Engineering an Accident

If a ship can stay intact for over a thousand years by accident, we at the CC Lab Biologic Team ask whether we can engineer those same conditions intentionally. That's the idea behind Terrestrial Storage of Biomass (TSB), a type of carbon capture that uses waste organic material to trap carbon underground.^[7] Waste wood is being produced every year in the United States through tree thinning in efforts to prevent wildfires. This material needs to be removed and disposed of in order to reduce wildfire risk caused by small-diameter trees and shrubs. The most common practice now is piling and burning, which solves the wildfire risk problem, but results in thousands of metric tons of greenhouse gas emissions per pile burned at a great cost.^[8] TSB approaches aim to divert this waste and trap carbon underground, contained in the woody biomass, for the long term. This approach only works if the biomass remains intact, minimally touched by the decomposers.

This September marks the beginning of the third and final harvest of the Wood Cookie Experiment. Wood cookies have already been collected and analyzed for decay in 2022 and 2024 from our sites, located in New Hampshire, Maine, Connecticut, California, Nevada, and Mississippi, where we begin this season’s field campaign. Their climates and soil environments vary in annual temperature, rainfall, microbial diversity, and insect populations.

The Team Behind Decomposition

That range of climates and soils sets up very different conditions for the same decay agents to operate in. Our treatments test how simple, cost-effective measures can slow decay. But first, we needed to identify the types of decay we would likely see in our samples. Previous studies point to four main drivers: fungi, bacteria/archaea, insects, and physical weathering. 

Fungi are by far the fastest and most dominant decomposers of wood. Forty percent of a log could be gone in a month if exposed to fungi in optimal warm and moist conditions.^{[11],[12],[13]} If you were to take a hike in the woods anywhere in the world and step over a piece of fallen tree, you can safely assume that a fungus is responsible for its structural breakdown. Depending on the broad category of fungi, also called guilds, the appearance, texture, and chemical composition of the remaining wood will vary significantly.^{[14],[15],[16]}

Aptly named “biscuit rot,” crumbling brown cuboid pieces of wood are indicative of the guild brown-rot. Softwoods, like pine, are particularly vulnerable to this fungal guild.^[15] Brown-rot fungi use tiny chemical "scissors" to chop up the wood's structural fibers, leaving behind a crumbly, brown skeleton of a polymer called lignin. This leftover wood is not only carbon-rich but incredibly difficult to break down further.^[16] From our carbon lens, we can think of brown-rotted lignin as safe from further decay and release to the atmosphere.

If the piece of log you found in the woods was white and spongy, it was most likely consumed by white-rot. White-rot is the most common fungal guild in decaying hardwoods, although it can be found alongside brown-rot as well. These fungi use enzymes to eat every component of wood, including the lignin.^[17] A wood cookie fully consumed by white-rot will leave little to no carbon behind.

Soft-rot fungi are the last and slowest type of wood-rot fungi and occur only in extreme environments that are very cold or low in oxygen.^[14] These fungi are similar to bacteria in their mode of attack, which typically occurs cell by cell, from the inside out.^[17] Like white-rot, soft-rot consumes all of wood’s components, but it does so five times more slowly than the other fungal guilds.

Bacteria are everywhere and also consume wood with enzymes.^[3] They have a hard time breaking down lignin, so they are usually found alongside fungi that have already begun the breakdown process. Unlike fungi, bacteria are very slow and could take decades to even hundreds of years to completely destroy large pieces of wood.^[3] The archaeological finds of preserved wood are likely cases of bacteria-consumed wood, which exclude fungi entirely by being submerged underwater. The oxygen-depleted environment of water makes it hard for fungi to survive but is perfect for bacteria.^[19]

The right oxygen-free environments, like wetlands or shallow ponds, can also shift decay toward methanogenesis, a chemical process where archaea (microorganisms similar to bacteria) produce methane instead of carbon dioxide. Methane traps far more heat in the atmosphere than the carbon dioxide released by typical decay, which means the type of gas escaping matters as much as whether decay happens at all. A wood cookie that avoids fungi and bacteria doesn't necessarily avoid greenhouse gas emissions.^[20]

A piece of wood with carved-out, hollow spaces and scattered with clods of dirt throughout has almost certainly been attacked by termites. Termites are opportunistic and destructive insects that are often associated with doomsday pest-control issues in homes. They are some of the only animals that not only live in wood, but consume it.^[21] In southern regions of the U.S., with the native termite species R. flavipes, wood can be destroyed quickly and completely. They like moist, warm conditions^[22] and will attack and abandon wood in seemingly random events. Our Mississippi site has had lots of termite presence.

Not all agents of decay are alive. Rain, temperature swings, and even the sun will also degrade wood over time. If you have wooden patio furniture, you may have already seen how greying progresses over the years, which is the visual cue for a process called ultraviolet (UV) photodegradation. Similar to human skin, the outside layer of wood can be damaged when exposed to the sun.^[23] Moisture and temperature changes can cause swelling, cracking, and breaking. Physical weathering itself does not lead to much loss of carbon to the atmosphere, but may expose new areas of the wood to decay by our familiar fungi, bacteria, and insects.^[24]

What’s New Going Into the Final Harvest

We will spend this fall collecting all 210 wood cookies and bringing them back to the lab to measure how much decay occurred over 6 years. Our main measurement of interest is mass loss, which is found by weighing the cookies before and after the experiment. This tells us how much of the cookie has been lost over time, and since most wood mass is made up of carbon, we can estimate how much carbon is lost to the environment.

This year, we are particularly excited about the addition of some new tests that will expand our knowledge of the decomposition happening across treatments and sites. In the field, we will assess how low the oxygen levels are underground and underwater via oxidation-reduction chemistry potential at buried replicates and dissolved oxygen at submerged replicates. These are commonly used measurements for environmental oxygen readings, which are helpful for understanding whether our treatments actually reach low enough conditions to prevent fungal and insect decay.