Quantifying decomposition rates using the international "Tea Bag Index"
Can burying tea bags help us understand climate change? Yes! Tea bags can provide important information on the global carbon cycle. Soil contains more than three times the amount of carbon than what is found in the atmosphere. This carbon storage is driven primarily by the decomposition of organic plant material. Decomposition happens when tiny organisms in the soil, such as fungi and bacteria, consume organic matter and turn it into nutrients - through a process called mineralization. Through the process of decomposition, plants and small organisms in the soil are provided with food to grow and survive. Another product of decomposition is the greenhouse gas carbon dioxide, which can be taken up again by plants, but also determines global warming. Decomposers rely on different factors in the environment such as humidity, the amount of nutrients in the soil and climate. These factors also affect the rate of decomposition. For example, climate - where decomposition rates will be slower in colder climates compared to warmer climates. Consequently, less carbon dioxide will be released into the air and more will be stored in the soil in colder climates.
Our project explores the long-term effects of megacarcasses (being the carcasses of megaherbivores such as elephants) in African savanna ecosystems. This presents a unique opportunity to apply the "Tea Bag Index" or TBI at megacarcass sites - a first for the Global TBI network, to explore the effects of increased nutrients in the soil caused by a megacarcass on decomposition rates of organic material.
We aim to:
1. Quantify decomposition rate and litter stabilization at megacarcass sites.
2. Explore whether carcass age affects decomposition rate and litter stabilization.
3. Investigate whether soil type in combination with carcass age affects litter stabilization and decomposition rates.
Attempts to develop a standardized approach to obtain decomposition data have evolved from the use of cotton strips and standardized leaf litter mixtures to the more recent Tea Bag Index (TBI). The TBI is an internationally standardized method, developed as a citizen science project in the northern hemisphere, which allows for the collection of comparable, and globally distributed data on decomposition rate and litter stabilization using commercially available tea bags as standardized test kits. To date the TBI has been applied by researchers, citizen scientists and school learners in approximately 25 countries across the globe and has been used to determine decomposition rates in various systems ranging from greenhouse experiments, vineyards, European grasslands, agricultural systems, forests, riparian zones, coastal systems, the Brazilian cerrado and the arctic tundra. In an African context, the TBI has been applied in just 4 countries on the continent, only one of which being in the semi-arid savanna of South Africa.
The "Tea Bag Index" Explained
During a field work campaign in June 2023, we buried 640 tea bags, i.e. 40 green tea and 40 rooibos tea bags, at eight carcass sites. These were individually weighed before burial. Four of the sites were located on nutrient-poor granitic soil, and four on comparatively nutrient-rich basaltic soil. A paired tea bag design (one green and one rooibos) was applied at carcass sites using two grids of increasing distance from the center of the carcass site; one in the north-western and the other in the south-eastern quadrants.
In savannas, factors that can influence decomposition such as soil moisture and temperature can change over time. For this reason, we are going to remove tea bags at different time periods of 3, 6, 9 and 12 months since burial. Decomposition normally occurs in two phases. At first, decomposition is fast when all the material that is easy to decay is consumed. Second, decay is much slower as only material that is difficult to break down is left, and we call this stabilization. Green tea decomposes easily, and will therefore give us an indication of how much material is stabilized (S), whilst rooibos tea decays slowly, and after three months will still be in the initial phase of decomposition, the weight loss of rooibos tea after three months will therefore give us an indication of initial decay rate (k). The first set of teabags - which will have spent 3 months in the ground during the cool, dry winter will be retrieved in September 2023 and re-weighed. Weight loss can then be used to calculate decomposition parameters from a decay curve. Subsequent tea bag retrieval will take place in December 2023 and in March and June 2024.
Findings from this study will contribute to understanding organic decomposition at nutrient-enriched sites, which may represent important areas of nutrient cycling in the typically nutrient-poor savannas where mega herbivores still occur!