USDA - THE SOIL FOOD WEB
Natural Resources Conservation Service
By Elaine R. Ingham
The following is an excerpt. For full article go to: http://soils.usda.gov/sqi/concepts/soil_biology/soil_food_web.html
Soil Biology - Overview
The creatures living in the soil are critical
to soil quality. They affect soil structure and therefore soil
erosion and water availability. They can protect crops from pests
and diseases. They are central to decomposition and nutrient
cycling and therefore affect plant growth and amounts of pollutants
in the environment. Finally, the soil is home to a large proportion
of the world's genetic diversity.
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.
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.
How Do Food Webs Differ?
Each field, forest, or pasture has a unique
soil food web with a particular proportion of bacteria, fungi,
and other groups, and a particular level of complexity within
each group of organisms. These differences are the result of
soil, vegetation, and climate factors, as well as land management
practices..
The “structure” of a food web is the composition
and relative numbers of organisms in each group within the soil
system. Each type of ecosystem has a characteristic food web
structure.
.
What Is Food Web Complexity?
Food web complexity is a factor of both the
number of species and the number of different kinds of species
in the soil. For example, a soil with ten species of bacterial-feeding
nematodes is less complex than a soil with ten nematode species
that includes bacterial-feeders, fungal-feeders, and predatory
nematodes.
Land management practices can alter the number
of functional groups – or complexity – in the soil. Intensively
managed systems, such as cropland, have varied numbers of functional
groups. Crop selections, tillage practices, residue management,
pesticide use, and irrigation alter the habitat for soil organisms,
and thus alter the structure and complexity of the food web.
Disease suppression. A complex soil food web contains numerous
organisms that can compete with disease-causing organisms. These
competitors may prevent soil pathogens from establishing on plant
surfaces, prevent pathogens from getting food, feed on pathogens,
or generate metabolites that are toxic to or inhibit pathogens.
Degradation of pollutants
An important role of soil is to purify water.
A complex food web includes organisms that consume (degrade)
a wide range of pollutants under a wide range of environmental
conditions.
Biodiversity.
Greater food web complexity means greater
biodiversity. Biodiversity is measured by the total number of
species, as well as the relative abundance of these species,
and the number of functional groups of organisms.
Management and Soil
Health
A healthy soil effectively supports plant
growth, protects air and water quality, and ensures human and
animal health. The physical structure, chemical make-up, and
biological components of the soil together determine how well
a soil performs these services.
In every healthy system or watershed, the
soil food web is critical to major soil functions including:
- sustaining biological activity, diversity, and productivity;
-regulating the flow of water and dissolved nutrients;
- storing and cycling nutrients and other elements; and
- filtering, buffering, degrading, immobilizing and detoxifying
organic and inorganic materials that are potential pollutants.
Successful land management requires approaches
that protect all resources, including soil, water, air, plants,
animals and humans. Many management strategies change soil habitats
and the food web, and alter soil quality, or the capacity of
soil to perform its functions. Examples of some practices that
change the complexity and health of the soil community include:
Compared to a field with a 2-year crop rotation,
a field with a 4 crops grown in rotation may have a greater variety
of food sources (i.e., roots and surface residue), and therefore
is likely to have more types of bacteria, fungi, and other organisms.
A cleanly-tilled field with few vegetated
edges may have fewer habitats for arthropods than a field broken
up by grassed waterways, terraces, or fence rows.
Although the effect of pesticides on soil
organisms varies, high levels of pesticide use will generally
reduce food web complexity. An extreme example is the repeated
use of methyl bromide which has been observed to eliminate most
soil organisms except a few bacteria species.
The Food Web and Carbon
Sequestion
Land management practices can be chosen to
increase the amount of carbon sequestered as soil organic matter
and reduce the amount of CO2, a greenhouse gas, released to the
atmosphere.
As the soil food web decomposes organic material,
it releases carbon into the atmosphere as CO2 or converts it
to a variety of forms of soil organic matter. Labile or active
fractions of organic matter stay in the soil for a few years.
Stable forms reside in the soil for decades or hundreds of years.
Physically stabilized organic matter is protected inside soil
aggregates that soil organisms help create. Humified organic
matter is stable because bacteria and fungi have helped form
molecules that are too complex and large for soil organisms to
decompose.