Smelling your way down the redox ladder: wetland ecology in a bottle

“The act of smelling something, anything, is remarkably like the act of thinking. Immediately at the moment of perception, you can feel the mind going to work, sending the odor around from place to place, setting off complex repertories through the brain, polling one center after another for signs of recognition, for old memories and old connection.” – Lewis Thomas

Students experiencing olfactory "thrills" while measuring dissolved oxygen and redox potential of soil microcosms after flooding. The rotten-egg odor was intense in several of these samples.

Students experiencing olfactory “thrills” while measuring dissolved oxygen and redox potential of soil microcosms after flooding. The rotten-egg odor was intense in several of these samples.

Incorporating multiple senses into the learning process is a hallmark of experiential learning, and has long been viewed as a successful education strategy.  In a classroom setting, combining activities like observing, listening, speaking, writing, and drawing can help students to acquire, synthesize, and reinforce their knowledge of the world.  In a field course, the senses of smell and even taste can also inform and enrich the educational experience. Smelling the twig of a black birch, the leaves of spicebush, the flowers of skunk cabbage, or the wonderful rotten-egg aroma of a salt marsh are ecological observations that lead to questions of “why?” and “how?”  Furthermore, the sense of smell seems to be strongly linked to memory, albeit in poorly understood ways (i.e., the Proust effect).  Incorporating these sorts of sensory experiences into laboratory and lecture-based courses is challenging. However, I recently discovered a laboratory activity that was developed to explicitly appeal to the students’ sense of smell.  Well, perhaps “appeal” is the wrong word here.  The activity nicely demonstrates some important aspects of wetland biogeochemistry, a topic that my wetland ecology students often struggle with, and it does this while providing some considerable olfactory “thrills.”

Setup of two experiments. Each experiment included six microcosms, flooded for different lengths of time. Six experiments were done in total, allowing us to assess the influence of sulfate and organic matter quality and quantity on biogeochemical changes induced by flooding.

Setup of two experiments. Each experiment included six microcosms, flooded for different lengths of time. Six experiments were done in total, allowing us to assess the influence of sulfate and organic matter quality and quantity on biogeochemical changes induced by flooding.

The lab was developed for a soil science class by R.S. Dungan, B.D. Lee, and C. Amrhein. It can be downloaded here.  A set of microcosms are created by the students, each containing a soil which is flooded for a different length of time. A simple gaslock is used to prevent oxygen from entering the microcosms. We used six microcosms, representing flooding durations of 20 minutes, 1 day, 7 days, 14 days, 21 days, and 35 days.  In the original activity, the soils were amended with a small amount of gypsum (for a source of sulfate) and nitrogen-rich organic matter (alfalfa).  Students then measure changes in dissolved oxygen, iron, nitrate, and the presence of hydrogen sulfide.

We modified and expanded the lab for an upper-level wetland science course.  For example, we ran experiments with and without an added sulfate source, approximating the chemical environments of a salt marsh versus a freshwater wetland.  Within each of these environments, we also tested the effect that organic matter quality and quantity had on the biogeochemical changes induced by flooding.  To do this, one set of microcosms contained no added carbon (i.e., only the carbon that was present in the soil), one was amended with alfalfa (low carbon:nitrogen ratio), and one was amended with Sphagnum moss (high carbon:nitrogen ratio). In addition to measuring dissolved oxygen, iron, and nitrate, we also measured sulfate, redox potential, and pH.  Changes in concentrations were plotted against time and redox potential.

Photographs of the microcosms, after 35 days, for the different experimental setups.

Photographs of the microcosms, after 35 days, for the different experimental setups.

The results were fantastic, and some are summarized in the video and figures below.  I learned a few things by doing this lab; in particular, I think that with a little practice I could estimate redox potential using only my nose.  Certainly that would be a great skill for a wetland delineator to have!

The short video includes repeat photographs of a single flask, and provides a nice visual summary of the observed changes. Too bad you can’t send smells through the internet…

Figure showing all the data collected by the class, with concentrations plotted against redox potential measurements. Below are student comments along the redox potential gradient.

