This is Part 2 of a two-part blog by Harvard College student Forrest Lewis (College '17). Read Part 1.

When Taylor Swift wrote “Out of the Woods” as a tribute to the 2015 Harvard Forest Summer Program students, she probably wasn’t expecting to see some of us go straight back into them. After a quick five-day trip to California to see family and friends, I’m now writing Part 2 of my “promised-I’d-write-more-but-actually-it’s-only-gonna-be-two-parts” blog from a familiar location in Harvard Yard, gearing up for a week-long backpacking pre-orientation trip departing Tuesday.

The 11 weeks at the Harvard Forest flew by in a blurred frenzy. Though I was initially worried about the duration of the program—my freshman seminar spent five days at the Harvard Forest over the course of an entire semester, which felt like an eternity—I find myself now wishing I could spend just one more day out in the Petersham paradise.

I find myself now wishing I could spend just one more day out in the Petersham paradise.

When I last blogged, I was eager to set out on my first self-designed research project. To refresh your memory, my project aimed to document interannual variability in the soil organic layer carbon stock at Harvard Forest. That, unfortunately, failed somewhat, and in hindsight that was likely predictable.

The organic layer proved difficult to isolate in the wet cores due to high precipitation in June 2015.The organic layer proved difficult to isolate in the wet cores due to high precipitation in June 2015.

The issue with accurately documenting interannual variability is it requires precise methods that do not change year-to-year. As I began measuring the percent soil organic matter in my soil cores, I was getting drastically different numbers from last year’s student. I was finding that I was continuously having trouble in the field locating where the organic layer stopped and the mineral layer began because the cores were all sopping wet and had no distinct color and textural change. As it turned out, it had rained nearly three times as much in June 2015 as in June 2014, so I turned my head toward trying to design a method that would accurately measure soil organic carbon on a yearly basis in both dry and wet years.

The first alteration to the previous years' methods that I made was discovering that bringing the entire core back to the lab instead of discarding the mineral soil in the field allowed me to dry out the core and then more clearly see an organic and mineral distinction in the color and textural changes. But still I was testing some of the soil in cores below that line and finding that it was often times greater than 40 percent SOM, thus organic soil according to the formal definition. 

So, I created a formal set of methods which dictated that I would take each core from the field, split it where I believed that delineation might be and also approximately one centimeter below in the questionable/likely mineral soil and test both for SOM. If the lower core had greater than 40 percent SOM I would add that to the original core total carbon content and then test another centimeter of soil below until we were clearly in the mineral layer and everything above that point would count toward the total carbon content of the organic layer.

Using these methods allowed us to create a confidence interval with upper and lower bounds that gives wiggle room in case our determined boundary between the organic and mineral layers is slightly off. The confidence interval was created in R by writing a script that analyzed the effect of misjudging the delineation and having either cut contain some quantity of the other soil.

Using the new methods, cores were split based off color and textural change, as well as 1 cm intervals below to test for percent soil organic matter.Using the new methods, cores were split based off color and textural change, as well as 1 cm intervals below to test for percent soil organic matter.The error bars turned out to be satisfyingly quite small, indicating a high level of confidence in isolating the organic layer using this method.

Averaging my total carbon measurements from 31 soil cores across seven plots, I found the mean value to be 42.03 megagrams carbon per hectare. The average uncertainty value, or half the difference between the upper and lower bounds, was only 1.04 MgC/ha, meaning about a 2.81 percent uncertainty.

My patience was tested and my motivation was questioned, but I must say I’m pleased with the end result.

To put these numbers into perspective, the above ground woody biomass stored in trees, plants, leaves, etc. at the Harvard Forest was measured last year to be 131.11 MgC/ha. Thus the top 4-6 inches of soil have 30 percent as much carbon as the entire above ground biomass with trees over 100 feet tall!

Through the trials and tribulations of research, I have learned more than I ever hoped to, and more than I’ll likely ever want to know about soil carbon. My patience was tested and my motivation was questioned, but I must say I’m pleased with the end result. It was a pleasure being able to present my results at the 2015 Harvard Forest Research Symposium before departing and setting my eyes on the next academic year.

I’ll miss the friends, food, and fun of the Harvard Forest and couldn’t imagine a better way to have spent the last three months. 

I’ll miss the friends, food, and fun of the Harvard Forest and couldn’t imagine a better way to have spent the last three months. Thanks for following the blog and until the next time, happy trails!