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The soil is alive……with microorganisms


Tuesday, July 1, 2014, by Ada Vilches
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Have you ever thought about what is under your feet? What is the ground you step on composed of? Well, no matter where you are on land, you are most likely either standing on soil or on something that rests on it. Thus, all terrestrial life is supported by soil at some level, but what really lives in it? This is precisely the question I am trying to answer this summer, and, it turns out, soils are made up of a vast multitude of organisms. As luck would have it, the ones I'm looking for are too small to be seen.

[Ada - front and center wearing a hat and blue jacket - and the soil-warming team.]This summer I am working with Dr. Jeffrey Blanchard from UMass Amherst in the Harvard Forest on warming plots. Long story short, these plots, or study areas, are 1 x 1 m squares that have been marked off with flagging tape, and there are two different types of plots. One type is heated; these have cables that were buried under the soil, and what they do is keep all the soil that is inside this square plots 5°C hotter than the ambient temperature. Another type is the disturbance control, which means cables have been buried beneath these squares as well to equalize the amount of disturbance done to the soil in both plot types, but in the disturbance control, the cables are never turned on.


[Winter at the soil warming plots. The snow has melted above the plots that are being warmed by cables under the soil.]We took soil samples from these two types of plots in three different sites: Prospect Hill, Barre Woods and SWaN. Afterwards we separated each soil sample based on visible differences in the coloration of the soil, called soil horizons. We are focusing our research on the organic horizon (the top part) and the first 10cm of the mineral horizon (below the organic). What we are trying to determine about these horizons is the microbial community composition and how these microbes function in the heated and control plots. In other words, which microorganisms are there and what are they doing. In order to know this, we sequenced the samples using metagenomics (DNA-sequencing) and metatranscriptomics (RNA-Sequencing). The DNA data is used to see who is there and the RNA data to determine their functions.

[This graph shows how three different samples of soil (the vertical bars) can have very different assemblages of microbes. Each type of microbe is represented by a different color and the size of the color blocks shows the relative abundance of that microbe.]


Now you may be wondering: okay but why are microorganisms significant? Well, it's simple. Microorganisms have an important role when it comes to carbon cycling and CO2 fluxes. You see, carbon is stored in soils. In fact soils contain 3 times more carbon than the atmosphere and 4 times more than the biotic portion. Therefore, soils are a vital carbon sink. Microorganisms decompose the organic matter found in the soils and also use the carbon stored in them. Now, whether the microbial communities use the carbon efficiently or not controls the CO2 fluxes. This is why the microorganisms play such an important role in the cycling of carbon, including the release of CO2 into the atmosphere, which is one of the factors affecting global warming. Over the years there has been an acceleration in global warming. Therefore, a better understanding of how these temperature increases affect soil microorganisms, which in turn affect atmospheric CO2 can help make better predictions of future climate change.