There’s More to the Soil Than You Think

From Spring Hill, Tennessee to Petersham, Massachusetts, I am so thankful to have the opportunity to be here at Harvard Forest. I have learned so much in the short time that I have been here. There are so many outdoor activities to do around the area and there is always something fun going on. All the REU students stay together in the Fisher House and we all get along. It is interesting to get to know everyone’s unique background. So, you may be wondering what am I doing coming from Tennessee to be at the Harvard Forest. Working alongside with my mentors Jerry Melillo, William Werner, and Michael Bernard, I was put on a research project that is one of the longest running soil warming experiments in the world. The soil warming plots of Prospect Hill have been warmed to 5°C above the ambient temperature for the past 26 years! Maintaining and collecting all the data has been no easy task.

Carbon storage occurs in many places, but what may come as a surprise is that one of the largest carbon pools is in the soil! Soil stores the most organic carbon in the terrestrial ecosystem, and the carbon stored in the soil will eventually cycle back into the atmosphere through decomposers such as fungi and microbes. One specific part of the data that is important to my experiment is the CO₂ respiration rates—how fast decomposers release CO₂.  

Over the past 26 years, we have seen fluxes in the CO₂ respiration rates. Initially, the amount of soil respiration in the heated plots increased but then fell back to the pre-warming levels over the first decade. With the predicted warming of our climate, higher temperatures could cause the decomposers to work faster, therefore releasing more carbon back into the atmosphere. This increase of carbon back into the atmosphere will lead to more warming, creating a self-reinforcing feedback loop. 

As the earth gradually warms, my research project hopes to provide further information as to how prolonged warming will affect the terrestrial carbon cycle and what forces are driving it. More specifically, I will address whether reductions in the soil organic matter content and/or soil microbial biomass account for differences in the respiration rates that we have previously observed in the heated plots relative to the control plots. With my experiment, we are able to control fundamental factors such as the seasons, water moisture, and most importantly the temperature. As we incubate the soils at different temperatures, we expect to see variations in the amount of carbon respiration. The higher the temperature, the more respiration.

Now to get down to the dirt of what I do. For my summer project, we have taken soil cores from the heated and disturbance control plots at Harvard Forests Prospect Hill. We divided the samples into organic and mineral soil layers and proceeded to sieve the soil to remove any large amounts of debris. Finally, we separated the soils into labeled jars, recording each weight and putting them into one of six incubation temperatures (6°C, 12°C, 18°C, 24°C, 30°C, 36°C). 

I regularly take CO₂ samples for each of the incubation temperatures. I place a modified sampling lid on each jar and take CO₂measurements at different times to measure the rate of respiration. Using a needle to take the samples, I carefully mix the air inside the jar and take the sample before replacing it back into the incubation temperature. This is done every minute for 74 minutes for each temperature treatment. I also have to keep a close watch on the soil’s moisture level. The moisture level will affect the respiration rates for each temperature, so to control for this I weigh and add the appropriate amount of water so the soils reach their correct target weight.  

Along with just taking the samples, we have to analyze them and put them into data sheets. There is also a portion of the project where we are able to work in the lab, for example, when we are measuring the microbial biomass of the soils. It may seem like a lot but it is something I most definitely enjoy doing. Harvard Forest is such an amazing experience and you can learn so much from being here. On top of having fun with the group and enjoying the amazing meals they prepare for us, I am so happy to have this opportunity and I know this summer is going to be a life-long memory for me.

States Labrum is a rising sophomore at Columbia State Community College in Columbia, TN. Currently, he is earning prerequisites for an undergraduate in Biology. Upon completion of his undergraduate, he plans to continue his education and earn a master’s degree.