Subject: Soil Food Web

From: "Lawrence F. London, Jr." <[email-masked]>
Date: Sun, 01 Dec 2002 21:58:15 -0800

<http://soils.usda.gov/sqi/SoilBiology/soil_food_web.htm>

-- 
L.F.London
[email-masked]
http://market-farming.com
http://www.ibiblio.org/ecolandtech/pcwiki/index.php/london
The Soil Biology Primer

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Chapter 1:   THE SOIL FOOD WEB

By Elaine R. Ingham, Oregon State University

SOIL BIOLOGY AND THE LANDSCAPE

An incredible diversity of
organisms make up the soil food web. They range in size from the tiniest
one-celled bacteria, algae, fungi, and protozoa, to the more complex nematodes
and micro-arthropods, to the visible earthworms, insects, small vertebrates, and
plants.

As these organisms eat, grow,
and move through the soil, they make it possible to have clean water, clean air,
healthy plants, and moderated water flow.

There are many ways that the
soil food web is an integral part of landscape processes. Soil organisms
decompose organic compounds, including manure, plant
residue, and pesticides, preventing them from entering water and becoming
pollutants. They sequester nitrogen and other nutrients that might otherwise
enter groundwater, and they fix nitrogen from the atmosphere, making it
available to plants. Many organisms enhance soil aggregation and porosity, thus
increasing infiltration and reducing runoff. Soil organisms prey on crop pests
and are food for above-ground animals.

The soil environment.  Organisms
live in the microscale environments within and between soil particles.
Differences over short distances in pH, moisture, pore size, and the types
of food available create a broad range of habitats.

Credit: S. Rose and E.T. Elliott

THE FOOD WEB: ORGANISMS AND THEIR INTERACTION

The soil food web is the community of organisms living all or part of
their lives in the soil. A food web diagram shows a series of conversions
(represented by arrows) of energy and nutrients as one organism eats another
(see food web diagram).

All food webs are fueled by the primary producers:
the plants, lichens, moss, photosynthetic bacteria, and algae that use
the sun’s energy to fix carbon dioxide from the atmosphere. Most other soil
organisms get energy and carbon by consuming the organic compounds found in
plants, other organisms, and waste by-products. A few bacteria, called
chemoautotrophs, get energy from nitrogen, sulfur, or iron compounds rather than
carbon compounds or the sun.

As organisms
decompose complex materials, or consume other organisms, nutrients are converted
from one form to another, and are made available to plants and to other soil
organisms. All plants – grass, trees, shrubs, agricultural crops – depend on
the food web for their nutrition.

WHAT DO SOIL ORGANISMS DO?

Growing and reproducing are the primary activities of all living
organisms. As individual plants and soil organisms work to survive, they depend
on interactions with each other. By-products from growing roots and plant
residue feed soil organisms. In turn, soil organisms support plant health as
they decompose organic matter, cycle nutrients, enhance soil structure, and
control the populations of soil organisms including crop pests. (See table of
functions of soil organisms.)

ORGANIC MATTER FUELS THE FOOD WEB

Soil organic
matter is the storehouse for the energy and nutrients used by plants and other
organisms. Bacteria, fungi, and other soil dwellers transform and release
nutrients from organic matter (see photo).

These microshredders, immature oribatid
mites, skeletonize plant leaves. This starts the nutrient cycling of
carbon, nitrogen, and other elements.

Collohmannia sp. Credit: Roy A. Norton, College of Environmental
Science & Forestry, State University of New York

Organic matter
is many different kinds of compounds – some more useful to organisms than
others. In general, soil organic matter is made of roughly equal parts humus and
active organic matter. Active organic matter is the portion available to soil
organisms. Bacteria tend to use simpler organic compounds, such as root exudates
or fresh plant residue. Fungi tend to use more complex compounds, such as
fibrous plant residues, wood and soil humus.

Intensive
tillage triggers spurts of activity among bacteria and other organisms that
consume organic matter (convert it to CO2),
depleting the active fraction first. Practices that build soil organic matter
(reduced tillage and regular additions of organic material) will raise the
proportion of active organic matter long before increases in total organic
matter can be measured. As soil organic matter levels rise, soil organisms play
a role in its conversion to humus—a relatively stable form of carbon
sequestered in soils for decades or even centuries.

FOOD
SOURCES FOR SOIL ORGANISMS

“Soil
organic matter” includes all the organic substances in or on the soil. Here
are terms used to describe different types of organic matter.

Living organisms:
Bacteria,
fungi, nematodes, protozoa, earthworms, arthropods, and living roots.

Dead plant material; organic material; detritus;
surface residue:  All these terms refer to
plant, animal, or other organic substances that have recently been added to the
soil and have only begun to show signs of decay. Detritivores are organisms that
feed on such material.

Active fraction organic matter:
Organic compounds that can be used as food by
microorganisms. The active fraction changes more quickly than total organic
matter in response to management changes.

Labile organic matter:
Organic
matter that is easily decomposed.

