Soil Science: Sifting, Sampling, and CO2

When most people think of greenhouse gases they think of smoke stacks, car exhaust, and fossil fuels. These all lead to more carbon dioxide (CO₂) entering the atmosphere, driving climate change. But there’s another source of CO₂ doesn’t come to mind quite as readily, and it’s right beneath your feet: soil.

Soil isn’t all bad news. As plants grow, they store carbon from the atmosphere. Their fallen branches and leaves decompose, moving that carbon into the soil. In fact, soils end up storing the most organic carbon in the terrestrial ecosystem. This carbon eventually cycles back into the atmosphere via decomposers such as microbes and fungi. However, the predicted warming of our climate could cause these decomposers to work faster and release more carbon. More carbon in the atmosphere leads to more warming, beginning a self-reinforcing feedback loop. 

The accuracy of future climate predictions relies on our understanding of these complicated relationships between the land, the oceans, and the atmosphere. The goal of my research is to provide further insight into how prolonged warming will affect the terrestrial carbon cycle.

In recent years, scientists have attempted to gain a better understanding of soil’s response to warming through field experiments. One of the longest running soil warming experiments is here, at the Harvard Forest! Designed and maintained by Jerry Melillo and his talented coworkers, the soil is heated to 5° C above the ambient temperature. CO₂ respiration rates--how quickly decomposers release CO₂--have been monitored for the last 25 years. The respiration in the heated plots initially increased, but then fell back to pre-warming levels over the first decade. This means that if both plots were at the same temperature, the soil in the heated plot would respire less than that in the control plot. My goal is to pinpoint the mechanism behind this acclimation.

My experiment consists of incubating jars of soil for five weeks at a range of constant temperatures and taking regular CO₂ measurements. Pairing a lab experiment, such as mine, with the field experiment already running at the Forest allows us to control variables such as temperature and moisture more carefully. By measuring the mass of the microbes in the soil, how much organic material is in each sample, and their moisture levels, I’ll be able to determine what differences between the heated and control plots are influencing the acclimation.

After a lot of preparation and paper reading, we finally got dirty last Monday. We took soil cores from the heated and disturbance control plots and divided them into two different soil layers. We then sieved the soil to remove roots and other debris. Finally, the soil was sorted into jars and placed into different temperature incubation chambers.

A lot of effort goes into maintaining what may not look like much more than a bunch of jars of dirt. Moisture affects respiration rates, so to control for this I’m constantly weighing the jars and misting them with water to get them back up to their target weight. I also monitor the temperature in each chamber to ensure it’s remaining constant. Finally, I sample gas from the jars twice a week. I attach a special sampling lid and take CO₂ measurements at different times to measure the rate of respiration. This is done by drawing air through an opening in the jar with a needle. 

I’m going to be honest: I hate needles. Every fall I go out of my way to get my flu vaccine as a nasal spray instead of a shot. When I was told that the gas sampling was going to be done using a lot of needles, I was reluctant, to say the very least. However, there wasn’t much I could do besides bite the bullet and start sampling. After the first round of sampling last Friday, I have the whole process down to an art. The choreography of placing the sampling lids on the jars, guiding the needle through the septum, mixing the gas inside, and drawing the sample all in under a minute is something I’ve begun to take great pride in.

The amount of equipment needed to run this incubation is amazing. I have boxes of jars, syringes, lids, and more jars piled around my desk. Soil incubations really make you appreciate the power of multiplication.

2 initial soil plot types x 2 soil horizons x 5 temperatures x 6 replicates = 120 jars of soil

120 jars x 3 gas samples x 2 times a week x 5 weeks = 3600 syringes to analyze

Hopefully that’s 3600 steps closer to fully understanding how a warmer climate could be affected by something as seemingly innocuous as the dirt beneath our feet.