Methane (CH4) is a recognized greenhouse gas, roughly 30 times as potent as CO2 (over a 100-year timespan). Globally, the vast majority of all CH4 that makes it into the atmosphere is produced via microbial origins, as a group of archaea known as methanogens emit it as a metabolic byproduct. While these microbes can grow in a diversity of environments, wetlands are a niche of particular importance, as they constitute the largest natural CH4 source.
A large portion of my PhD came to focus on the study of these wetlands, and potentials controls over their CH4 emissions. While this could’ve involved extensive fieldwork, study far away from a proper lab can come with limitations and drawbacks. For this reason, and given some of the research questions we were focused on, we opted instead to construct a wetland study system at Yale by using field-site analog called mesocosms.
Mesocosm: An environmental sample (e.g. a sediment core) which, by capturing the physical and biogeochemical features of an environment, serves as a proxy for study within a laboratory or greenhouse
By using a mesocosm study system, the natural complexity of a field-site can essentially be brought into the lab, enabling high resolution, quick study while avoiding the oversimplifications of many lab analogs and logistical drawbacks of field work. The first step to developing a mesocosm system was to design and fabricate mesocosm sampling containers, as well as devise a methodology of how to sample from our field site.
The design constraints for the mesocosms shells were:
- Made of an affordable and durable material
- Large enough to take a robust core of target area, to a depth that spans both oxic and anoxic layers
- Watertight to hold waterlogged wetland sediment
- Built-in sampling ports to take solid and liquid samples from multiple depths
- Built-in septum for characterization of aquatic chemistry using needle probes
- Includes option for installation of gas-tight headspace attachment for methane flux measurements
- The shell itself must also act as the coring tool, thus requiring an attachable cutting head and handle
The mesocosms were constructed out of durable and cheap 3″ OD PVC pipe and included an extensive sampling port infrastructure to enable comprehensive study of internal conditions. As these shells would hold waterlogged cores of wetland sediment, everything had to be watertight, and thus all ports needed either a pierceable or resealing septum.
The mesocosms also included cutting attachments, made from sharpened sheet metal, that slotted into the base of the PVC pipe, and were held in via a taper. A t-handle was then threaded into the top of the pipe, making it so the whole shell could be rotated while pushing down. This was necessary to cut through the fibrous mat of roots and sphagnum that covered the top of our site. By making the mesocosms into the coring instrument, the wetland cores would see a minimal amount of disturbance during their extraction.
As wetlands have markedly stratified biogeochemistry, it was important that during coring and transport, the samples remain upright to avoid unnatural mixing between the layers. This meant that, after sampling, the mesocosms remained upright the entire ~500 miles from the site in Maine back to Yale.
After their return to Yale, they were transferred to a climate-controlled greenhouse and connected to a custom-made automated CH4 flux monitoring system. This system, which consisted of a Los Gatos greenhouse gas analyzer, a bank of switching valves, and a Campbell microcontroller, could continuously monitor the methane emissions from up to 15 mesocosms concurrently. This high-resolution flux tracking enabled real-time monitoring of the response of gas fluxes to changes in environmental conditions. This proved critical for related work evaluating the efficacy of different soil additives for suppressing methane emissions.
