IntroductionSoils are great systems for studying the interplay of geology, chemistry, and biology. To simplify, geologists and chemists often ignore the biology.

The following information comes from a soils class taught in the Geology Dept. at Beloit College. Though biology was not a focus, we explored biocomplexity in a number of ways:

•One project was in a deciduous forest (mainly maples), whereas another was in a wet prairie

•We had three activities/demonstrations that investigated the general effects of organisms on soils. These are the focus of the poster.

How do Invertebrates influence soil properties?Approach: Look at a closed system over time; observe changes due to earthworms.

How? Layer sand and soil in a 2-l soda bottle that has the neck part cut off. Add worms + water and food scraps as needed. Wrap sides with dark plastic (worms like the dark). Observe periodically.

Results

•Layers get mixed as the worms burrow. This mixing is more rapid and can be in the opposite direction predicted from geological and physical processes.

•Organic matter and organic-rich mud gets concentrated and redistributed due to feeding and defecation activities of the worms; mud and organic-rich castings can accumulate at the surface.

•Conduits for the transport of water and air form as worms burrow thereby increasing permeability and aeration of the substrate.

Can microbes influence the appearance of the soil?Approach: Construct a Winogradsky column, a closed system to study communities of microbes.

How? Collect muddy sediment from a pond, lake, or stream; collect some water from the same area. Mix sediment with shredded paper, calcium sulfate, and calcium carbonate. Pour into clear cylinder (we used a 50-cm glass cylinder); add pond water; seal with parafilm; and place near a window. Monitor over time.

Theory: Due to (1) initial distribution of sediment, organic matter (mainly the added paper), and organisms, (2) distribution of light, and (3) continue biological activity, chemical gradients will be established. Microbes will distribute themselves along these chemical gradients. The typical result is layers defined by colors, the colors of which are determined by the dominant microbes present.

Results: Layering is definitely evident, but gradients are apparently more complex. See if you can explain some of the complexities!

What kinds of microbes are present? Do they differ from one part of the column to another?Approach: The soils class joined with Marion Fass’ microbiology class. Microbiology students needed to culture different organisms from different environments. The soils students served as consultants, suggesting parts of the core where the appropriate organisms would most likely be found. Example: if microbiology students were interested in investigating anaerobic microbes, soil students could suggest sampling soil from a particular area (deeper, blue-to-black part of the core).

How: Microbiology students demonstrated standard culturing practices. Students jointly monitored cultures over time.

Results: We did not carry out detailed identification of the cultures, but students could

•see relative abundance of different types of microbes on the basis of macroscopic appearance of the cultures

•test predictions concerning culture conditions and abundance of organisms.

Geological implicationsThe study of paleosols—fossil soils—is important in reconstructing past environments. The organisms themselves are rarely preserved. The preserved texture and mineralogy, (particularly secondary minerals such as iron oxides and sulfides), however, can reflect biological processes, which in turn shed light on the paleoenvironment. Geologists commonly explain these with chemical gradients; we must keep in mind that many of the chemical gradients result from biological processes.

Example of a core from the wet prairie.

•the top is organic rich with many roots and burrowing invertebrates.

•the lower part is organic poor and shows different areas with distinct coloration.

Some of the questions that we had:

•What causes the different colors?

•Why are there vertical cracks?

These questions can be addressed in an indirect way through the following activities.

After about one month, a light gradient is obvious: photosynthesizers coat the glass away from the light (to the right in both photos), except in the area just above the sediment surface, which has a red tinge.

7.5 cm

Carol Mankiewicz, Beloit College

C6H12O6 + 6H2O + 12S

Sulfate reducers use simple organic compounds (produced by heterotrophic bacteria) + sulfate (as an electron donor) to produce hydrogen sulfide.

Most of sediment is oxygen poor or oxygen depleted

After about 16 months, the effects of light is still obvious. Some vertical gradients are apparent, but “splotchiness” in the sediment column is more of the norm. Areas of color (green, purple, black, white, and orange) reflect different colonies of bacteria, which in turn reflect the chemical gradient. Irregular pores in the sediment seem to correlate with green and purple colonies; white and black seem to be in areas where paper was originally present. Thus, the column sheds light on how irregular concentrations of minerals might occur. For example, if iron is present in the sediment where anaerobic bacterial photosynthesis is taking place, pyrite could form. The pyrite is preserved, whereas the bacteria will degrade leaving no direct trace.

Aerobic heterotrophic bacteria?

Area expanded to right

Side view; light to right

well lit side

Other bacteria degrade paper (cellulose) to glucose, which is fermented to simple organic compounds like acetate, lactate, and ethanol. Soil tends to be black.

Oxygen is not required for photosynthesis. Many bacteria, like purple sulfur bacteria that store sulfur in their bodies and green sulfur bacteria thrive in lighted anaerobic environments and utilize hydrogen sulfide produced by sulfate reducers.

Water remains aerated due to photosynthesis by algae and cyanobacteria. Invertebrates in this column include grazing snails and a variety of zooplankton.

photosynthesis

aerobic respiration

6CO2 + 6H2O

C6H12O6 + O2

Small worms(?) (0.5-mm in diameter) burrow the upper few cm of sediment, aiding to aerate upper sediment.

bacterial photosynthesis (anaerobic):

6CO2 + 12H2S