Download Biocomplexity and Soils

Carol Mankiewicz, Beloit College
How do Invertebrates influence soil properties?
Approach: Look at a closed system over time; observe changes due
to earthworms.
Introduction
Soils 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.
Example of a core from the wet prairie.
•the top is organic rich with many
roots and burrowing invertebrates.
Results
•the lower part is organic poor and
shows different areas with distinct
coloration.
•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.
Some of the questions that we had:
•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.
•What causes the different colors?
•Why are there vertical cracks?
•Conduits for the transport of water and air form as worms burrow
thereby increasing permeability and aeration of the substrate.
These questions can be addressed in an
indirect way through the following
activities.
Can microbes influence the
appearance of the soil?
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.
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.
Water remains aerated due to photosynthesis by algae
and cyanobacteria. Invertebrates in this column include
grazing snails and a variety of zooplankton.
Approach: Construct a Winogradsky
column, a closed system to study
communities of microbes.
well lit side
Side view; light
to right
Geological
implications
photosynthesis
6CO2 + 6H2O
C6H12O6 + O2
Aerobic
heterotrophic
bacteria?
aerobic respiration
Small worms(?) (0.5-mm in diameter) burrow the upper
few cm of sediment, aiding to aerate upper sediment.
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!
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.
What kinds of microbes are present? Do they differ
from one part of the column to another?
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.
Most of
sediment is
oxygen
poor or
oxygen
depleted
7.5 cm
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.
Area
expanded to
right
bacterial photosynthesis (anaerobic):
6CO2 + 12H2S
C6H12O6 + 6H2O + 12S
Sulfate reducers use simple organic
compounds (produced by heterotrophic
bacteria) + sulfate (as an electron donor) to
produce hydrogen sulfide.
Other bacteria degrade paper (cellulose)
to glucose, which is fermented to simple
organic compounds like acetate, lactate,
and ethanol. Soil tends to be black.
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.
The 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.