Figure showing all the data collected by the class, with concentrations plotted against redox potential measurements. Below are student comments along the redox potential gradient.

Biogeochemical changes with soil flooding, showing selected data from the class. Soils included a small amount of gypsum as a sulfate source, and the three lines indicate the results with organic matter of varying quality and quantity.

Biogeochemical changes with soil flooding, showing selected data from the class. Soils included a small amount of gypsum as a sulfate source, and the three lines indicate the results with organic matter of varying quality and quantity.

Biogeochemical changes with soil flooding, showing selected data from the class. No sulfate source was added, and the three lines indicate the results with organic matter of varying quality and quantity.

Biogeochemical changes with soil flooding, showing selected data from the class. No sulfate source was added, and the three lines indicate the results with organic matter of varying quality and quantity.

Questions for the students

A. Write a paragraph for each of the following questions, citing the appropriate figures:

  1. Describe the sequence of biogeochemical changes that occured after soil flooding. What chemical transformations take place?  Why do these changes occur?
  2. Explain the observed differences between the experiments with and without the added sulfate source. Why did these differences occur? What implications do these results have for understanding energy flow in salt marshes and freshwater wetlands?
  3. What is the likely effect of organic matter quality and quantity on the pattern and rate of biogeochemical changes after flooding? Why?

B. Write a sentence (or  equations) for each of the following questions:

  1. Hydrogen sulfide was produced in the experiment that reached a highly negative redox potential. What other gases were likely produced first?
  2. What visual changes occurred in the experiment (added sulfate, low C:N) between day 15 and 20 (see video)? What caused these changes?
  3. Why does nitrate increase in the first few days? What process is taking place?
  4. If we allowed these experiments to continue longer, what gas might be released eventually?
  5. Write the chemical equations for the redox transformations involving oxygen, nitrate, iron, and sulfate.

Literature Cited

Dungan, R.S., B.D. Lee, and C. Amrhein. 1999. Stinking Mud: An Introductory Soil Science Laboratory Exercise Demonstrating Redox Reactions in Flooded Soils. J. Nat. Resour. Life Sci. Educ. 28:89–-92.

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Posted on March 8, 2013, in Original Posts, Teaching, Wetland ecology (EES-386) and tagged , , , , , , , , . Bookmark the permalink. 5 Comments.

  1. Neat posting and class project. I bet the students liked it. I would like to see the answers and have you educate me. Really interesting. DAD

    From: Among The Stately Trees Reply-To: Among The Stately Trees Date: Friday, March 8, 2013 4:08 PM To: Bruce Booth Subject: [New post] Smelling your way down the redox ladder: wetland ecology in a bottle

    WordPress.com rkbooth posted: “”The act of smelling something, anything, is remarkably like the act of thinking. Immediately at the moment of perception, you can feel the mind going to work, sending the odor around from place to place, setting off complex repertories through the brain,”

  2. Hello! I think the involving of smell is awesome. I´m interested in set it up, so I would like to know if the nitrate test strips are specially designed for gas measuring or if test strip for solutions will do.
    Also I was wondering why the release of nitrate occurs only in low carbon:nitrogen ratio conditions.
    I hope you can attend my doubts. Thank you!

  3. Thanks for the comment Ruben GR! The nitrate test strips measure nitrate in solution. In the experiment, nitrate was detected with any organic matter source (just not when no organic matter was used). However, nitrate was greatest when the organic matter had high amounts of nitrogen relative to the amount of carbon (i.e., low C:N ratio). More nitrate was released via decomposition with this higher-quality organic source. Within a few weeks of anaerobic conditions the nitrate is converted to atmospheric nitrogen via denitrification.

  4. I have never used any of these test strips before. Except for the Lead acetate test strip are you dipping them in the water above the soil? How deep and for how long?

    • Yes, the test strips are used by dipping them in the water. Only takes a few seconds, but the times are indicated on the test strip directions and vary a bit among the different tests.

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