as the total decay of the tea by microbial agents, resulting in type I statistical errors. With these concerns in mind, here, we test the Tea Bag method to assess its performance in Bornean tropical rain forest (a hot and humid environment) with a high density of termites (Ashtonet al. 2019).