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).