Ecosystems: What Are They and How Do They Work? - Environmental Science - page 50

Terrestrial Ecosystems

Swamps and marshes
Tropical rain forest
Temperate forest
Northern coniferous forest (taiga)
Savanna
Agricultural land
Woodland and shrubland
Temperate grassland
Tundra (arctic and alpine)
Desert scrub
Extreme desert
Aquatic Ecosystems
Estuaries
Lakes and streams
Continental shelf
Open ocean

800 1,600 2,400 3,200 4,000 4,800 5,600 6,400 7,200 8,000 8,800 9,600

2

Average net primary productivity (kcal/m /yr)

Figure 3-20 Natural capital: estimated net primary productivity per unit of area in major life zones and
ecosystems, expressed as kilocalories of energy produced per square meter per year (kcal/m 2 /yr). (Data from
R. H. Whittaker, Communities and Ecosystems, 2nd ed., New York: Macmillan, 1975)

Since the beginning of agriculture, human activi-
ties have led to rapid soil erosion, which can convert
this renewable resource into a nonrenewable resource.
Entire civilizations have collapsed because they mis-
managed the topsoil that supported their populations.

3-5 SOILS

What Is Soil and Why Is It Important?
Soil is a slowly renewed resource that provides most
of the nutrients needed for plant growth and helps
purify water.
Soil is a thin covering over most land that comprises a
complex mixture of eroded rock, mineral nutrients, de-
caying organic matter, water, air, and billions of living
organisms. Study Figure 3-21, which shows a profile of
different-aged soils. Although soil is a renewable re-
source, it is renewed very slowly. Depending mostly
on climate, the formation of just 1 centimeter (0.4 inch)
can take from 15 years to hundreds of years.
Soil is the base of life on land. It provides the bulk
of the nutrients needed for plant growth. Indeed, you
are mostly composed of soil nutrients imported into
your body by the food you eat. Soil is also the earth's
primary filter, cleansing water as it passes through. It
helps decompose and recycle biodegradable wastes
and is a major component of the earth's water re-
cycling and water storage processes. In addition, it
helps control the earth's climate by removing carbon
dioxide from the atmosphere and storing it as carbon
compounds.

Layers in Mature Soils
Most soils developed over a long time consist of
several layers containing different materials.
Mature soils, which have developed over a long time,
are arranged in a series of horizontal layers called soil
horizons, each with a distinct texture and composition
that varies with different types of soils. A cross-
sectional view of the horizons in a soil is called a soil
profile. Most mature soils have at least three of the
possible horizons shown in Figure 3-21. Think of them
as floors in the building of life underneath your feet.
The top layer is the surface litter layer, or O horizon.
It consists mostly of freshly fallen undecomposed or
partially decomposed leaves, twigs, crop wastes, ani-
mal waste, fungi, and other organic materials. Nor-
mally, it is brown or black.
The topsoil layer, or A horizon, is a porous mixture
of partially decomposed organic matter, called humus,
and some inorganic mineral particles. It is usually