The Global Soil Biodiversity Initiative recently blogged about The Tea Bag Index, a citizen science project to measure rates of decomposition across the world. Their blog was a good reminder to share this project with EFAO members and talk about the importance of estimating decomposition.
Decomposition is a critical function performed by soil microbes that recycles nutrients, forms humus and stores carbon. It is important to understand decomposition so we can improve soil nutrient status and take carbon out of the atmosphere. Said again: understanding what controls rates of decomposition is central to soil health and global climate change mitigation. (Check out this video on humus formation!)
Historically, researchers measure decomposition using “litter bags”: mesh bags, sewn by frantic graduate students, and stuffed with pre-weighed amounts of dry material like leaves, stover, straw, needles, etc. depending on the research question. Bags are buried in the soil for a pre-determined amount of time and then dug up and re-weighed. The difference in mass between the bag before and after burial is used to estimate decomposition rate. The trouble with the litter bag method is that it’s hard to compare from ecosystem to ecosystem, site to site. Is the decomposition of wheat straw in Saskatchewan different than in Ontario due to differences wheat variety, length of time buried, or soil/climate and soil microbes etc.? Another issue is that estimates of decomposition rate require two measurements after burial to get change over time.
Photo taken from globalsoilbiodiversity.org
To reduce the number of unknowns and simplify the method to just one measurement, researchers at The Tea Bag Index in Europe standardized and simplified the litter bag method using two types of tea bags – Lipton Green tea and Lipton Rooibos tea – with both types buried at specific depth and for a specified amount of time. This means anyone with access to these specific tea bags (which can be found here in Ontario!) and a scale can collect data on decomposition that can be compared to all other experiments that follow the protocol.
The Tea Bag Index isn’t just for scientists.
Yes, it is great to participate in citizen science projects because standardized datasets from around the world are incredibly powerful. Even more, the Tea Bag Index is a cheap and easy way to help you better understand microbial life in the soil on your farm. You can compare different materials (e.g. different pastures, different crops) or the same material in different soils/fields or at different times of year, or a combination.
HOW IT WORKS
It is generally recognized that there are two types of organic material: labile, easily decomposable material that is broken down in months, and recalcitrant, less decomposable material that takes many months or years to break down. This results in two stages (phases) of decomposition following exponential decay: rapid decomposition of labile material occurs in the first stage; decomposition slows down and levels off in the second stage when only recalcitrant material remains.
The Lipton Green Tea is composed of labile, or easily decomposed organic material, while the Lipton Rooibos Tea has more recalcitrant, or less decomposable organic material. After 90 days buried you assume:
- labile Green tea decomposes so quickly that it is in the second stage of decomposition and;
- Rooibos tea is still in the first stage of decomposition because the little amount of labile material it has is enmeshed in recalcitrant material and harder for microbes to access and decompose.
[An important caveat: not all labile material will decompose! Some will be stabilized. More on this below.]
You can use weight loss of the Green tea bag that you measure to determine how much of the labile fraction is decomposed and how much is stabilized. This gives you a stabilization factor that is valid for both Green and Rooibos tea and is used to calculate the decomposable portion of the Rooibos tea. Finally, the decomposable portion of the Rooibos tea and the mass loss of the Rooibos tea bag that you measure (weight before burying – weight after burying) are used to calculate the decomposition rate.
Full details on the mathematics of decomposition can be found in this publication by researchers at The Tea Bag Index.
Interested in collecting data for The Tea Bag Index? The simple step-wise protocol can be found here.
Questions about The Tea Bag Index? Contact the organisers directly or contact Sarah (firstname.lastname@example.org).