Let’s play a word association game: Climate change
Rising sea level, acidifying oceans, species migration and extinction, extreme weather, and an ever-warming climate. Yes, all of these things and more. And while these are all of global concern, how about the impacts of climate change on a smaller, more regional level?
Here at Harvard Forest, we’re asking exactly that. Currently, Harvard Forest has three soil warming experiments running, each of which has cables installed under the soil to warm the plots 5° C above ambient temperature in an attempt to measure how a warmer climate may impact New England hardwood forests.
Through routine measurements of gas fluxes, nutrient concentrations in the soil, and tree growth for over more than a decade, my mentor Jerry Melillo and his crew have been able to put together a really interesting story, which can be summed up in a few bullet points:
- Warmer soils made for a more productive soil microbe community. This means that microbes were able to burn through soil organic matter – basically plant and animal debris in various stages of decomposing – and release a surplus of nutrients into the surrounding soil.
- In particular, the breakdown of soil organic matter led to a huge spike in nitrogen availability. Because nitrogen is typically seen as a limiting nutrient in these ecosystems, having an abundance of free nitrogen meant that the trees in the heated plots were able to grow at a faster rate and store more woody biomass in comparison to the control plot.
- Once nitrogen was freely available, trees stopped investing as much energy and carbon into forming fine root systems. Seeing as many species of fungi colonize fine root biomass by forming a sheath over the root tip, decreasing the number of fine roots also has the potential to impact root tip colonization
While these trends held up for the first 10 years of the experiment, in the past two years, one aspect has noticeably changed. The trees. Their growth spurt has come to an end.
And this is a really interesting observation, as it indicates that nitrogen might not be the only limiting factor in this ecosystem anymore. Perhaps a decade of soil heating has desynchronized nutrient cycles or depleted the system of another critical nutrient. And we’re hedging our bets on phosphorus.
To test out our hypothesis, my time at the Harvard Forest REU has been a whirlwind of different experiments and approaches that aim to tackle this question from as many angles as possible. And these can be summed up in a four-part game plan:
- We want to know what the availability of phosphate is in both the heated and control plots. To do this, we deployed charged membranes in the soil, which essentially act as roots to absorb free phosphate in the soil over the course of a two-week incubation.
- In addition to the availability of phosphate, we’re interested in the overall storage of phosphorus in the forest. This involved doing a series of sequential extractions to determine the size of labile and recalcitrant phosphorus pools in the soil.
- And what makes all this phosphate available? Phosphatase! So another part of my project involved measuring the potential enzymatic activity of phosphatase. On top of that, because the big time commitment in this experiment is preparing the soils, not running the assay, we also decided to measure the activity of beta-galactosidase, cellobiohydrolase, N-acetylglutamate, and a handful of other enzymes as a bonus.
- Finally, while those cover the parts of my project look at soil, I’m also trying to determine if decreasing fine root biomass and nitrogen availability has had any impact on the fungal community. Different species of fungi utilize different exploration strategies, which makes them more or less carbon expensive for the tree to maintain a relationship with. So our thinking is that if there is a surplus of nutrients available in the heated plots, then maybe some species of fungi will be more or less favorable for the tree to support.
While I’ll likely be posting a few more updates as the summer goes on, if you’re interested in following the rest of my adventure this summer and all the nitty-gritty details of my projects, I also keep a science blog, which you can find here: environmentalbrigade.wordpress.com/ !