Agriculture Reference
In-Depth Information
5
The Living Soil
Soil and the Carbon Cycle
Soil is the living “skin” of the earth's terrestrial ecosystem. Like skin, it is bom-
barded by the sun's radiation, wind, and rain and abraded by all manner of objects
scraping its surface. Unlike skin, after an initial phase of weathering, soil develops
primarily from the surface downward as plant and animal residues are continu-
ally added to the surface layer. These organic residues nourish a diverse popula-
tion of organisms, feeding on the dead residues and on each other. In turn, they
release mineral nutrients in an interminable cycle of growth, death, and decay,
commonly called the carbon (C) cycle. Figure 5.1 is a diagrammatic representa-
tion of the C cycle in a vineyard.
Green plants use energy from the sun to make carbohydrates, and subse-
quently proteins, lipids (fats), and other complex molecules, for their own growth
and reproduction. As plant tissue matures, biochemical changes take place that
lead to senescence; leaves yellow and eventually fall. In a perennial plant such as
the grapevine, leaves are shed in winter but roots grow, age, and die all the time.
Prunings may also be returned to the soil. Collectively, the above-ground plant
material returned to the soil is called litter. Below ground, root fragments that are
“sloughed off” and C compounds that leak from living roots constitute rhizo-C
deposition. This below-ground C material is a readily accessible substrate (food)
for microorganisms, which proliferate in the cylinder of soil surrounding each
plant root, a zone called the rhizosphere. The rhizosphere is also the zone from
which roots take up nutrients, as described in “The Absorbing Root,” chapter 3.
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