Root exudates:
Soluble
sugars, amino acids and other compounds secreted by roots.

Particulate organic matter (POM) or Light fraction
(LF) organic matter:  POM
and LF have precise size and weight definitions. They are thought to represent
the active fraction of organic matter which is more difficult to define. Because
POM or LF is larger and lighter than other types of soil organic matter, they
can be separated from soil by size (using a sieve) or by weight (using a
centrifuge).

Lignin:
A
hard-to-degrade compound that is part of the fibers of older plants. Fungi can
use the carbon ring structures in lignin as food.

Recalcitrant organic matter:
Organic matter such as humus or lignin-containing
material that few soil organisms can decompose.

Humus or humified organic matter:
Complex organic compounds that remain after many
organisms have used and transformed the original material. Humus is not readily
decomposed because it is either physically protected inside of aggregates or
chemically too complex to be used by most organisms. Humus is important in
binding tiny soil aggregates, and improves water and nutrient holding capacity.

Components of Soil
Organic Matter

WHERE DO SOIL ORGANISMS LIVE?

The organisms
of the food web are not uniformly distributed through the soil. Each species and
group exists where they can find appropriate space, nutrients, and moisture.
They occur wherever organic matter occurs – mostly in the top few inches of
soil (see figure), although microbes have been found as deep as 10 miles (16 km)
in oil wells.

Soil organisms
are concentrated:

Around roots. The
rhizosphere is the narrow region of soil directly around roots (see photo). It
is teeming with bacteria that feed on sloughed-off plant cells and the proteins
and sugars released by roots. The protozoa and nematodes that graze on bacteria
are also concentrated near roots. Thus, much of the nutrient cycling and disease
suppression needed by plants occurs immediately adjacent to roots.

Bacteria are abundant around this root tip
(the rhizosphere) where they decompose the plentiful simple organic
substances.

Credit: No. 53 from Soil Microbiology and Biochemistry Slide
Set. 1976 J.P. Martin, et al., eds. SSSA, Madison WI.

In litter. Fungi
are common decomposers of plant litter because litter has large amounts of
complex, hard-to-decompose carbon. Fungal hyphae (fine filaments) can “pipe”
nitrogen from the underlying soil to the litter layer. Bacteria cannot transport
nitrogen over distances, giving fungi an advantage in litter decomposition,
particularly when litter is not mixed into the soil profile. However, bacteria
are abundant in the green litter of younger plants which is higher in nitrogen
and simpler carbon compounds than the litter of older plants. Bacteria and fungi
are able to access a larger surface area of plant residue after shredder
organisms such as earthworms, leaf-eating insects, millipedes, and other
arthropods break up the litter into smaller chunks.

On humus.
Fungi are common here. Much organic matter in the soil has already been
decomposed many times by bacteria and fungi, and/or passed through the guts of
earthworms or arthropods. The resulting humic compounds are complex and have
little available nitrogen. Only fungi make some of the enzymes needed to degrade
the complex compounds in humus.

On the surface of soil
aggregates.

Biological activity, in particular that of aerobic bacteria and fungi, is
greater near the surfaces of soil aggregates than within aggregates. Within
large aggregates, processes that do not require oxygen, such as denitrification,
can occur. Many aggregates are actually the fecal pellets of earthworms and
other invertebrates.

In spaces between soil
aggregates. Those
arthropods and nematodes that cannot burrow through soil move in the pores
between soil aggregates. Organisms that are sensitive to desiccation, such as
protozoa and many nematodes, live in water-filled pores. (See Figure page 1.)

WHEN ARE THEY ACTIVE?

The activity
of soil organisms follows seasonal patterns, as well as daily patterns. In
temperate systems, the greatest activity occurs in late spring when temperature
and moisture conditions are optimal for growth (see graph). However, certain species are
most active in the winter, others during dry periods, and still others in
flooded conditions.

Not all
organisms are active at a particular time. Even during periods of high activity,
only a fraction of the organisms are busily eating, respiring, and altering
their environment. The remaining portion are barely active or even dormant.

Many different
organisms are active at different times, and interact with one another, with
plants, and with the soil. The combined result is a number of beneficial
functions including nutrient cycling, moderated water flow, and pest control.

THE IMPORTANCE OF THE SOIL FOOD WEB

The living component of soil, the food web, is complex and has different
compositions in different ecosystems. Management of croplands, rangelands,
forestlands, and gardens benefits from and affects the food web. The next unit
of the Soil Biology Primer, “The Food Web & Soil Health,” introduces the
relationship of soil biology to agricultural productivity, biodiversity, carbon
sequestration and to air and water quality. The remaining six units of the Soil
Biology Primer describe the major groups of soil organisms: bacteria, fungi,
protozoa, nematodes, arthropods, and earthworms. For more information about the
diversity within each organism group, see the list of readings at the end of
“The Food Web & Soil Health” unit.

Go to the next chapter:
"The Food Web & Soil Health"

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