Wednesday, March 13, 2013

Meltwater Seep Patches: The Results

During my field seasons, I blog about the field work we are conducting and the samples we collect. What do we do with all of the information we gather? When we finish measuring everything we want on the soil, we analyze the data, make graphs that show our results, and draw conclusions based on what we find. Here is an example:

A few seasons ago, you read about one of our field projects in which we were sampling meltwater seep patches. You can read about the field project from my blog post back in December of 2009. Seep patches appear as random wet spots in the soil. They're strange shapes and come in all sorts of sizes.
Here's a photo of me standing next to one of the larger seep patches.
The water making those wet patches is from ice. When snow, glaciers, or permafrost melt, that water percolates down through the soil. The water then moves through the bottom of the soil active layer with gravity. Because the air is so dry, the water can get drawn to the surface (when soil conditions are just right!) through capillary action. That's what makes a seep patch!

Recently, some summers have had major heat-waves, which causes extra melt and the appearance of more seep patches than usual. We wanted to know how those seep patches were changing the soil and the microscopic organisms living there. Obviously they're wetter, and that extra water can be very important for soil biology in a desert. But, what else gets put into the patches with that water? Nutrients can be dissolved in that water, which would also help fertilize soil biology. But, a lot of salts can also be dissolved in that water, which makes those seep patches not only wet, but also very salty. Salty environments can be harmful to soil biology.

To find out what conditions were like for soil biology inside the seep patches, we samples soil from inside seep patches to find out how much water, nutrients, and salts  were in the patches. That would tell us how good of a habitat they are for soil biology. We dug soil pits inside the seep patches to collect soil. We also dug soil pits outside the seep patches to see how inside compared to outside.
This is one of the seep patches we sampled. You can see little bags of soil at each soil pit we dug. After we took the soil samples, we filled the pits back in with the soil to minimize the damage we cause.
We sampled six seep patches that year. Here's a photo I took from a helicopter showing the area we sampled. Everywhere red number represents a seep patch that we sampled.

The inset at the bottom right shows our sampling method. The dark gray area represents a seep patch. At each of the six seep patches, we dug two bits at the Center of the seep patch (abbreviated Ca and Cb), two at the Edge of the seep patch (Ea and Eb), and two Outside the seep patch (Oa and Ob).

We took the soil from those pits back to the lab, and measured nutrient content and salt content. We learned that soil from inside the patch is a lot saltier than soil outside the patch.The graph below is made by an analysis called "principle components analysis" (abbreviated PCA). It's a bit difficult to explain, but in simple terms, it shows how each soil sample is related to the others in terms of salt content. Each symbol represents an individual soil sample. The soils from Center locations are circles. Soils from the Edge are squares, and soils from the Outside locations are diamonds. (Each sample is also labeled by the patch number and whether it was the "a" or "b", shown in the map above.) 
This is the graph made by an analysis called "Principle components analysis". It shows how each soil sample is related to the others in term of salt content

You can see in the graph that the Center and Edge samples cluster more towards the left side of the graph. There are also more arrows pointing towards the left side of the graph. Those arrows represent the salts that were measured in the soils. So, samples on the left side of the graph have higher amounts of those salts. The samples from Outside (the diamonds) cluster more towards the right, which means they're lower in those salts (because they're away from the arrows). The Outside samples also spread out a bit more. That means dry soils outside the seep patches are less salty, but pretty variable in salt content. Seep patches tend to make soils more similar to each other in that they are all very salty.

How does the soil biology respond to that saltier, wetter environment? What's more important for biology: getting the much-needed water or having to deal with the harmful salts? We took soil from the "Center" locations and the "Outside" locations and measured the amount of CO2 being respired. (Remember, respiration produces CO2, and we can measure respiration to tell us about how active the soil biology are.) We learned that not all patches are the same! Sometimes, there's more activity inside the seep patch, sometimes there's more activity outside the seep patch, and sometimes there's no difference!
This bar graph shows the amount of respiration (measured as "carbon mineralization") from inside and outside each of the six seep patches. Dark bars are the center of the patch, and the white bars are from outside the patch. The taller the bar, the more CO2 was respired from the soil, meaning the soil biology are more active.

We noticed that when the patch increased respiration (like Patch #1), the patch was much wetter but only somewhat saltier. That means the positive influence of water could overpower the negative influence of salts. When the patch decreased respiration (like Patch #2, 3, and 5), the patch was somewhat wetter but much saltier. That means the negative influence of salts can overpower the positive influence of water. So, the influence that seep patches have on soil biology depends on the relative size of the increase in water and the increase in salts.

Therefore, we are able to conclude that these meltwater seep patches aren't all the same. They will make the soils wetter and saltier at those spots, but they vary a bit in how much wetter or how much saltier the soil becomes. Since the relative increases in water and salt can cause the biology to respond in different ways, that means we can't predict how exactly a new seep patch will influence the soil biology. Future heat-waves and future climate warming will create more seep patches, which will create a lot of variability in the soil habitat and soil biological activity.

The citation for the paper publishing these results is: 
Ball, B. A. and R. A. Virginia. 2012. Meltwater seep patches increase heterogeneity of soil geochemistry and therefore habitat suitability. Geoderma 189-190:652-660.
You can also read more details about it in a poster presenting the results by clicking this link.