What happened here hundreds of years ago?

We can look at soil samples to figure out what has grown here in the past. We can also find out when any fires occured. Both of these things help us put together the story of a certain place and give us a better understanding of how global warming influences treeline forests.

 

Soil samples may record hundreds of years of soil accumulation with hints of what's happened since then.

What is soil? How does it form?

Soil is a combination of four main things: minerals (from rock), organic matter (from dead and decaying organisms), water, and air. Often soil can be seen as a series of different layers from the bedrock (the deepest) to the organic layer (the top layer that you can see when you walk on it).

Soil has more living organisms (bacteria, small plants and animals, fungi) per handful than anything else-- probably more than a million individual life forms in a bucketful! Soil is important to many living things, especially plants. Plants rely on soil for three main things: water, nutrients (food), and oxygen. Plants can also change the soil by growing in it.

Soil is formed in a number of different ways: growing plants can add material to the soil when they die and decay, rocks and minerals can break down and mix with soil, erosion by wind or water can remove certain elements of the soil while leaving others, and particles within soil layers can move and mix.

But soil differs from location to location. Several factors affect the type of soil found in a certain place, including: climate (precipitation, temperature, etc.), organisms present (plants, animals, fungi, etc.), elevation, slope, depth to the water table, the type of bedrock underneath, geologic history (were there once glaciers scouring the area? How about oceans or lakes?)

In Alaska, the climate is generally cold. How does this affect the soil? The normal processes of breaking down dead plant and animal material (the organic material) happen much more slowly in Alaskan soils than in temperate areas to the south because the colder temperatures keep things from breaking down as fast. The organic layers tend to build up and accumulate into deep layers of slowly decomposing organic material. In fact, the presense of permafrost-- a year-round frozen layer of soil just below the surface-- keeps some plant and animal material from decomposing at all.

This layer of cold organic material is like a library of stored information. Since we want to learn what plants grew in these places a long time ago, we can look below the surface at pieces of plant fruits, leaves, stems, and seeds that haven't decomposed yet. We can also learn if there were any recent fires by looking for charcoal pieces. Charcoal lasts for a long time in the soil, so if there are big (greater than 1 millimeter) charcoal pieces, then a fire may have burned on or near the area at some point in time, perhaps hundreds of years ago. We can determine how long ago a fire burned by using radio carbon dating (or carbon 14 dating). Click here to learn about radio carbon dating.

Because the organic layer is made up of dead plant and animal material, and because plants and animals (and all living things) are made up of carbon (an element), the organic layer in Alaska holds lots of carbon underground. (And this may have influences on the climate-- click here to learn more.)

How did we collect soil samples? Follow the pictures and listen to the audio below.

Listen to Dr. Chris Fastie describe how we collected soil samples. Click here for text

Click here to download a free RealPlayer so you can listen.

 

 
There are lots of low tundra shrubs here. We use a knife to cut through these plants and into the soil so that we can remove a block of soil that we call a "monolith." This piece is nearly done. You can see the dark top layer-- or organic layer-- where the dead plant material accumulates before it decomposes.

 

 
We remove the soil monolith carefully so that the layers stay intact... ...and measure how deep the soil layers are before we seal them in bags to send back to the lab. Can you notice the different layers? Some are dark brown (to the right) and some are gray (to the left).

 

 
This block of soil probably took hundreds of years to accumulate. Although they may take some time to develop, the cold temperatures of Alaska keep the soil from breaking down quickly. Layers of soil-- "horizons"-- are created by soil processes (see the introduction to soil, above). The living horizon is on top, and the deep organic "O" horizons "O" are further down. At the bottom of this monolith is the mineral layer, right about the bedrock.

 

 
The organic matter is deep here and contains the remains of different plant species. We can look at the leaves, stems, fruits, and seeds to figure out what has grown here in the past. Were the same species of plants growing here many years ago? Are there new species now? We are also looking for signs of charcoal-- the lower streak on my hand is charcoal. Charcoal lasts a long time in the soil. If we find large pieces of charcoal, we know that this area probably burned. We use radio-carbon dating to figure out how long ago the fire was. (Click here to learn more.)

Listen to Dr. Chris Fastie explain what we see in this soil. Click here for text.

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